(placoderm fishes) from the Aztec Siltstone (late ...

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Australian Journal of Zoology, 2014, 62, 44–62 http://dx.doi.org/10.1071/ZO13070

New arthrodires (placoderm fishes) from the Aztec Siltstone (late Middle Devonian) of southern Victoria Land, Antarctica Gavin C. Young A,C and John A. Long B A

Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia. School of Biological Sciences, Flinders University, PO Box 2100, Adelaide, SA 5001, Australia. C Corresponding author. Email: [email protected] B

Abstract. A small collection of arthrodire remains is described from the Devonian Aztec Siltstone of southern Victoria Land, Antarctica. Barwickosteus antarcticus, gen. et sp. nov., is a small phlyctaeniid arthrodire probably closely related to Barrydalaspis from the Bokkeveld Group of South Africa. Grifftaylor antarcticus, gen. et sp. nov., is a generalised phlyctaeniid resembling Phlyctaenius and Neophlyctaenius. New specimens of Boomeraspis show that it had a high-spired trunk-armour with a median dorsal plate of similar proportions to Tiaraspis, Mithakaspis, Turrisaspis or Africanaspis. Other fragmentary median dorsal plates are provisionally referred to Turrisaspis and Mulgaspis. With these new taxa the vertebrate assemblage from the Aztec Siltstone comprises at least 37 genera and 50 species, making it one of the most diverse of Middle–Late Devonian age.

Received 2 September 2013, accepted 30 December 2013, published online 26 May 2014 Introduction Devonian fossil fish remains were discovered in Antarctica on 31 December 1911 by geologists Frank Debenham and T. Griffith Taylor of Captain Robert Falcon Scott’s British Antarctic ‘Terra Nova’ Expedition (1910–13). These were the first fossil vertebrates, and the first Devonian fossils, found on the Antarctic continent. Griffith Taylor was the leader of the western geological party of the British Antarctic Expedition, which in the summer of 1910–11 manhauled up the coast of McMurdo Sound to Granite Harbour, at the mouth of the Mackay Glacier (Taylor 1913, 1916). The fish remains were found in glacial moraine at Mount Suess, but their provenance somewhere in the hinterland was uncertain. It was assumed that they derived from within the thick sequence of sedimentary rocks called the ‘Beacon Sandstone’ (now Beacon Supergroup), well exposed in the region of the lower Ferrar and Taylor Glaciers (Debenham 1921). The first discovery of Devonian fish fossils has always been meticulously attributed to Frank Debenham, as acknowledged in the species Byssacanthoides debenhami Woodward 1921. They were found when establishing a campsite on the moraine of Gondola Ridge on New Year’s Eve, 1911, and would have been noticed by any trained geologist within a short time of arrival. Debenham subsequently made scant mention of these fossils, but from the outset Griffith Taylor, already a trained and published palaeontologist (Taylor 1910), was interpreting the fragmentary remains. In January 1912 the party was marooned for three weeks at Cape Roberts (southern side of Granite Harbour), when Griffith Taylor wrote up the first account (his journal was left there and not retrieved until the following Journal compilation Ó CSIRO 2014

spring). This stated: ‘Debenham had the honour to find some fossils first . . . bluish plates crustacean in nature’ (Hanley 1978: p. 167). In the ‘Narrative of the western geological parties’, written up by Griffith Taylor at Tewkesbury (Raymond Priestley’s home) in 1913, he had revised his crustacean idea, likening the remains to ‘a certain Mesozoic fish’ although ‘not yet . . . submitted to a specialist’ (Hanley 1978: p. 126). He described ‘plates shaped like the tiles carrying a roof ridge’, clearly remains of the very common antiarch Bothriolepis (Young 1988), and others ‘with a beautiful bluish lustre’ which can be ascribed to one of the cosmine-covered lobe-finned fishes such as Koharalepis Young et al. (1992); both taxa are readily identified from Griffith Taylor’s sketches (e.g. Taylor 1930, fig. 6). Griffith Taylor first announced the Devonian rather than Mesozoic age for these fish, based on determinations of the British Museum expert A. S. Woodward, in his presentation to the Royal Geographical Society in London (15 January 1914). However, Griffith Taylor was not the first to publish on this ‘extremely important discovery’ (David and Priestley 1914: p. 243), this being a note added by Edgeworth David to proofs after he heard Taylor’s presentation, the latter not being published until the end of that year (Taylor 1914). The Devonian age was confirmed by Woodward (1916), and the first description (Woodward 1921) identified eight taxa of Devonian fishes, including two types of placoderms (the antiarch Bothriolepis, and an ‘undetermined coccostean’). During the Trans-Antarctic Expedition of 1955–58 (Gunn and Warren 1962) the first in situ Devonian fish material was collected in the Skelton Névé region, some 150 km south of www.publish.csiro.au/journals/ajz

Devonian fossil fish from Antarctica

Australian Journal of Zoology

Granite Harbour (Fig. 1). White (1968) described this material. Many new fossil localities were discovered in the same area during the 1968–69 and 1970–71 summer field seasons of the New Zealand Antarctic Research Program (NZARP) by two Victoria University of Wellington Antarctic Expeditions (VUWAE 13, 15; see McKelvey et al. 1972; Ritchie 1972; McPherson 1978). Later expeditions (1976–77, 1988–89, 1991–92; M. A. Bradshaw, NZARP Event 33; J. A. Long, NZARP-ANARE expedition) collected material from new localities 100 km to the south in the Cook Mountains (Woolfe et al. 1990) but these predominantly represent upper zones of the fish assemblage (Young and Long 2005). The Aztec Siltstone is the most fossiliferous formation within the Devonian Taylor Group of the Beacon Supergroup (see Bradshaw 2013), and apart from the very diverse fossil fish assemblage, is notable for its preservation of fossilised soil horizons (McPherson 1979), some of which have been interpreted as the oldest to have formed in a forest environment (Retallack 1997). The Aztec fauna includes representatives of most of the major Devonian fish groups (Table 1): thelodont agnathans (Turner and Young 1992), chondrichthyans (Young 1982; Long and Young 1995; Burrow et al. 2009), placoderms (Ritchie 1975; Young 1988; Long 1995; Young and Long 2005), dipnoans 77°

25 km

N

78°

45

(Campbell and Barwick 1987, fig. 2; Young 1989a, 1991), other sarcopterygians (Young et al. 1992; Young 2008a), acanthodians (Young 1989b; Young and Burrow 2004; Burrow et al. 2009), and actinopterygians (Young 1989a; Long et al. 2008a). Devonian fishes also occur in West Antarctica, in the Ohio Range and Ellsworth Mountains (V. T. Young 1986; Young 1992). The placoderm fishes are the most diverse group of Devonian vertebrates, and the placoderm order Arthrodira is the most diverse placoderm subgroup (Young 2010a). The arthrodire material described below was mainly collected in 1970–71 (VUWAE 15). White (1968) had erroneously determined as an acanthodian spine what is clearly a placoderm spinal plate (Denison 1978), which he called ‘Cosmacanthus? sp.’, a taxon first described from the Upper Devonian of Scotland by Agassiz (1844), and likely to belong to the arthrodire Groenlandaspis (A. Ivanov, pers. comm.). In addition, from the 94 specimens at his disposal, White described two new arthrodire genera (Antarctaspis, Antarctolepis), and identified ‘arctolepids indet.’ from six of the eight collected localities. Denison (1978) interpreted Antarctaspis as closely related to the Phyllolepida, but Young and Goujet (2003) documented a range of shared characters with Yujiangolepis from China, and Toombalepis from Australia. Ritchie (1975) described a new species Groenlandaspis antarctica, and noted additional species yet to be described. Young (1991, fig. 15.10b) illustrated a left posterior dorsolateral plate of an ‘undetermined arthrodire’ from Portal Mountain, and noted other isolated arthrodire plates that did not belong to Groenlandaspis, including one from the Lashly Range said to resemble Barrydalaspis Chaloner et al. 1980 from the Devonian of South Africa. This material is described below. Elsewhere in Antarctica, arthrodire remains have been recorded from the Horlick Formation (Early Devonian) of the Ohio Range (Miles, in Doumani et al. 1965; Young 1986). With the new descriptions below, the Aztec fish fauna contains at least 37 genera and 50 species (Table 1), making it the most diverse fossil vertebrate assemblage from Antarctica, and one of the most diverse fish faunas of Givetian–Frasnian age known anywhere (cf. Schultze and Cloutier 1996). The diversity of some other important Devonian fossil fish sites may be compared: Miguasha (Quebec, World Heritage site), 21 genera; Canowindra (New South Wales, Australia), 8 genera; Burrinjuck, New South Wales, 64 genera; Gogo, Western Australia, 51 genera. The last two represent Devonian tropical marine reef environments (Long and Trinajstic 2010; Young 2011) and higher diversity would be expected. The dual connection between Richard Barwick and Griffith Taylor, first as pioneers in the exploration of the Transantarctic Mountains, where the Barwick and Taylor Valleys are named after them (Fig. 1), and second for their research on, and interest in, Devonian fishes, is ample justification for naming some of the fish taxa from the Aztec Siltstone in their honour. Localities

Fig. 1. Twenty-four localities for the Devonian Aztec fish fauna of southern Victoria Land (from Young 1988, fig. 3).

Localities producing the arthrodire material described below are indicated in Fig. 1 (locality numbers from Young 1988).

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G. C. Young and J. A. Long

Table 1. Current faunal list for the Aztec Siltstone fish fauna Includes new taxa described in this paper Agnatha Gnathostomata

Osteichthyes

Dipnoi

Rhipidistia

Actinopterygii Chondrichthyes

Acanthodii

Placodermi

Antiarcha

Arthrodira

Phyllolepida

incertae sedis

Previous arthrodire material described by White (1968) came from the Boomerang Range (Antarctolepis, loc. 20), Lashly Range (Antarctaspis, loc. 8), and ‘arctolepids indet.’ from moraine at locs 1, 2, and also the Lashly and Boomerang Ranges (locs 8, 20). Ritchie (1975) described Groenlandaspis antarctica

Turinia antarctica Turner & Young, 1992 ?Eoctenodus sp. Howidipterus sp. ?ctenodontid indet. Gyroptychius? antarcticus (Woodward) Koharalepis jarviki Young, Long & Ritchie, 1992 Mahalalepis resima Young et al., 1992 Platyethmoidia antarctica Young et al., 1992 Vorobjevaia dolonodon Young et al., 1992 Notorhizodon mackelveyi Young et al., 1992 Aztecia mahalae Johanson & Ahlberg, 2001 porolepiform indet. Donnrosenia schaefferi Long, Choo & Young, 2008 ?palaeoniscoid indet. [type III of White 1968] Mcmurdodus featherensis White, 1968 Antarctilamna prisca Young, 1982 Anareodus statei Long & Young, 1995 Aztecodus harmsenae Long & Young, 1995 Portalodus bradshawae Long & Young, 1995 ?Elasmobranchii gen. et sp. indet. Burrow et al., 2009 Gyracanthides warreni White, 1968 Antarctonchus glacialis White, 1968 Byssacanthoides debenhami Woodward, 1921 Culmacanthus antarctica Young, 1989 Milesacanthus antarctica Young & Burrow, 2004 Pechoralepis juozasi Burrow, Long & Trinajstic, 2009 Nostolepis sp. cf. N. gaujensis Burrow et al., 2009 acanthodid? gen. et sp. indet. Burrow et al., 2009 Bothriolepis antarctica Woodward, 1921 Bothriolepis alexi Young, 1988 Bothriolepis askinae Young, 1988 Bothriolepis barretti Young, 1988 Bothriolepis karawaka Young, 1988 Bothriolepis kohni Young, 1988 Bothriolepis macphersoni Young, 1988 Bothriolepis mawsoni Young, 1988 Bothriolepis portalensis Young, 1988 Bothriolepis vuwae Young, 1988 Bothriolepis spp. indet. 1–13 Venezuelepis antarctica Young & Moody, 2002 Antarctolepis gunni White, 1968 Barwickosteus antarcticus, gen. et sp. nov. Boomeraspis goujeti Long, 1995 Grifftaylor antarcticus, gen. et sp. nov. Groenlandaspis antarctica Ritchie, 1975 Groenlandaspis spp. ?Mulgaspis sp. indet. ?Turrisaspis sp. indet. Austrophyllolepis quiltyi Young & Long, 2005 Placolepis tingeyi Young & Long, 2005 Yurammia sp. nov. Young, 2005 Antarctaspis mcmurdoensis White, 1968

from four localities: loc. 11 (Portal Mountain), loc. 14 (southern end, Mt Metschel), loc. 16 (northern Boomerang Range) and loc. 24 (southern Warren Range), but abundant undescribed groenlandaspid material occurs at many other localities (A. Ritchie, pers. comm.). Long (1995) described a new


groenlandaspid Boomeraspis from loc. 21 (Alligator Peak, Boomerang Range), and additional localities for this taxon are documented below. The locality details for the arthrodire material described in this paper are summarised below for each taxon with reference to the locality numbers of Fig. 1. Detailed discussion of measured sections and fossil fish horizons at each locality are given in Young (1988). Biostratigraphy and age Woodward’s (1921) initial age assessment of ‘Upper Devonian’ was queried by White (1968), and the Aztec Siltstone fish fauna is now considered to be somewhat older, and referred mainly to the late Middle Devonian zonal scheme for East Gondwana (Givetian) (see Young 1993, 1996; Young et al. 2010; Burrow et al. 2010). Turner (1997) considered the thelodont Turinia antarctica to be of early Givetian age; this taxon identifies the lowest two zones in the biostratigraphic scheme for the Aztec Siltstone sequence of Young (1988). The upper two ‘Pambulaspis’ and overlying ‘phyllolepid’ zones were combined by Young (1993: p. 228), partly on the evidence that the antiarch Pambulaspis co-occurs with phyllolepids in south-eastern Australia (G. C. Young 1983). The Antarctic ‘Pambulaspis’ was assigned to the genus Venezuelepis Young & Moody, 2002, but this taxon is also associated with phyllolepid remains in the Venezuelan fish fauna (Young et al. 2000; Young and Moody 2002), and fragmentary phyllolepids occur with an unnamed species of Venezuelepis at the Victorian Tatong locality (Long 1989; Young et al. 2010, fig. 8). Young and Long (2005) referred the phyllolepids from the Aztec assemblage to Australian genera Placolepis and Austrophyllolepis, known only from the Givetian–Frasnian. These come from the uppermost biostratigraphic zone of Young (1988). Previously, the oldest known phyllolepid remains were referred to Zone 6d (karawaka zone) of the Aztec Siltstone sequence (Young 1993, fig. 9.3), but Young and Long (2005) revised this downward to at least to Zone 6c. The lowermost Zones 6a–b of the Aztec Siltstone sequence are defined by the unique co-occurrence of the turiniid thelodont Turinia antarctica with the first Bothriolepis species (B. askinae, B. kohni). Most of the arthrodire material described below comes from these and the next higher portalensis zone of Young (1988), and thus would approximate to the middle Givetian (Zone MAV6) (Young et al. 2010, fig. 6). Currently there is no independent biostratigraphic control of these ‘non-marine’ macrovertebrate zones relative to the Middle–Late Devonian boundary. Material and methods Fossils are mainly preserved as white bone in a sandstone/ siltstone matrix. Well preserved bone was retained and margins exposed by mechanical preparation where necessary. Poorly preserved bone was removed mechanically or by acid etching, with impressions studied using latex rubber casts whitened with ammonium chloride. Material is housed in the College of Science, Australian National University, Canberra (prefix ANU V), the Australian Museum, Sydney (prefix AMF), Geoscience Australia, Canberra (prefix CPC), the Western Australian


47

Museum, Perth (prefix WAM), and the Natural History Museum, London (prefix NHM P). Standard abbreviations for placoderm dermal bones used in the text and figures, and other figure abbreviations are listed here alphabetically: ADL, anterior dorsolateral plate; AL, anterior lateral plate; AMV, anterior median ventral plate; AVL, anterior ventrolateral plate; cfPDL, area in contact with opposite PDL plate; dent, denticles; dep, depression; dlr, dorsolateral ridge of trunk-armour; gr.eh, groove for epihyal element; ifpr, infrapectoral process of AVL plate; IL, interolateral plate; llc, groove for main lateral line sensory canal; MD, median dorsal plate; MxL, mixilateral plate of Bothriolepis; n, notch, n1,2, notches in ventral margin of MD plate; oaADL, area overlapped by ADL plate; oaAL, area overlapped by AL plate; oaAVL, area overlapped by AVL plate; oaMD, area overlapped by MD plate; oaPDL, area overlapped by PDL plate; oaPL, area overlapped by PL plate; pc, posterior corner; pect, embayment for pectoral fin insertion; PMD, posterior median dorsal plate; PMV, posterior median ventral plate; PVL, posterior ventrolateral plate; SM, submarginal plate; SP, spinal plate; th, thickening; tr.r, transverse ridge (keel); vlr, ventrolateral ridge of trunk-armour. Systematic palaeontology Class Placodermi McCoy, 1848 Remarks Since the 1930s, when Gross (1931) and Stensiö (1931) independently demonstrated that the Antiarchi (a major subgroup) are placoderms, rather than jawless agnathans, as previously suggested, there has been a consensus that the Placodermi is a monophyletic group. However several recent papers have challenged this. Johanson (2002) partially resurrected the agnathan idea for antiarchs by proposing similarities in the shoulder girdle not seen in other gnathostomes, and Brazeau (2009) proposed that, uniquely amongst jawed vertebrates, the antiarchs Bothriolepis and Pterichthyodes primitively lacked pelvic fins, a condition otherwise known only in agnathans (e.g. osteostracans). Accepting these and other characters as valid has produced phylogenies with the antiarchs (i.e. Bothriolepis and Pterichthyodes) placed as a sister group to other placoderms plus other gnathostomes, making the Placodermi a paraphyletic group (Brazeau 2009, fig. 3; Davis et al. 2012, fig. 4). However, Young (2008b) showed that Johanson (2002) had misinterpreted antiarch morphology, and thus the resemblances to agnathans were invalid, whilst Zhu et al. (2012) have demonstrated pelvic fins in early antiarchs from China, bringing them into line with other placoderms, and all other gnathostomes. Most recently, a new form from China, Entelognathus Zhu et al. 2013, uniquely combines a placodermlike dermal armour with osteichthyan-like dermal bones on the jaws (premaxilla, maxilla, dentary, gulars, etc.). This confirms other evidence from several Chinese forms indicating that the common ancestor of placoderms and osteichthyans possessed large dermal bones (see discussion in Young 2010a: p. 540). In complete contrast, the phylogeny of Davis et al. (2012) implies shark-like conditions in the last common ancestor of

48


modern gnathostomes. Any new taxon (such as Entelognathus), combining characters previously thought to be unique to two major groups, could imply that one or other of those groups (in this case either placoderms or osteichthyans) is paraphyletic. However, most taxa within both groups still display many other characters supporting their monophyly (for placoderms see Young 2010a: p. 539), and reinterpretation of the full character set in the light of the new taxon Entelognathus is beyond the scope of the present paper. Order Arthrodira Woodward, 1891 Family Phlyctaeniidae Fowler, 1947 Genus Barwickosteus, gen. nov. Type species: Barwickosteus antarcticus, sp. nov.

Etymology Commemorating Richard Essex Barwick, participant of the original Victoria University Antarctic Expeditions, and ‘osteus’, after the Greek for ‘bone’.


at collecting site MC2 of Young (1988, fig. 4). The Portal Mountain (P2) material (Locality 11, Fig. 1) is from a higher fossil horizon (Unit 26, Section 10 of Barrett and Webb 1973) than the original Portal Mountain collecting site discovered by VUWAE 13 in 1968–69 (from Unit 17). Diagnosis As for genus (only species). Remarks The new taxon is a generalised phlyctaeniid of small size known only by its trunk-armour. It is distinguished from various groenlandaspids by ADL shape, the longer spinals with large recurved mesial denticles, relatively longer PVLs and PMV, the latter embaying the PVLs, and a more narrow AMV. It resembles Barrydalaspis in the broad pectoral embayment and recurved mesial denticles on the elongate spinals, but differs in the less divergent spinals, which are relatively longer, and especially in the longer PVLs and larger PMV. Description

Diagnosis A phlyctaeniid of small size with a trunk-armour characterised by a broad pectoral embayment, spinal margin length ~65% of AVL width, and angle between ventrolateral ridge and spinal margin of nearly 50; spinals diverging ~30 from the midline in the restored armour, slender and curved, with 5–9 recurved denticles on inner margin of spines, which reach almost as far back as the end of the ventral armour. AVLs and PVLs of similar length; AVL with B/L index of ~127; PMV deeply separating PVLs anteriorly; subanal lamina of PVL relatively narrow. ADL triangular, with angled dorsolateral ridge, pointed dorsal margin and narrow overlap for MD plate; posterior angle of ADL situated ventral to lateral line sensory groove. Ornament of fine crowded tubercles on all dermal bones. Barwickosteus antarcticus, sp. nov. Figs 2, 3, 4A Synonymy ‘material . . . shows . . .resemblance to the genus Barrydalaspis’ (Young 1991: p. 549) ‘phlyctaeniid arthrodire’ (Young 2003, fig. 1E)

Etymology From Antarctica. Material examined Holotype. ANU V886, associated ADL, AVL, AMV, PVL and SP plates presumed to come from one individual. Referred material. ANU V887–890 from Mount Crean, Lashly Range (MC2); ANU V850–853 from Portal Mountain (P2).

Localities and horizon The type material, from Mount Crean (Locality 8, Fig. 1) comes from the lower Aztec Siltstone (Section L2 of Askin et al. 1971)

V886 is an association of small trunk armour bones (AVL, AMV, SP, ADL, two PVLs) that fit together, and evidently came from the same fish. The right AVL (Fig. 2B) is immediately adjacent to an incomplete SP plate, but this is too large to fit against it (Fig. 2G). A smaller SP 37 mm away (Fig. 2A) is the correct size. The AVL has a broad pectoral embayment and divergent spinal margin reminiscent of Barrydalaspis Chaloner et al., 1980. A suggested suture indicates that part of the AMV is attached (Figs 2B, 3A), as confirmed by a better preserved AVL (ANU V887; see below). Part of the bone surface is abraded, but fine ornament is seen on much of the ventral lamina, posterior part of the lateral lamina, and the anterior part of the infraspinal process (ifpr, Fig. 3A). Behind this, adjacent to the pectoral embayment (pect), is a smooth depressed area (dep), its anterior edge ~90 from the ventrolateral ridge (vlr). The attached AMV is preserved asymmetrically, 4 mm at its widest, but very incomplete anteriorly (Fig. 3A). It is displaced partly over the AVL, so broken margins of the two plates are difficult to distinguish, but evidently it was broader anteriorly. The posterior margin appears notched, with a slightly convex right lateral margin preserved in front (crushed onto the edge of the AVL, which always overlaps the AMV here). The second AVL (ANU V887; Fig. 2E) is better preserved, with the same shape, broad pectoral embayment and long divergent spinal margin as in the holotype. It is slightly smaller than the opposite AVL of the holotype (Fig. 3B), so must represent a second individual. B/L index is 127 (L 13; B 16.5); i.e. within the range given for Barrydalaspis (135 in smaller specimens; 125 in larger: Chaloner et al. 1980: p. 130). These authors gave no details of size range of their material, but a cast of the holotype (ANU V3588) is more than twice as large (AVL length 31 mm). The spinal margin length in V887 (11.5 mm) is 65% the width of the plate, whereas in Barrydalaspis it is 44–48% (smaller in larger specimens). The angle between the ventrolateral ridge and the spinal margin measures 48–49 in both AVLs, but is less (~30) in the restored armour, and not as divergent as in Barrydalaspis


(A)


(E)

(B)

49

(F )

(C)

cfPDL

llc

th

(I)

(D)

(G) A–F G –H I 5 mm

(H)

Fig. 2. (A–H) Barwickosteus antarcticus, gen. et sp. nov. (A–D) Holotype, ANU V886: (A) SP plate; (B) right AVL with AMV; (C) left ADL; (D) two PVLs. (E) ANU V887, left AVL. (F–H) Isolated SP plates (ANU V888, V886a, V890). (I) Groenlandaspid PDL plate from the same beds, internal view, referred to ?Turrisaspis sp. indet. (ANU V889).

(~45: Fig. 4). As noted by Chaloner et al. (1980), this divergent angle is reminiscent of petalichthyids, and amongst phlyctaeniids is otherwise approached only in Phlyctaenius acadica, which, however, has a shorter SP plate (Denison 1958, fig. 112) (longer in other species: V. T. Young 1983). The SP of Barwickosteus is clearly longer than in the holotype of Barrydalaspis (Fig. 4A, B). The right SP of the Barwickosteus holotype (Fig. 2A) is 4 mm at maximum width and at least 30 mm long (missing its distal end). It is more slender and curved than the holotype spinal of Barrydalaspis, but has similar ornament. Five recurved denticles are preserved on the

mesial border of the free part. The nearby larger spinal (Fig. 2G; 6 mm maximum width, preserved length 40 mm) shows at least eight denticles (about one-third of the distal end is missing). ANU V888 (Fig. 2F) is a smaller example (preserved L 16 mm; maximum B 2.5 mm), incomplete proximally, with a row of fine denticles on the leading edge, and at least seven large recurved denticles posteriorly. V890 is a larger example with nine recurved denticles (Fig. 2H), whereas only five are shown on Barrydalaspis (Chaloner et al. 1980, fig. 3). Two PVLs still attached together in the midline (Fig. 2D) can be attributed to the holotype, as they are the correct size and

50



(A)

(C)

ifpr

oaMD AMV

dlr

pect dep

AVL

llc

cd pc

vlr oaAL (B)

AL 5 mm ifpr (D)

pect

vlr

oaAVL

Fig. 3. Barwickosteus antarcticus, gen. et sp. nov. Camera lucida drawings of trunk-armour bones of ANU V886 (holotype: A, C, D) and V887 (B). (A) right AVL with AMV, ventral view; (B) left AVL, ventral view; (C) left ADL, lateral view; (D) right PVL, lateral lamina in lateral view.

located 60 mm from the holotype AVL. This association permits a reliable approximation of the whole ventral armour (Fig. 4A). The large triangular space for the PMV (Fig. 2D) demonstrates that it was considerably larger than in Barrydalaspis (Fig. 4B), and more deeply separated the PVLs than in Groenlandaspis (Fig. 4D). The right lateral lamina of the PVL has a rounded dorsal margin, concave anteriorly with a rounded process between it and the overlap for the AVL (Fig. 3D). In the restored ventral armour of Barwickosteus the spinals reach almost as far back as the end of the PVLs. Spinals of similar length occur in Arctolepis and Neophlyctaenius, and in other forms (Elegantaspis, Heintzosteus) they are both straighter and longer, extending back past the ventral trunk-armour (Denison 1978, fig. 39; Goujet 1984, figs 88, 105). However, Barwickosteus differs from all of these in the more elongate PMV and PVLs, and the broader more transverse pectoral embayment, which approaches the condition in Barrydalaspis (Fig. 4A, B). The left ADL assigned to this taxon (22 mm from the PVLs) is quite different from that attributed to Barrydalaspis. The condyle is poorly preserved (cd) and its shape is uncertain

(Figs 2C, 3C). The bone overall is triangular in shape, with a pointed dorsal margin and a narrow overlap for the MD plate (oaMD). In general shape it most resembles the ADL of Tiaraspis (Denison 1958, fig. 108). The posterior margin is not quite complete, but evidently the posterior angle (pa) lies ventral to the lateral line sensory groove (llc), an unusual configuration that differs from all other ADLs. A distinct dorsolateral ridge (dlr) is angled upwards and backwards above the similarly oriented sensory groove. The overlap for the AL plate (oaAL) evidently still has part of that plate attached (AL). The external surface where well preserved shows fine crowded tubercles as on the AVL. Genus Grifftaylor, gen. nov. Type species: Grifftaylor antarcticus, sp. nov.

Etymology Commemorating T. Griffith Taylor, leader of the Western Geological Party of the British Antarctic Terra Nova Expedition



(A)

51

(B)

AMV AVL AVL SP

SP PMV

PVL

PVL (D)

AMV

AVL AMV

(C)

AVL (E)

PVL

PVL

PMV

AVL

SP

PVL

Fig. 4. Comparison of various arthrodire trunk-armours in ventral view. (A) Barwickosteus antarcticus, gen. et sp. nov., based on ANU V886, 887; (B) Barrydalaspis theroni, modified from Chaloner et al. (1980, fig. 2); (C) Grifftaylor antarcticus, gen. et sp. nov., based on ANU V852; (D) Groenlandaspis antarctica, after Ritchie (1975); (E) ‘phlyctaenaspid arthrodire’, modified from Chaloner et al. (1980, fig. 5).

(1910–13), the first to discover Devonian fossil fish remains in Antarctica on 31 December 1911.

ornament of crowded tubercles, coarser on PVL than AVL, some areas showing strongly reticulate ridges formed by aligned tubercles. Grifftaylor antarcticus, sp. nov.

Diagnosis A phlyctaeniid with a trunk-armour in which the PDL is longer than high (L/B index ~125); sensory groove reaches or almost reaches the posterior margin, with slight inflection at PDL ossification centre, passing anteriorly off the bone at or near ventral edge of ADL overlap area. Relatively large subrectangular PMD plate (approaching half the length of PDL), which is about as wide as long, and lacks lateral corners. Length of AL plate ~130% its height. AVL plate with long spinal margin oriented at ~22 to the midline; lateral lamina of AVL enclosing pectoral fenestra posteriorly. PVL relatively short, ~70% AVL length; AMV probably narrow, and of similar length to PMV. Dermal

Figs 4C, 5, 6

Synonymy ‘phlyctaeniid arthrodires’ (Young 1988: p. 15; pl. 11, fig. 5) ‘phlyctaeniid arthrodires indet.’ (Young 1989a, table 1 (pars)) ‘undetermined arthrodire from Portal Mountain’ (Young 1991, fig. 15.10(b))

Etymology From Antarctica.

52



(B)

(A)

(D)

IL SP

AV L (C)

PMD SP (E)

AL

PVL

PDL H D A,E B,F,G C 10 mm g r.e h

(F)

dep

(G)

(H)

Fig. 5. Grifftaylor antarcticus, gen. et sp. nov. (A) left PDL, external view (ANU V2612); (B) right PDL, impression of external surface (ANU V2320); (C) left PDL (with CPC 26344); (D) articulated ventral armour, ventral view (ANU V852); (E) Associated bones, external view (ANU V851); (F, G) isolated SM plate, external and internal views (ANU V2147); (H) right AL plate partly preserved as bone, internal view (ANU V850). All except (B), (H) are latex casts whitened with ammonium chloride.



53

portalensis has been recorded from all these localities (Young 1988).

oaMD (A) oaADL

Diagnosis As for genus (only species). Remarks llc

oaMD (B)

oaAL

dlr

oaPL oaMD

(C)

oaADL llc

oaAL

oaPL oaMD (D)

llc

oaAL

oaPL

dlr A,C B

llc D

10 mm

oaAL

oaPL

Fig. 6. Grifftaylor antarcticus, gen. et sp. nov. Camera lucida drawings of PDL plates in external view. (A) left PDL (ANU V2612); (B) left PDL (ANU V2320); (C) right PDL (ANU V851); (D) left PDL (with CPC 26344).

Material examined Holotype. ANU V2162, a left PDL plate preserved in part and counterpart (Figs 5A, 6A). Referred material. ANU V850–853, 2147–2161, 2180, 2328–2330, and CPC 26344 (associated with Bothriolepis portalensis Young, 1988).

Localities and horizon The type locality (Locality 11, Fig. 1) is the original Portal Mountain fish locality discovered by VUWAE 13 in 1968–69. This material comes from two horizons, V2147–62 from the original collecting site (Unit 17, Section 10 of Barrett and Webb 1973), and V850–53 from a higher fossil horizon (Unit 26; ‘P2’ collecting site), ~30 m above Unit 17. V2328–30 come from the northern Boomerang Range (Locality 16, Fig. 1; Unit 4, Section A2 of Askin et al. 1971). The PDL with CPC 26344 comes from the southern Warren Range (Locality 23, Fig. 1), from a sandstone near the base of Section A5 of Askin et al. (1971). Bothriolepis

The new taxon is known only by bones of the trunk-armour. It is distinguished from all other phlyctaeniids by the PDL plate being longer than high. Otherwise the known trunk-armour bones resemble generalised phlyctaeniids like Phlyctaenius acadica or Neophlyctaenius sherwoodi, but the new taxon differs from the former in the longer spinal margin on a more elongate AVL, probably smaller PMV than AMV, and relatively shorter PVL (~71% of AVL length); from the latter it also differs in the relatively shorter PVL compared with the AVL, probably shorter and less curved spinals, and shallower pectoral embayment on the AL plate. The configuration of the anteroventral overlap area on the holotype PDL indicates that this plate cannot belong to Antarctolepis, the type specimen of which is an AL plate with a much more angular posterodorsal margin (White 1968, pl. 3, fig. 1). Antarctolepis also had finer and crowded tubercular ornament. Description The holotype (Fig. 5A) is a left PDL plate previously figured by Young (1991, fig. 15.10b). The internal impression has almost complete margins, permitting a good restoration (Fig. 6A). It is generally lower and broader than comparable PDL plates figured by Denison (1958, fig. 109). Its coarser morespaced tubercles resemble somewhat the ornament of the ‘undetermined arctolepid plate’ from the Lashly Range of White (1968, pl. 3, fig. 3), but an undetermined PDL plate from the Boomerang Range (White 1968, fig. 13) has a completely different shape. The PDL is not known in Barrydalaspis, and the ADL attributed to that form (Chaloner et al. 1980, fig. 4) resembles the ADL of Groenlandaspis (cf. Ritchie 1975). The same applies to an ADL referred by Forey et al. (1992, fig. 5b) to Wajidosteus. ANU V851 shows a very similar PDL with several associated bones (Fig. 5E). The almost complete right PDL has a similar shape to ANU V2162 (Fig. 6C), although tubercles of the ornament are more crowded, with less alignment. It is ~70% the size of V2162, so this could be size related. A second incomplete PDL nearby is also a right plate, so at least two individuals are represented in the sample. A left PDL (V2320) (Fig. 5B) is also smaller (L ~21 mm, preserved height 20 mm, but the posterodorsal edge is missing); it differs from the holotype in having a more pronounced dorsolateral ridge and indented overlap for the ADL (Fig. 6B). The smallest example of the PDL (Figs 5C, 6D) also has non-aligned tubercles, consistent with increasing alignment with growth. This specimen was previously illustrated in association with bones of Bothriolepis portalensis (Young 1988, pl. 11, fig. 5). Compared with V2162 the overlap for the AL plate appears to be more extensive posteriorly, judging by a notch in the dorsal edge of the overlap area. This suggests that it was as long as, or slightly longer than, the overlap for the PL plate, whereas in larger

54


specimens the PL overlap is much longer (Fig. 6A–C). There is also a stronger ridge (dlr) on the ADL overlap area, also expressed in V2320 (Fig. 6B), the next smallest example. More marked dermal ridges or crests on young individuals is well documented in other placoderms (e.g. Bothriolepis: Young 1988; Materpiscis: Long et al. 2008b), so these are considered juvenile features. We provisionally include all these remains in a single species. These PDLs are low and long compared with other phlyctaeniids. The length of the exposed part (overlaps often being incomplete) is 1.5–1.8 times the height, whereas in most others the height is greater than the length (e.g. Denison 1958, fig. 109), or about equal in Phlyctaenius acadica (Denison 1978, fig. 42). The latter species is otherwise similarly developed to our new taxon with respect to overlaps and sensory groove configuration, but its total length is only ~0.8 times the height, whereas this is 1.26 in the holotype of Grifftaylor antarcticus. The PDL of Neophlyctaenius also has length less than height (0.9: Denison 1950, fig. 3). The PDL of Denisonosteus is even higher, with a much reduced sensory groove visible only near the front of the bone (Young and Gorter 1981, fig. 24D). The sensory groove in Grifftaylor antarcticus has only a slight inflection, similar to that of Phlyctaenius acadica, in marked contrast to Groenlandaspis or Boomeraspis, both of which have a very strong dorsal inflection typical of groenlandaspids. The sensory groove inflection is less pronounced in Tiaraspis (Gross 1962), but its PDL is dramatically different in proportions, being much higher than long, as in all groenlandaspids. Immediately above the PDL in V851 is a smaller rectangular plate (PMD, Fig. 5E), clearly a posterior dorsal plate, as were probably generally developed in primitive arthrodires and other placoderm groups, even if preserved in only a small number of taxa. This element is relatively large with respect to the size of the PDL, and has a more rectangular outline than in Aethaspis (Denison 1958, fig. 93), and it lacks the lateral corners of a posterior dorsal plate figured by White (1968, fig. 11). An associated right PVL shows only the anterior overlap for the AVL, and the mesial overlap for the left PVL, but the ventral armour is well preserved in a second specimen from the same locality and horizon (Portal, Unit 26). ANU V852 shows a well preserved associated right AVL and PVL with incomplete SP and IL plates (Fig. 5D). The spinal margin of the AVL is well preserved, straight, and long, forming a strong prepectoral corner. It is more laterally directed than in Groenlandaspis, and proportionately much longer than in the ‘phlyctaeniid arthrodire’ restoration from the Bokkeveld Group of South Africa by Chaloner et al. (1980, fig. 5). A clear angle at its anteromesial extremity enclosing ~110 is mirrored by concentric growth lines in the ornament. Although incomplete mesially, a very broad AMV, as in Groenlandaspis, could not be accommodated in a restoration based on this specimen (Fig. 4C). The pectoral embayment is well preserved, with the upper edge of the pectoral fenestra also seen (formed by the AL plate). The AVL in lateral view has an upward projection posteriorly, evidently abutting the PVL lateral lamina, and enclosing the pectoral fenestra posteriorly, a different arrangement to Groenlandaspis (Ritchie 1975), but seen in more generalised phlyctaeniids (e.g. Dicksonosteus: Goujet 1975). The PVL, slightly displaced from the AVL to reveal its anterior


overlap, is relatively short (71% of AVL length). An even shorter PVL (55%) occurs in the ‘phlyctaeniid arthrodire’ illustrated by Chaloner et al. (1980). As far as preserved, the subanal lamina in V852 was rounded posteriorly, and the aligned tubercles indicate that the left PVL overlapped the right in the midline (Fig. 4C). The distal part of the SP is preserved as several broken pieces at the edge of the specimen (SP, Fig. 5D), including ~10 mm of free mesial margin with a row of mesial denticles (total length unknown). The anterior edge shows another elongate bone impression with a small subcircular overlap mesially, assumed to be the other end of the SP as it lies subparallel to the spinal margin of the AVL. At the edge is the displaced lateral margin of another bone, sitting adjacent to the subcircular overlap, evidently the IL plate (IL, Fig. 5D). The ornament of this ventral armour is variably developed, mostly showing strong lineation of tubercles concentric to the plate margins, with ornament coarser on the PVL than the AVL, except over the ossification centre of the latter, which is also marked by a patch of coarser unaligned tubercles. Ornament on the small preserved parts of the SP and IL also seem much coarser than adjacent parts of the AVL. Inside the pectoral margin are finer tubercles coalesced in distinct rows oriented more normal to the margin, rather than subparallel to it as in other parts of the bone. A similar arrangement is depicted in Ritchie’s (1975) restoration of Groenlandaspis antarctica. A restoration of this ventral armour (Fig. 4C) can be compared with other taxa. Grifftaylor differs from Groenlandaspis (Fig. 4D) in several characters: (1) the AMV was smaller and not as expanded anteriorly, as indicated by the shape of the anteromesial margin of the AVL; (2) the presence of mesial spines on the SP plate, and (3) the rounded subanal lamina of the PVL. The last two features are also found in Boomeraspis, but differences to the right PVL figured by Long (1995, fig. 2A) include the strongly convex lateral border of the subanal lamina with a well developed keel, which is relatively narrow in Boomeraspis goujeti, as shown in an isolated example from A5 (ANU V2318) (Fig. 7I). The new taxon seems generally similar to the Phlyctaenius restoration of Heintz (1933) and Denison (1978, fig. 42), but with a longer spinal margin, probably smaller PMV than AMV, and relatively shorter PVL (71% of AVL length; cf. 87% for Phlyctaenius acadica, although proportionately longer in other species: V. T. Young 1983, fig. 15). The AVL of Denisonosteus has a much shorter spinal margin (Young and Gorter 1981, fig. 24D), clearly separated by an angle from a transverse anterior margin of similar length, as in Phlyctaenius (Denison 1978, fig. 42), and the sensory groove is visible only near the front of the bone. A more posteriorly directed spinal margin on the AVL is also seen in Turrisaspis and Mulgaspis (Daeschler et al. 2003; Ritchie 2004). Two other arthrodire plates from the same locality are less well preserved. ANU V850 (Fig. 5H) is a right AL plate preserved mainly as bone of its inner surface. Impressions of ornament show this was finer than on the ventral armour just described. Partly preserved are the ventral and posterior margins with a shallow pectoral embayment. The anterior margin is incomplete, but this bone was clearly not as long and low as the AL plate of Groenlandaspis (Ritchie 1975). ANU V853 is similarly preserved in visceral view, showing concentric growth around an



55

(B)

MD

(C)

(D) (A)

(E)

dlr (G)

dent

oaMD (F )

llc llc

A B,C,I E,F D,G,H 10 mm

(H)

(I ) Fig. 7. Various groenlandaspid arthrodire bones, referred to the following taxa: (A–D, I) Boomeraspis goujeti Long, 1995; (E, F) ?Turrisaspis sp. indet.; (G) ?Mulgaspis sp. indet.; (H) Groenlandaspis sp. (A) Associated bones of AMF 54312; (B) incomplete MD plate, left lateral view, partly preserved as an impression (ANU V2152); (C) incomplete MD plate, left lateral view (ANU V2151); (D) right ADL plate, partly preserved as an impression (ANU V2154); (E) right ADL plate, partly preserved as an impression (ANU V2153); (F) right PDL plate, preserved as bone in counterpart to E (ANU V2153); (G) incomplete MD plate, left lateral view (ANU V798); (H) MD plate, right lateral view (ANU V2150), previously illustrated by Young (1989a, fig. 5A); (I) right PVL plate, external view (ANU V2318; latex cast whitened with ammonium chloride).

ossification centre but very incomplete margins. It is probably a left PVL, perhaps from the same individual as V850. Two other right AL plates, V2155 from Portal and V2329 from the northern

Boomerang Range, have similar proportions to V850, and are provisionally included here. V2329 has the best preserved margins, being only ~127% longer than high, in contrast to

56


Groenlandaspis antarctica, where this proportion approaches 190% (Ritchie 1975). Finally we figure an interesting SM plate that may belong to this taxon, based on ornament. However, since the SM has never been described for Groenlandaspis, we cannot exclude that possibility. ANU V2147 has a distinctive ornament of tubercles coalesced in rows to form a reticulate pattern (Fig. 5F), reminiscent of the antiarch Merimbulaspis Young 2010b. ANU V2180 from Portal is an indeterminate bone also with this ornament (first thought to indicate the presence of Merimbulaspis in the Aztec assemblage). However, V2147 has the wrong shape to be an antiarch SM plate, and reticulate ornament approaching that of V2180 is also developed on the AVL of V852. The external surface of V2147 has smooth margins anteriorly on the dorsal edge, where it would have underlapped the lateral skull margin. The inner surface shows the standard groove for an ‘epihyal’ element (gr.eh, Fig. 5G), best documented by Goujet (1984, pl. 9) for Spitzbergen arthrodires. Highly unusual is a deep pit or depression just inside the ventral margin (dep), the function of which is unknown. Family Groenlandaspidae Obruchev, 1964 Remarks Ritchie (1975) contrasted the lower MD of Groenlandaspis antarctica with the high pointed MD of Tiaraspis from the Early Devonian of Germany (Gross 1962; Schultze 1984), suggesting a progressive decrease in height from Early to Late Devonian. Since then several other high-spired groenlandaspid taxa have been described, to which the new material described below could belong: Africanaspis Long et al., 1997, Mithakaspis Young & Goujet, 2003, and Turrisaspis Daeschler et al., 2003. The medium-spired Mulgaspis Ritchie, 2004 may also be a contender, as some high MDs were included in this taxon. Ritchie (2004) recognised seven genera in the family: Africanaspis, Boomeraspis, Groenlandaspis, Mithakaspis, Mulgaspis, Tiaraspis, and Turrisaspis. The best known of the additional high-spired taxa is Turrisaspis, represented both by articulated specimens and many isolated measurable bones. Its skull is unusual in having anterior and posterior pineal bones, but this could also occur in both Tiaraspis and Groenlandaspis (Ritchie 2004: p. 63). If confirmed by future description, this might be a character of the family, and would cease to be taxonomically useful for distinguishing genera. Daeschler et al. (2003) suggested that Turrisaspis may be more closely related to Groenlandaspis than to Tiaraspis, and Janvier and Clément (2005) noted that the high-spired taxa could be nested within Groenlandaspis (with the name itself a junior synonym of Cosmacanthus). The material described below comprises only trunk-armour bones, so neither skull features, nor broader questions of generic inter-relationships, are considered further. Regarding the trunkarmour, Turrisaspis was distinguished by three characters (Daeschler et al. 2003): (1) MD higher than its length (shared with Tiaraspis, Africanaspis and Mithakaspis); (2) MD about the same height as PDL; and (3) posterolateral process of PDL angled upward, with the MD overlap not reaching the posterior margin. Africanaspis shares (1) and (3), as also does Mulgaspis, if high


MDs like that figured by Ritchie (2004, fig. 5C) are included. However, very limited variation in MD shape in the type species of Turrisaspis, based on 29 MD plates from the largest sample size of any groenlandaspid (Daeschler et al. 2003, fig. 10), suggests to us that probably additional species may be represented in the material assigned to Mulgaspis. Some other MD features also distinguish the named groenlandaspid genera, e.g. the overlap area for the ADLs meeting in front of the MD in Mithakaspis, and the fact that Tiaraspis MD plates normally have only one large notch in their ventral margins (Gross 1962, fig. 1). Turrisaspis has two notches (where the ADL and PDL push up under the MD), these separated by a ventral process where the MD extends down over the ADL/ PDL suture, presumably strengthening the interlocking of these bones. The ADL and PDL also overlap the MD in Tiaraspis, but this overlap area is absent in Turrisaspis. The MD from the Famennian of Belgium interpreted after the Early Devonian Tiaraspis by Gross (1965) was interpreted as possibly just a broken tip of Groenlandaspis thorezi by Janvier and Clément (2005). We suggest that this is less likely because very similarshaped high MDs are also known from the Famennian of Australia (Young et al. 2010, fig. 4K), but these also differ from Tiaraspis in having two ventral notches (M. Otto, pers. comm.). The number of notches is not always reliable for distinguishing genera based on isolated MDs, two notches (and an overlap area) occurring in some species of Groenlandaspis (Young et al. 2010, fig. 4G), but the separating process being absent or very slight in others, e.g. G. antarctica or G. riniensis (Ritchie 1975; Long et al. 1997). Given these uncertainties, and until the full Antarctic material of Groenlandaspis is described, we propose to use the groenlandaspid generic names as form taxa defined by the characters just discussed to interpret the isolated plates described below. These indicate that three genera and species of high-spired groenlandaspids may be present. Boomeraspis goujeti Long, 1995 Figs 7A–D, I; 8A–C Material examined Holotype. WAM 93.7.2, impression of a right PDL plate. New material. AMF 54312, an incomplete high-spired MD with associated IL, PL and SP plates; ANU V2151, 2152, incomplete MD plates, the latter with an associated IL plate; ANU V2318, a complete right PVL external impression. ANU V2319 (incomplete PL, on same sample as CPC 26343) and V2154 (ADL) are provisionally included in this taxon.

Localities and horizon The type material of Long (1995) comes from the south-eastern spur of Alligator Ridge in the Boomerang Range (loc. 21, Fig. 1; Section 3 of Barrett and Webb 1973) but from a lower horizon than previously described material, ~30 m above the base of the Aztec Siltstone. Associated scales of Turinia antarctica suggested that the type horizon belonged to the askinae or kohni biozones of Young (1988). However the species Bothriolepis portalensis Young, 1988 was identified from higher horizons (81–111 m above base) in this section. Most of the new material comes from Unit 17, Section 10 of Barrett and Webb (1973), the



(A)

57

(B)

IL MD PL MxL

MD

SP (C)

tr.r

oaAL n2

n1 MD oaPVL ADL

D A

MD

PDL

B 10 mm

C

dlr PL PDL PL

(D)

n

(E)

AL

(F )

Fig. 8. (A–C) Boomeraspis goujeti Long, 1995. (A) Various bones of AMF 54312; (B) camera lucida drawing of ANU V2152, restored after the MD plate of AMF 54312, left lateral view; (C) camera lucida drawing of right PL plate, lateral view (AMF 54312). (D) Mithakaspis lyentye, MD plate in left lateral view (after the holotype: Young and Goujet 2003, fig. 32B). (E, F) Partial trunk-armours in left lateral view (not to scale) of Turrisaspis elektor (E) (modified from Daeschler et al. 2003, fig. 11) and Africanaspis doryssa (F) (modified from Long et al. 1997, fig. 13).

original locality at Portal Mountain (loc. 11, Fig. 1), discovered in 1968–69 by VUWAE 13. This is the type locality and horizon for the species Bothriolepis portalensis Young, 1988, placed biostratigraphically immediately above the lower zones characterised by thelodont scale association. Two specimens (ANU V2318, 2319) come from Section A5 (loc. 16, Fig. 1). In summary, Bothriolepis portalensis Young, 1988 was identified at all three localities (11, 16, 21) producing specimens now referred to Boomeraspis goujeti, and its stratigraphic range evidently includes at least the portalensis and the underlying kohni zones (the Bothriolepis species from the type locality has not been determined).

Description The key new specimen (AMF 54312) (Figs 7A, 8A) comprises an incomplete high-spired MD with associated IL, PL and SP plates, mostly preserved as impressions. These are on the same sample as antiarch bones referred to Bothriolepis portalensis by Young (1988, fig. 37D, pl. 8, fig. 10). The MD impression is incomplete ventrally (maximum preserved height 42 mm and width 23 mm). The anterior margin is very slightly convex, and the posterior edge slightly concave. The narrow slightly rounded tip is 2 mm across, and the anterior and posterior margins enclose an angle of 37, comparable to that recorded in Turrisaspis elektor (30–40: Daeschler et al. 2003). As far as preserved the MD

58


seems closer to the shape of Turrisaspis, and not as high and narrow as in Africanaspis doryssa (Long et al. 1997). This complete distal impression has been used to restore the upper part of a second MD (Fig. 7B). ANU V2152 is an elongate element, the lower part preserved as bone showing a tuberculate surface, the upper part (dorsal tip missing) as an impression demonstrating ornament on both sides and that it must be a MD plate. The ornament of randomly distributed fine tubercles is interrupted in the impression by several round ring-like depressions; these could be either healed wounds on the external surface, or some form of lesion caused by parasites during life or scavengers after death. Similar lesions occur on Bothriolepis bones from the same locality (see Young 1988, pl. 10, figs 3, 5, 8). Ornament is more crowded with enlarged tubercles on a thickened ridge along the anterior margin of the impression, and its distal edge shows that the spine tip was broken off before embedding in the sediment. Extending the preserved margins indicates a similar enclosed angle to the previous specimen (~35). Anteroventral and posteroventral extremities are missing, but the ventral margin clearly shows two large notches (n1, n2; Fig. 8B). As restored, the measured plate height (after Daeschler et al. 2003, fig. 10) is ~147% times the breadth, i.e. closely similar to proportions in Turrisaspis elektor, but less high than the MD of Africanaspis doryssa (Fig. 8E, F). However, the gently convex anterior margin is a resemblance to Africanaspis (straight to gently concave in Turrisaspis, and also Mithakaspis) (Fig. 8D). ANU V2151 is another less complete high-spired MD, preserved in a sawn off-cut that shows the spongy internal bone texture of the spine (Fig. 7C). The preserved base is 20 mm wide, narrowing to 10 mm wide 8 mm from the preserved tip (preserved length 30 mm). The tip of the spine may be slightly rounded, but it may not be complete. As preserved, it is reminiscent of a specimen (AMF 54152) from Mt Jack Station in western New South Wales referred by Ritchie (2004, fig. 5C) to Mulgaspis evansorum, but with a quite different high and distally rounded spine compared with the more typical triangular MD restored for that species (Ritchie 2004, fig. 7). The ornament of V2151 is poorly preserved, but shows clusters of distally directed pointed tubercles near the tip, also evident on the holotype. In Turrisaspis elektor ornament was said to form rows parallel to the ventral margin, and some Africanaspis doryssa specimens also show this (Long et al. 1997, fig. 12C). There is no suggestion of this in these Antarctic specimens. Ornament variation in Mithakaspis was interpreted by Young and Goujet (2003, p. 61) to possibly indicate species differences within two groupings: (a) tubercles aligned in antero-posterior rows, tending to coalesce in ridges in larger examples, and (b) non-aligned tuberculation tending to reduce to smooth lateral surfaces with tubercles only at the margins in larger examples. The few examples of the MD described here show no indication of antero-posterior alignment, nor the strong posterior denticles seen in type species of both Turrisaspis and Africanaspis. Instead, some enlarged tubercles and crowded ornament form an anterior thickening or ridge, in this respect resembling the type material of Mithakaspis lyentye, which also lacks posterior denticles (Young and Goujet 2003, figs 31B, 32).


The high-spired MD of AMF 54312 has an associated arthrodire PL plate of odd shape less than 10 mm away, and assumed to come from the same individual (Fig. 8A; preserved H 32 mm and L 29 mm). The dorsal process of the bone seems complete, but the edge is obscured by the associated MxL plate of Bothriolepis, so could have extended slightly further. This bone differs from the PL of Groenlandaspis antarctica in its more upright posterior border, the shape of overlaps for the AL and PVL plates (oaAL, oaPVL, Fig. 8C), and in the strongly developed keel (tr.r). The PL of Turrisaspis is even more triangular in shape (Fig. 8E), and, like Groenlandaspis, has the ventral process carrying the PVL overlap projecting outwards and backwards, rather than vertically as in this specimen. The area between the overlaps for AL and PVL may have participated in the border of the pectoral fenestra. The PL of AMF 54312 is higher than long because of its strongly developed ventral process, and in this respect it differs from all other ‘arctolepid’ PLs, which are longer than high (Denison 1958, fig. 111). This is also a difference to the PL previously described for Boomeraspis (Long 1995, figs 2C, 3C), but the other differences to Groenlandaspis cited above also apply to the Boomeraspis type PL, as noted by Long (1995). There are also several specific similarities to AMF 54312 in the PL from the Boomeraspis type locality. It has a very similar keel (tr.r, Fig. 8C), which expands anteriorly to form an elongate raised triangle, this corresponding to an indented margin to the overlap area for the AL plate. Above this the overlap area has a concave edge in both specimens. Both specimens also show lineations in the ornament, reflecting the shape of the dorsal process, but this is more angled in the type PL, with a stronger vertical ridge crossing the bone to its ossification centre. In addition, the transverse ridge or keel in AMF 54312 has more distinct ornamental ridging formed by tubercles in slightly radiating longitudinal alignment. Finally, the ventral process is much stronger. As a consequence, the type PL of Boomeraspis is longer than high, the reverse proportion to AMF 54312. Assuming that the ventral process is not missing from the type PL, this could be a specific difference, but with only two good specimens this is hard to assess, and for the moment we attribute these minor differences to individual variation. There is another incomplete PL plate (ANU V2319) from A5 that suggests a similar shape (same sample as Bothriolepis portalensis, CPC 26343) and may also belong here. Comparing the PL plate to other high-spired taxa, Turrisaspis elektor has a markedly triangular PL with a posterior border sloping backwards (Fig. 8E). The PL shape of Africanaspis doryssa was restored only from overlaps of surrounding bones (and the PVL is unknown), but the restoration (Fig. 8F) is also higher than long, with a vertical posterior margin. A few similarities were also noted above in the structure of the MD plate in Boomeraspis and Africanaspis. Whilst the Aztec material is considerably older than the Famennian Africanaspis doryssa, a close relationship between these Gondwana taxa may be demonstrated with future discoveries. Between the MD and PL of AMF 54312 is an elongate impression (length 27 mm; maximum preserved height 5 mm) with a smooth lateral overlap, the rest preserved with strong longitudinal ridges. This is probably a right IL plate (IL, Fig. 8A); it was evidently lower and less complex than that illustrated for Mulgaspis by Ritchie (2004, figs 6D–E, 12). Another elongate



ridged impression associated with the MD of V2152 may also be an IL, but it is not certain that it lies in the same sedimentary lamina. The third associated bone on AMF 54312 is an incomplete arthrodire SP plate with fine tubercular ornament. It suggests elongate proportions (preserved L 46 mm; maximum breadth 6 mm), but both ends are incomplete. Africanaspis doryssa was restored with an elongate SP (bone not known), but the less elongate SP of Turrisaspis, covered with fine tubercles (Daeschler et al. 2003, fig. 9S) is quite comparable with the Antarctic example as far as preserved. ANU 2154 is a small right ADL, poorly preserved mainly as an impression (Fig. 7D), but showing a very distinct dorsolateral ridge (dlr) running up and back at an angle from the sensory groove (llc). This is quite different from the ADL of Groenlandaspis antarctica (Ritchie 1975). The ADL was not found at the type locality of Boomeraspis (Long 1995), but a distinct dorsolateral ridge is well developed on the ADL of Africanaspis doryssa (Fig. 8F). Possibly ANU 2154 represents the same taxon as the high MD plates just described, and is provisionally referred also to Boomeraspis. ANU V2318 from A5 (Fig. 7I) is the impression of a small right PVL 28 mm long, which closely resembles that figured by Long (1995, fig. 2A). The subanal lamina has the same shape, with a gently curved lateral margin and narrow ridge formed by fusion of a single tubercle row; this is continuous at the ossification centre with a ventrally directed keel. The posterior border of the subanal lamina is gently rounded, in contrast to that of Groenlandaspis antarctica, which is concave, with a posterior angle protruding at the lateral edge (Fig. 4D). In Tiaraspis this region has a narrow and pointed extension along the ventrolateral ridge (Gross 1962; Ritchie 1975), and Turrisaspis shows a similar configuration (Daeschler et al. 2003, fig. 9R). The latter is a shared character of these two taxa in addition to the high-spined MD plate. The lateral lamina of V2318 is crossed by another thin ridge running upwards diagonally to the anterodorsal lamina of the plate. Again, this is very similarly developed to the type B. goujeti. The ventral lamina of V2318 is more complete than the previously figured PVL, and shows a rounded anteromesial margin, rather differently developed from the angled margin of Groenlandaspis (Ritchie 1975, fig. 2b). In Phlyctaenius the margin for the PMV plate is longer, and concave rather than convex. The anterior overlap area for the AVL in V2318 extends partly on to the lateral lamina. In Turrisaspis this overlap seems to have a slightly different position, extending equally onto both lateral and ventral laminae (Daeschler et al. 2003, fig. 9R). ?Turrisaspis, sp. indet. Figs 2I, 7E, F Material examined Referred material. ANU associated PDL.

V2153,

an

incomplete

ADL

and

Description This sample comprises a right ADL, fractured in two, preserved mainly as an impression (Fig. 7E), and on the counterpart a preserved bone positioned immediately behind it with its ventral margin missing (Fig. 7F). The anterior edge of the ADL above the condyle slopes upwards and backwards to a high posterodorsal angle, more steeply inclined than the ADL of Groenlandaspis antarctica (Ritchie 1975), but similar to that of Turrisaspis elektor (Fig. 8E). Also similar is the orientation of the sensory groove (llc) running straight across the plate, whereas in Groenlandaspis (and Africanaspis) it slopes strongly downwards and backwards. The bone behind the ADL is clearly a high and narrow PDL plate. Its posterior margin is complete, and the anterior margin broken dorsally but complete ventrally, indicating that the bone was at least twice as high as long, and thus of similar proportions to the PDL of Turrisaspis elektor. However, it differs in lacking the anterior upward projection, the overlap for the MD plate being highest posteriorly (Fig. 7F). In addition to proportions, this PDL differs in several respects from the PDL of Groenlandaspis antarctica (Ritchie 1975). The vertical posterior border is straight to slightly convex (markedly concave in Groenlandaspis to form a prominent posterior projection), whilst the overlap for the MD is highest in the middle of the dorsal margin in Groenlandaspis, not highest posteriorly as in this specimen. The provisional assignment to Turrisaspis is based on the points of resemblance in the ADL and PDL just described. However, differences (e.g. the configuration of the dorsal margin of the PDL) might be expected, given the difference in age of this material (Givetian) compared with Turrisaspis elector (Famennian). V889 (Fig. 2I) is another isolated PDL plate, from the same beds as the type material of Barwickosteus, but much smaller than the PDL just described. It is preserved in inner view, and shows a posterior thickening (th), the roughened dorsal area for contact with the opposite PDL, and a slight ridge and notch in the anterior margin suggesting a relatively high position for the lateral line canal passing forward onto the ADL (llc, Fig. 2I). The bone is ~2.2 times higher than long, and thus of very different proportions to the PDLs of Groenlandaspis antarctica, Boomeraspis and Mulgaspis (Ritchie 1975, 2004; Long 1995), all of which are longer relative to height, with a concave posterior margin formed by a prominent posterodorsal projection. The PDL of Tiaraspis is not as elongate as in Groenlandaspis antarctica (Gross 1962, fig. 2G–L), but still less than twice as high as long. The PDL of Africanaspis has similar proportions to V889 (slightly more than twice as high as long: Long et al. 1997), whereas that of Turrisaspis is more than three times as high as long. Both differ in the low position that the lateral line canal passes off the anterior margin, but both are also considerably younger in age (Famennian). For the present this small PDL is grouped with the previous specimen and provisionally referred to Turrisaspis. ?Mulgaspis, sp. indet. Fig. 7G

Locality and horizon Portal Mountain (loc. 11, Fig. 1), from Unit 17, Section 10 of Barrett and Webb (1973).

59

Material examined Referred material. ANU V798, a very incomplete MD plate.

60


Locality and horizon From Portal Mountain (loc. 12, Fig. 1), Unit 4, Section P1 of Askin et al. (1971). Description This very incomplete piece of bone, 40 by 20 mm in size, has an ornament of undulating widely spaced tubercle rows, with enlarged denticles along one margin (dent, Fig. 7G). The broken section on the sample edge shows tubercles on the opposite surface, demonstrating that this is a very incomplete MD of groenlandaspid type. The MD of Groenlandaspis antarctica has quite different ornament of crowded non-aligned tubercles (Ritchie 1975), but similar tubercle rows occur in other Antarctic specimens, e.g. ANU V2150 (Fig. 7H), presumably representing a different Groenlandaspis species from the Aztec fauna. However ANU V798 differs conspicuously from that example. It has a non-pointed tip, which is reminiscent of some MDs referred to Mulgaspis by Ritchie (e.g. Ritchie 2004, fig. 10A). The anterior and posterior margins enclose an angle of ~84, a smaller angle than in the Antarctic Groenlandaspis (exceeding 90), but also close to Mulgaspis MD plates (67–82 for most examples: Ritchie 2004). This angle is much more acute in the other MDs just described, and in the type species of Mithakaspis, Turrisaspis and Africanaspis (Fig. 8D–F), so we provisionally refer this specimen to Mulgaspis. It is noteworthy that the large sample of MD plates for Turrisaspis elektor always have a pointed tip (Daeschler et al. 2003), as is the case with all the MDs with varying ornament assigned to Mithakaspis (some with aligned ornament very similar to ANU V798, e.g. Young and Goujet 2003, fig. 31A). Only the Mulga Downs Group assemblage of western New South Wales has produced non-pointed MD plates, all of which were referred by Ritchie (2004) to two species of Mulgaspis, although it seems likely that more taxa are represented (see above). The fact that we have a MD plate with a truncated tip from the Aztec fauna may indicate a closer age correlation to the Mulga Downs Group than applies to the other localities. Unit 4 of Section P1 is only 16 m above the base of the Aztec Siltstone, and has produced the lowest remains of Bothriolepis from Portal Mountain (Young 1988, p. 78). Thelodont scales also occur at this level, so it is relevant that scales of Turinia may also be abundant in samples from the Mulga Downs Group (e.g. Turner et al. 1981, fig. 16). Acknowledgements Drs P. J. Barrett and A. Ritchie are thanked for arranging GCY’s participation in the 1970–71 VUWAE 15 expedition, when he worked for the Bureau of Mineral Resources (secondment approved by J. N. Casey). Field and logistic support was provided by Scott Base staff, US Navy Squadron VXE-6, and other members of the VUWAE 15 team (R. Askin, P. Barrett, R. Grapes, B. Kohn, J. McPherson, D. Reid, A. Ritchie). GCY acknowledges a Trans-Antarctic Association travel grant to London (May 1988), the Alexander von Humboldt Foundation for a Humboldt Award in Berlin (2000–03), and advice from Professor H.-P. Schultze and Dr M. Otto on Tiaraspis. R. J. Tingey (formerly BMR) and Dr R. E. Barwick gave information on early Antarctic exploration and fossil fish discoveries, and J. Burnett provided images of the letter to C. S. Wright in Griffith Taylor’s papers in the National Library of Australia. Dr A. Ritchie (formerly Australian Museum) provided unpublished information on Antarctic


Groenlandaspis; Dr Yong Yi Zhen lent specimens in the Australian Museum collection. Ben Young is thanked for preparation and photography of Antarctic specimens. JAL acknowledges financial support for Antarctic fieldwork in 1991–92 through ASAC Grant 136, and assistance in the field from colleagues Drs Margaret Bradshaw, Fraka Harmsen and Brian Staite. This research was a contribution to IGCP Projects 328, 406, 410, and 491, and supported by ARC Discovery Grants DP 558499, 772138, and 1092870. Finally, we acknowledge the artistic inspiration of the late Richard Barwick, from his legendary coloured chalk diagrams for teaching first-year Zoology classes to the exquisite shaded pencil representations of fossil bones (a weak imitation of which was employed for this paper).

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Erratum The published version of this paper (p. 48) lists ‘ANU V850-853 from Portal Mountain (P2)’ under Referred Material for Barwickosteus antarcticus. This was an error; these specimens belong to Grifftaylor antarcticus (p. 53 of published paper).