Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/209
Comunicações Geológicas (2016) 103, Especial I, 7-12 ISSN: 0873-948X; e-ISSN: 1647-581X
The ichnological importance and interest of the Geological Museum of Lisbon collections: Cladichnus lusitanicum in continental facies from the Lower Cretaceous of the Lusitanian Basin (Portugal) A importância e interesse icnológicos das colecções do Museu Geológico de Lisboa: Cladichnus lusitanicum em fácies continental do Cretácico Inferior da Bacia Lusitânica (Portugal) C. Neto de Carvalho1*, A. Baucon1,2, A. Bayet-Goll3 Artigo original Original Article © LNEG – Laboratório Nacional de Geologia e Energia IP
Abstract: The revision of the classic collections of trace fossils housed in the Geological Museum of the former Geological Survey of Portugal is bringing new information to well established ichnogenera. Through the revision of the type material of Taenidium lusitanicum Heer, 1881 and other specimens housed in the Geological Museum of Lisbon and the National Museum of Natural History and Science we reinstate the ichnospecies Cladichnus lusitanicum (Heer). This is the only ichnospecies of Cladichnus occurring in environment typical from the Scoyenia ichnofacies in otherwise typical turbidite forms. Keywords: Branched meniscate burrows; Scoyenia ichnofacies; Lower Cretaceous; Lusitanian Basin Resumo: A revisão das colecções clássicas de icnofósseis albergadas no Museu Geológico dos antigos Serviços Geológicos de Portugal está a trazer nova informação para icnogéneros bem estabelecidos. Através da revisão do material-tipo de Taenidium lusitanicum Heer, 1881 e de outros espécimes incluídos nas colecções do Museu Geológico de Lisboa e do Museu Nacional de História Natural e da Ciência, restabelecemos a icnospécie Cladichnus lusitanicum (Heer). Esta é a única forma de Cladichnus que se conhece em ambientes típicos da icnofácies de Scoyenia, um icnofóssíl característico de ambientes marinhos profundos turbidíticos. Palavras-Chave: Galerias meniscadas ramificadas; icnofácies de Scoyenia; Cretácico Inferior; Bacia Lusitânica Geology and Paleontology Office of Centro Cultural Raiano, Geopark Naturtejo Meseta Meridional – UNESCO Global Geopark, Portugal. Dipartimento di Scienze Chimiche e Geologiche, University of Modena. Via Campi, 103 - 41125 Modena , Italy 3 Department of Geology, Faculty of Science, Ferdowsi University of Mashhad, Iran *Corresponding author/Autor correspondente:
[email protected] 1
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lusitanicum and interpreted it as marine algae despite being found in the Almargem Formation together with a diverse continental flora (Heer, 1881; Teixeira, 1950). Wilckens (1947) found T. lusitanicum in the upper Aptian of Stromness Bay, Annenkov island, Antarctica and described it as a trace fossil. Teixeira (1950), despite ignoring the interpretations of Wilcken (1947) and being a paleobotanist, considered T. lusitanicum of difficult attribution and doubtful systematic position. For this author the inclusion in the (ichnogenus) Taenidium is not appropriate justifying change of genus (Teixeira, 1950). D’Alessandro and Bromley (1987) based on the true branched meniscate morphology of “Taenidium” fischeri erected the ichnogenus Cladichnus. Nevertheless, they revised all branched “Taenidium”, including T. lusitanicum, which tempted to attribute to their newly ichnogenus Cladichnus owing to the primary successive branching pattern and meniscate fill. However, as these authors recall, its palmate form is highly distinctive and represents a fundamentally different burrowing behavior from the radiating forms (D’Alessandro and Bromley, 1987). Nevertheless, Fu (1991) included both T. lusitanicum from Portugal and from Antarctica into Cladichnus fischeri without discussing the fundamental morphological/behavioral differences. In the present study we reinstate the ichnospecies Cladichnus lusitanicum by revising the type material of Heer (1881) and other material collected in the same period at the Geological Museum of Lisbon. According to these occurrences in continental facies from the Lower Cretaceous of the Lusitanian Basin, the typical deep sea character of Cladichnus can no longer be sustained neither the chemosymbiotic interpretations (Fu, 1991; Wetzel and Uchman, 2013) at an ichnogeneric level.
2. Geological summary and paleoenvironment 1. Introduction Meniscate backfilled trace fossils are diverse and occur in many environments from fluvial to the deep sea. Perhaps the best known among them, Taenidium, was described by Heer (1877) and includes variably oriented, unwalled, straight, winding, curved, or sinuous, cylindrical meniscate backfilled trace fossils without true branching (emended by Keighley and Pickerill, 1994). Heer (1881) described a branched meniscate backfilled burrow system as Taenidium
The Lusitanian Basin is an aborted rifting basin that resulted from the opening of the North Atlantic by the break-up of Pangaea. It shows a general N-S trend bordered by Variscan granite and metamorphic massifs that fed this otherwise carbonate basin of siliciclastics during and after the subsidence pulses. Major discontinuities and cycles during the Lower Cretaceous were controlled by the opening of the Atlantic to W from the Lusitanian Basin and the installation of a passive margin in the post-rifting stage (e.g., Dinis and Trincão, 1991; Rey et al., 2006; Dinis et al., 2008). A regional uplift was created by
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Fig. 1. Geological setting and stratigraphy of type location (square) of Cladichnus lusitanicum within the Lusitanian Basin (LB), western Iberian margin (adapted from the geological map of Sintra 1:50000 published by the Geological Survey of Portugal). Fig. 1. Enquadramento geológico e estratigrafia da localidade-tipo (na caixa) de Cladichnus lusitanicum na Bacia Lusitânica (LB), na margem ocidental Ibérica (adaptado do mapa geológico de Sintra na escala 1:50000 publicado pelos Serviços Geológicos de Portugal).
the onset of seafloor spreading in the Iberian sector (Shillington et al., 2004). Three transgressive-regressive 2nd order cycles separated by unconformities are related with the northward propagation of the Atlantic opening. From this tectonic event resulted the wide deposition of sandstones and conglomerates in a braided river system during the upper Barremian-lower Aptian (Dinis and Trincão, 1991). This evolution was recorded in several other Atlantic-related basins (Jacquin et al., 1998; Dinis et al., 2008). The trace fossils under study come from the Almargem Formation, where Teixeira (1950) described a diverse flora in the region of Belas (Fig. 1). Almargem Formation is a continental, siliciclastic unit 40 m thick, composed of three members deposited in lake, river and floodplain environments, respectively (Rey, 1972): the lower member is made of clay, sandstone and lignite beds (3 to 8m thick); the middle member is composed by lenticular coarse-grained sandstones and conglomerates with cross-stratification (10 to 15 m thick) corresponding to ephemeral shallow and highly ramified channels; the upper member is defined by an alternation of clays and planar sandstones with oblique stratification and plant debris, sometimes with rhizolites and nodules revealing the presence of paleosols (20 to 30 m thick). The holotype of Taenidium lusitanicum was described by O. Heer in fine-grained, mica-rich sandstone in thin tabular beds showing oblique stratification sets, asymmetric ripples oriented N-S, plant debris and bioturbation (Rey, 1972). This is found in the lowermost member corresponding to floodplain deposition. To W the Almargem Formation passes laterally to the shallow marine formations of Regatão, Cresmina and Rodízio (Rey, 1992). The Almargem Formation in the
region of Lisbon is dated from upper Barremian to Aptian (Rey, 1992), but can reach the end of the Cretaceous in the north-easternmost part of the Lusitanian Basin (Dinis and Trincão, 1991). The studied trace fossils have been found together with other trace fossils, although not described, and plant debris (Rey, 1972).
3. Systematic Ichnology Ichnogenus Cladichnus D’Alessandro and Bromley, 1987 Diagnosis: Annulated or monilliform burrow fills composed of meniscus-shaped segments, comprising primary successive branched and radiating systems; wall lining lacking or very thin (after D’Alessandro and Bromley, 1987). Type ichnospecies: Cladichnus fischeri (Heer, 1876/77) Cladichnus lusitanicum (Heer) 1881 Taenidium lusitanicum HEER, p. 12, pl. XX. 1947 Taenidium lusitanicum Heer, WILCKENS, p. 41-45, pl. 6, figs. 1 and 2. 1950 Taenidium lusitanicum Heer, TEIXEIRA. Locality and Age: The holotype selected by Heer (1881; Fig. 2a) came from Quinta do Grajal, Belas, in the “Grés superiores” unit of sandstones and clays dated from upper Aptian; all the material known was collected in the Almargem Formation at Belas region to which “Grés superiores” is included, dated as a whole between upper Barremian and Aptian (Rey, 1992). Diagnosis: Horizontal, palmate burrow systems consisting of meniscate, curved tunnels departing from both sides of an axial
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Fig. 2. Cladichnus lusitanicum. a – Holotype of Taenidium lusitanicum showing the successive branched meniscate burrow system characteristic of Cladichnus, in a palmate pattern (specimen n. 23738). b – Cross-cutting relationship of a denser patch of C. lusitanicum. c – C. lusitanicum forward-, and upward-branching burrows with two rows of meniscate backfilling. d – Smooth undertrace of C. lusitanicum resulting from lining and reminding the bulbous nature of Asterosoma side branches. e – Lined meniscate side branches show different preservational types. f – Regular side branching (specimen n. MNHN/UL.I.Icn01). Scale bar is 10 mm. Fig. 2. Cladichnus lusitanicum. a – Holótipo de Taenidium lusitanicum evidenciando um sistema de galerias sucessivamente ramificadas características de Cladichnus, num padrão em palma (espécime nº. 23738). b – Relações de intersecção num aglomerado mais denso de C. lusitanicum. c – Galerias ramificadas para a frente e para cima de C. lusitanicum, com duas direcções de retropreenchimento. d – Sub-impressões lisas de C. lusitanicum resultantes da existência de uma parede construída e lembrando a natureza bulbosa das ramificações laterais de Asterosoma. e – Ramificações laterais preenchidas e alinhadas mostrando diferentes tipos preservacionais. f – Ramificação lateral regular (espécime nº. MNHN/UL.I.Icn01). A escala gráfica é de 10 mm.
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tunnel. Only one order of branching is present. Tunnels are characterized by a massive lining and an inner meniscate fill with a single or double row of ovate, cylindrical or monilliform meniscate packets. Branching is developed in two opposite directions and arching backwards; distal part of the tunnels are parallel to each other and bending upward. Material: 7 specimens (including the holotype) housed in the Geological Museum (n. 23738 to 23744) and 1 specimen in the National Museum of Natural History and Science (MNHN/UL.I.Icn01). All are figured in the present work. Most of the specimens show more than one burrow system (i.e., Fig. 2b), all but one incomplete. Description: Palmate retrusive branching pattern developed from a single vertical-to-inclined tunnel that, when visible, it shows only the horizontal-to-oblique section of the tube, 10 to 12 mm in diameter (e.g., Fig. 2a). Successive primary branching develops outwards from a central horizontal axis, 115 to 220 mm long. Branches are close to one another, 10 mm wide and rather constant, but show different lengths, less commonly linear or bending backwards and upwards to the distal part (sharp tips; Fig. 2c). The angle of bifurcation is quite variable and the branches can be emplaced in different levels. Lining is present providing a smooth outer sculpture, as can be seen in natural casts (Fig. 2d). Homogeneous, packeted meniscate backfill made of fine sediment shows much less mica than the surrounding rock. Menisci are concave towards the proximal direction and measure 8 to 10mm. There are crossovers between close burrow systems. Discussion: The studied material is included in Cladichnus because the main taxobases of this ichnogenus are present. The branching pattern of the studied burrows is however different from the previously described ichnospecies of Cladichnus (revised by Wetzel and Uchman, 2013): (a) C. fischeri (Heer, 1876/77) is composed by a vertical or inclined tube that branches radially in the lower end, being the centre of radiation above the level of horizontal tubes; (b) C. radiatum (Schröter, 1894) consists of an inclined to vertical tube and radiating horizontal, sometimes bifurcate tunnels, and the center of radiation is within the plan of the branches; (c) C. aragonensis Uchman, 2001 is similar in geometry to C. radiatum but filled with tangentially arranged pellets; (d) C. parallelum Wetzel and Uchman, 2013 shows branching and some radiation within the downward burrow component, but the horizontal tubes are arranged parallel to each other and do not branch. In C. lusitanicum successive branching is developed horizontally from a single vertical-to-inclined shaft, but instead of radiating the branches are organized in a palmate pattern. This is clearly diagnostic and separates this ichnospecies from the type ichnospecies C. fisheri (Fu, 1991), and any other form of Cladichnus. The horizontal tubes are parallel but more prone to bend back- and upwards (Figs. 2c, f). At present, there is no documented modern example of Cladichnus lusitanicum, therefore ethology and burrowing dynamics are interpreted from the following characteristics of the studied specimens: 1. Axial tunnel cross-cuts lateral ones. Cross-cutting relationships indicate that the axial tunnel was filled after the lateral ones. The branching pattern of Cladichnus lusitanicum shows a preferential direction of development that makes it different from the remaining forms of Cladichnus; 2. Constant size of the tunnels. Tunnel diameter is constant suggesting short residence period (Wetzel and Uchman, 2013); 3. Self-avoiding geometry. The lateral tunnels are subparallel to each other, thus displaying an overall self-avoiding geometry that is suggestive of systematic exploration of the sediment (Seilacher, 2007; Baucon, 2010; Sims et al., 2014). In turn, this systematic character is commonly linked to deposit feeding (Seilacher, 2007), although feeding on meiofauna or brooding could be alternative mechanisms to explain it;
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4. U-shaped morphology of branchings. The distal parts of the lateral tunnels are bended upwards. This U-shaped morphology could indicate that lateral tunnels have been connected with the sediment surface; 5. “Lining”. The presence of a mantle and an inner packeted meniscate core shows that the tracemaker was backfilling the burrow right after bulldozing through the sediment. An outer lining and a meniscate core is displayed by Ancorichnus (Baucon and Carvalho, 2008); 6. Meniscate fill. Meniscate packets indicate that bioturbation was developed during discrete events of backfilling (Keighley and Pickerill, 1994). Specifically, bioturbation was developed in sandy softgrounds by the active redeposition of sediment during the forward burrowing of the animal (D’Alessandro and Bromley, 1987); 7. Backfill similar to the host rock. Backfilled structures are the result of (a) several cycles of ingestion and excretion (ingestionand-excretion backfilling) or (b) actively manipulating the sediment with rigid limbs (excavation backfilling) (Baucon et al., 2014). Because the texture of the backfill is similar to the host rock, Cladichnus lusitanicum is likely to have been produced by excavation backfilling. This contrasts with typical examples of Cladichnus, the filling of which contrasts with the host rock (Monaco et al., 2012, Pl. 2.3; Uchman, 2007, Fig. 6A, B) and therefore suggests ingestion-and-excretion backfilling. Successive probings were made after previous probes were filled (D’Alessandro and Bromley, 1987). The probes were emplaced in different levels as it can be seen from different branching lengths and sharp tube endings. 8. Retrusive meniscate fill. Meniscate fill consists generally of cylindrical meniscate packets from which burrowing direction is not discernible. However, some specimens (Fig. 2a) show lateral tunnels with arcuate menisci, indicating that the tracemaker moved in direction opposite from the axial tunnel; 9. Large size of the meniscate packets. The length of each meniscate packet (see Keighley and Pickerill, 1994) is similar to or exceeds the burrow diameter; undisturbed, inter-menisci sediment is relatively thin. This shows that a large quantity of material was processed during each backfill (Keighley and Pickerill, 1994). Meniscate packets are produced by modern burrowing insects, i.e. masked chafer beetle larvae (Counts and Hasiotis, 2009). Analogous size relationships have been observed in Ancorichnus and Parataenidium (Baucon and Carvalho, 2008); 10. Double meniscate packets. Some specimens double rows of meniscate packets for each lateral tunnel. Left and right rows of each tunnel are bilaterally symmetrical. This suggests that laterally contiguous meniscate packets have been emplaced during the same episode of backfilling. This could also indicate that both sides of the animal served contemporaneously to excavate the sediment, similarly to the burrowing mechanisms invoked for Rusophycus (Seilacher, 2007). The above described evidences suggest that the tracemaker of Cladichnus lusitanicum explored systematically the sediment for nutrients (deposit feeding) or meiofauna. The burrowing dynamics consisted of a three-step burrowing cycle: (1) advancement of the axial tunnel, which was maintained open with the sediment surface; (2) digging and backfilling of a lateral tunnel until the sediment surface is reached; (3) homing, that is return to axial burrow through the sediment surface, and restart of a new burrowing cycle (Fig. 3). However, this model of burrowing dynamics is unsatisfactory because it is not energetically efficient behavior for deposit feeding: the tracemaker had to expose itself to the risks of predation on the seafloor after each burrowing cycle. In addition, the axial tunnel seems to be backfilled as well; therefore the third step of the suggested burrowing cycle would have been not feasible. Similarly,
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Fig. 3. Burrowing dynamics interpreted for Cladichnus lusitanicum leave two open possibilities as feeding behavior: Model 1 – “Rotational” (valid if the axial burrow has two rows of packets); Model 2 – “Alternate” (valid if the branching burrows have two rows of packets). Fig. 3. A dinâmica da arquitectura das galerias interpretada para Cladichnus lusitanicum deixa em aberto duas possibilidades como comportamento de alimentação: Modelo 1 – “Rotativo” (válido se a galeria axial apresenta duas direcções de material de retropreenchimento; Modelo 2 – “Alternado” (válido se as galerias ramificantes apresentam duas direcções de material de retropreenchimento).
tracemaker identity is difficult to ascertain because a modern Cladichnus lusitanicum and the behavior that encloses has not been documented yet. The ichnogenus Cladichnus is commonly attributed to deposit-feeding or chemosymbiotic vermiform tracemakers (Uchman, 2007), although some Cladichnus-like structures are also interpreted as plant-derived structures (Gregory et al., 2004). However, an arthropod tracemaker for Cladichnus lusitanicum is also likely because of excavation backfilling, that is commonly derived from manipulation with rigid limbs (Baucon et al., 2014). General morphological similarity with the meniscate packets of the masked chafer beetle (Counts and Hasiotis, 2009) supports this hypothesis. For these reasons, further studies have to be carried out with new findings in loco to decipher completely the behavior and burrowing dynamics of Cladichnus lusitanicum.
4. Discussion and conclusions The branched meniscate backfilled burrow systems with a palmate pattern referred to Cladichnus lusitanicum are reinstated here as a valid ichnospecies. These forms, occurring in different units of the upper-Barremian-Aptian Almargem Formation from the Lusitanian Basin, are also special when the continental paleoenvironment where they occur is considered. The ichnogenus Cladichnus was previously reported mostly in turbidite marls (Savdra, 2012) spanning from Late Cretaceous to Palaeogene. More specifically, Cladichnus fischeri ranges from the Coniacian to the Eocene (Uchman, 2004). C. aragonensis was described from the Hecho Group Eocene flysch of the Pyrenees (Uchman, 2001). C. parallelum was found in the Cretaceous and Eocene distal turbidites (Wetzel and Uchman, 2013). However, there are some few reports about the occurrence of Cladichnus in shallower storm-related and wave-dominated deltaic deposits from the Carboniferous (Chisholm, 1970; Gluszek, 1998). Wilkens (1947)
describes C. lusitanicum together with typical marine body fossils and plants evidencing nearshore environments. C. lusitanicum in the Portuguese Lusitanian Basin is found in floodplain deposits resulting from a fast displacement of a braided river system. Another feature that apparently is common in C. lusitanicum to all the previous known ichnospecies of Cladichnus is the usual relation with oxygen-depleted levels and the frequent sharing of similar tiers with Chondrites (Monaco et al., 2012). Also the chemosymbiotic feeding mode demonstred for Chondrites has been progressively compared with Cladichnus (e.g., Keighley and Pickerill, 1994). However, C. lusitanicum occurs in beds including plant debris that may have been exploited by the producer in a systematic and oriented foraging pattern and shallow tier, without any evidence for farming or phobotactic behaviour. This is also true for the same forms described by Wilckens (1947) in the Aptian of South Georgia Island. Hopefully more findings of Cladichnus lusitanicum in the Lusitanian Basin and elsewhere may consolidate the paleoethological interpretation as pascichnion for this ichnospecies and respond to the question whether it is stratigraphically restricted to Lower Cretaceous, and mostly to Aptian age.
Acknowledgements We wish to express our gratitude to the operational directors of the National Museum of Natural History and Science, Dr. Vanda Faria dos Santos, and the Geological Museum of Lisbon, Dr. Miguel Ramalho, the authorization to study the collections. To José António Anacleto and Eng. Jorge Sequeira from the Geological Museum, and João Paulo from the National Natural History Museum, to help to find the material now studied. The photographer Luis Quinta is highly appreciated for his photo of the Fig. 2f that clearly sets the difference with our amateur photos. The work of Andrea Baucon has been supported by the ROSAE project.
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