Saws, scissors and sharks: Late Paleozoic

Saws, scissors and sharks: Late Paleozoic experimentation with symphyseal

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dentition Leif Tapanila

Department of Geosciences, Idaho State University and Idaho Museum of Natural History, Pocatello, ID 83209, USA corresponding author: email: [email protected], phone: 1-208-282-5471, Fax: 1-208-282-4414

Jesse Pruitt Idaho Virtualization Lab, Idaho Museum of Natural History, Pocatello, ID 83209, USA Cheryl D. Wilga Department of Biological Sciences, University of Alaska Anchorage, AK 99508, USA Alan Pradel CR2P UMR 7207 (MNHN, CNRS, UPMC, Sorbonne Université), Département Origines et Evolution, Muséum National d'Histoire naturelle, CP 48, 57 rue Cuvier, F-75231 Paris cedex 05 (France) Department of Vertebrate Paleontology, American Museum of Natural History, New York, NY, USA

Grant sponsors and numbers: NSF IOS 1631165 to CW

ISU College of Science and Engineering grant (no number) to LT

Thi s article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/ar.24046 This article is protected by copyright. All rights reserved

ABSTRACT Sharks of Late Paleozoic oceans evolved unique dentitions for catching and eating soft

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bodied prey. A diverse but poorly preserved clade, edestoids are noted for developing biting teeth at the midline of their jaws. Helicoprion has a continuously growing root to accommodate

more than 100 crowns that spiraled on top of one another to form a symphyseal whorl supported

and laterally braced within the lower jaw. Reconstruction of jaw mechanics shows that individual serrated crowns grasped, sliced, and pulled prey items into the esophagus. A new description and interpretation of Edestus provides insight into the anatomy and functional morphology of another specialized edestoid. Edestus has opposing curved blades of teeth that are segmented and shed

with growth of the animal. Set on a long jaw the lower blade closes with a posterior motion, effectively slicing prey across multiple opposing serrated crowns. Further examples of symphyseal whorls among Edestoidae are provided from previously undescribed North American examples of Toxoprion, Campyloprion, Agassizodus, and Sinohelicoprion. The symphyseal dentition in edestoids is associated with a rigid jaw suspension and may have arisen

in response to an increase in pelagic cephalopod prey during the Late Paleozoic.

INTRODUCTION Chondrichthyans have a deep evolutionary history starting more than 455 million years

ago (Miller et al., 2003). Two major clades are chiefly defined by how the upper jaw connects to the chondrocranium. Selachians opted for a flexible connection of the upper jaw, and comprise the more speciose group of chondrichthyans today. The other clade, Euchondrocephala,

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developed a tight joint between upper jaw and chondrocranium, and found its greatest diversity in oceans of the late Paleozoic Era, especially from the Mississippian to Permian periods (330 to

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250 million years ago). Their modern representatives, chimera and ratfish, are unusual chondrichthyans by modern selachian standards, but they share a feature with ancient euchondrocephalans: the growth of symphyseal dentitions with a common base for the crowns. Growing functional teeth at the midline of the jaw (symphyseal dentition) is the typical

dental arrangement in euchondrocephalans. In modern taxa these toothplates are supported by the ethmoid, whereas in Paleozoic forms they are typically supported by the palatoquadrate (Zangerl, 1981; Stahl, 1999). Symphyseal teeth often have button- or plate-shaped crowns presumably for crushing shells. However, one fossil clade, Edestoidea Hay 1930 (within the clade Eugeneodontidae Zangerl 1981), developed symphyseal whorls having laterally compressed, elongate crowns with serrated edges for puncturing and slicing prey. Edestoids were diverse and dominant large-bodied chondrichthyans of late Paleozoic oceans. Known almost exclusively from tooth fossils with little cranial material, the form, function, and phylogeny of the clade remains enigmatic. Digital techniques of the last decade provide new ways to examine fossil collections and resolve this clade. Recently, the spiral-toothed edestoid, Helicoprion, was reassembled using CT scans (Tapanila et al., 2013; Ramsay et al., 2015). Here for the first time we describe and interpret the jaw function of a very different edestoid, Edestus, based on CT and Xray data of a nearly complete juvenile skull. Helicoprion and Edestus represent end-members in specialization among two „styles‟ of

cutting dentitions in the Edestoidea (Fig. 1). Following previous authors, we refer to these styles in terms of clades (Agassizodontidae - Zangerl 1981 and Edestidae - Jaekel 1899, respectively), however phylogenetic details are still in their infancy owing to a general lack of cranial and post-


cranial characters among these fossil sharks. We describe the symphyseal dentitions of several taxa in each clade with a focus on previously undescribed North American occurrences and

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compare their symphyseal dentitions. First, we will review the anatomy and functional morphology of the best described edestid, Helicoprion.

Helicoprion (Agassizodontidae): Review of an extreme dentition Helicoprion is a peculiar fossil with a nearly-global distribution and over a century of

collection and investigation. Tapanila and Pruitt (2013) provided a comprehensive inventory of all Helicoprion in public collections, which in North America includes sites from Arctic Canada and Alaska southward through the American West down to Mexico. The greatest number and quality of Helicoprion fossils are recovered from economic phosphate rocks in SE Idaho that were once part of the Phosphoria Sea, an embayment of the western coastline during the Permian time period, 265 million years ago. Although SE Idaho has produced more than 70 Helicoprion fossils, much of what is known about the animal‟s jaw anatomy and functional morphology is

derived from an exceptionally preserved specimen, IMNH 37899 (Idaho No. 4 in Bendix-

Almgreen, 1966). This specimen was CT scanned (Tapanila et al., 2013) and its description is

summarized here.

Description: The symphyseal whorl of Helicoprion consists of dozens of triangular crowns attached to a common, unsegmented root that forms a logarithmic closed-spiral having a spiral angle of 82°. Crown dimensions increase gradually from the inner whorl to the outer whorl. The first tooth at


the center of the whorl is hook-shaped, called the juvenile tooth arch (Bendix-Almgreen, 1966) Projections of the crowns point anteriorward and underlie two smaller (older) crowns. Geometric

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morphometry analysis of >100 Helicoprion specimens (Tapanila and Pruitt, 2013) identified three distinct species that varied primarily in the upper, middle, and lower crown proportions (see Figure 2 for crown terminology). Further, these species-level differences are evident only in mature individuals having crowns numbering more than 85. The height of the root below the crowns (shaft) also varies by species, but is typically wider than 1/12th of the volution height. CT scans of IMNH 37899 reveal a medial, symphyseal tooth whorl, which is associated

with the left mandibular arch elements in natural position (Fig. 3). The tooth whorl forms a complete spiral that measures 23 cm in diameter and is composed of 117 serrated tooth crowns borne by a continuous base. The series of crowns increase in size from the center of the spiral to the very large crowns along the outer margin of the spiral. The preserved mandibular arch cartilages consist of the upper (palatoquadrate) and the

lower (Meckel‟s) jaws around the tooth whorl, and a labial cartilage that extends from the lower

jaw and braces against the outermost root of the whorl. The palatoquadrate is lined with dozens of small (2 mm) platelike teeth (Bendix-Almgreen, 1966; Ramsay et al., 2015). The posteroventral quadrate process of the palatoquadrate possesses double articular

surfaces that match the corresponding articular surfaces of the Meckel‟s cartilage. A dorsal, basal socket is present posteriorly, probably for the articulation with the braincase. The palatoquadrate extends and tapers anteriorly as a palatine ramus that bears a medial dome-shaped ethmoid

process for the articulation with the braincase. Thus, Helicoprion likely possessed an autodiastylic jaw suspension characterized by an ethmoid and basal articulation of the upper jaw


to the neurocranium. Such jaw suspension is present in other Paleozoic euchondrocephalans and is present in early embryonic stages of modern chimaeroids (Grogan et al., 1999). The lateral

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surface of the upper jaw displays two fossa: a quadrate fossa posteriorly and a palatine fossa anteriorly for quadratomandibular muscle attachment. The lateral surface of the Meckel‟s cartilage displays a quadratomandibular fossa, which

is bordered by a lateral flaring of the cartilage anteroventrally. Anterior to the jaw joint is a dorsal process that abuts a descending process of the palatoquadrate. Helicoprion possesses a unique example of articulation involving a labial cartilage. The

latter forms a synchondrosis with the dorsal surface of the Meckel‟s cartilage. The labial cartilage is wider where it corresponds to the dorsal position of the successive roots in the whorl. In this way, the labial cartilage provides lateral support to the whorl and mediates alignment

between successive volutions of the whorl. With addition of new crowns and growth of root at the lingual end of the whorl, the

previous teeth are pushed forward (labially) along an arced path, eventually concealed within the

lower jaw. Earliest, central parts of the whorl are encased in tesselated cartilage. Helicoprion continuously added tooth crowns throughout its life, some estimated to include more than 180 in five volutions (Tapanila and Pruitt, 2013). Tooth crowns increase in size following an exponential function, which corresponds to an ever-increasing whorl diameter, and jaw length to

match. The largest whorl is 56 cm in diameter with the largest tooth crowns 14 cm in height.


Interpreted Jaw Function: The Helicoprion whorl could represent an extreme condition of the modern holocephalan

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toothplate that is composed of a single, continuous tooth base that supports fused tooth crowns, and which grows throughout life continuously from the lingual towards the labial surface of the jaw in a logarithmic spiral. A region at the lingual (posterior) end of the spiral, called the tooth

pit, was invoked by Bendix-Almgreen (1966) as the developmental site for root and crown

formation. In the modern holocephalan, the toothplate is formed from the fusion of the last two members of a reduced tooth family and grow throughout life (Patterson 1992; Stahl 1999:23). Helicoprion is exceptional for having a tooth whorl that spirals upon earlier crowns to complete

more than one full volution, and in mature individuals, exceeds four volutions (Tapanila and Pruitt, 2013). Both modern holocephalans and Helicoprion do not shed teeth nor toothplates in life. A biomechanical model of how the jaws and tooth-whorl functioned in the feeding

mechanism suggests that Helicoprion specialized on soft prey (Fig. 4; Ramsay et al., 2015). Hard prey would tend to slip out of jaws with tooth edges that are anterior-posterior directed as they closed, much like trying to cut a walnut with scissors. In contrast, soft prey tissue is trapped between the upper jaws and tooth whorl of the lower jaw. The distinctive outward curvature of the tooth-whorl creates a continuum of functional tooth crowns that, combined with tooth crown

position relative to the jaw joint, produces an effective mechanism for slicing soft prey. As the jaws close, the anteriormost tooth crowns quickly pierce and drag the prey into the mouth, while intermediate tooth crowns further slice into the prey, and the more posterior tooth crowns push

the prey towards the esophagus. Tooth crowns at the tip of the jaw have a velocity advantage during mouth closing (mechanical advantage from 0.150 to 0.289 from fully open to fully


closed), while tooth crowns at the jaw joint end have a force advantage (mechanical advantage from 0.234 to 0.597 from fully open to fully closed). Total estimated force output for the all

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quadratomandibularis muscle divisions (anterior, posterior, and ventral) is estimated at 2,391 N. Cyclical opening and closing of the jaws further disables prey, whereby the curved path traveled by each tooth crown resembles the arc-like slashing of many swords or a rotary saw blade.

The ventral position of the jaw joint relative to the tooth crown influences the direction of

force applied to the prey (Ramsay and Wilga, 2007). The convex shape of the tooth whorl

results in anteriormost tooth crowns that are at obtuse angles, intermediate tooth crowns that are

at or near right angles, and more posterior tooth crowns that are at acute angles relative to the

longitudinal axis of Meckel‟s cartilage. As the jaws close, tooth crowns move from a more anterodorsal to a more posterodorsal path. Enlarged fused labial cartilages connected to Meckel‟s cartilage would have functioned to prevent the lower jaw from elevating the tooth whorl into the floor of the cranium.

The laterally compressed jaws would be critical in bracing the tooth-whorl by providing

extensive contact area with Meckel‟s cartilage. However, the posterior end of the jaws expanded outward laterally not only for transport of prey to the esophagus, but also for passage of water to the branchial arches for ventilation. As in all aquatic vertebrates, the pharynx and this expanded

oral region would have generated the suction for transporting prey from the oral cavity into the esophagus for swallowing.


METHODS AND MATERIALS Descriptions and illustration of fossils in this report use a combination of external

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scanning methods, computed tomography (CT), and computer-generated graphics. CT volume data were reconstructed using Mimics v. 14.11. Post-processing and computer graphics software includes GeoMagic Studio 2012 & 2016, Blender v. 2.64a, Pixologic ZBrush 4r8 P2, and Luxion Keyshot 7. Terminology used throughout this paper is defined in Fig. 2. Description of tooth crown

morphology distinguishes upper, middle and lower parts on the basis of inflection points in the curvature of the crown, as viewed from lateral aspect (e.g., Tapanila and Pruitt, 2013).

Toxoprion Two specimens, USNM 330003 and 330004, were recently described as Campyloprion

sp. without the benefit of CT-scanning (Itano and Lucas, 2018, p.414-415, textfigs. 6, 16A, B). These are reinterpreted here as Toxoprion, as discussed below. Both USNM 330003 and 330004 were CT scanned using an Xradia microXCT Scanner

(University of Texas High-Resolution X-ray CT Facility). Both scans were conducted at 90kV, 10W for 3.0 to 3.5s acquisition time. For USNM 330003, a total of 928 slices were collected from 1441 views resulting in a final voxel resolution of 53.72 µm. For USNM 330004, a total of 961 slices were collected from 1441 views resulting in a final voxel resolution of 55.72 µm.


Agassizodus CT scanning of FMNH PF 8517 used a custom-built dual tube X-Ray scanner from GE

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(University of Chicago PaleoCT Facility). The specimen was scanned at 200kV at 1 second acquisition time. A total of 2001 slices were collected at a final voxel resolution of 29.685 µm.

Campyloprion Surface scanning of USNM PAL 443547 by Faro EdgeArm laser scanner at 100µm

resolution and photo-draped (Idaho Virtualization Lab, IMNH).

Edestus CT scanning of FMNH PF2204 used a GE Phoenix v|tome|x s 240 (AMNH) at a voxel

resolution of 55.72 µm. Surface roughness is an artifact of scanning resolution and compressed nature of the specimen. The cranial model was constructed in Zbrush using two complete blades (NUFV 22373 and 22392 of ANS collections, Philadelphia) scaled to match FMNH PF2204. Material supporting the Edestus description includes digital 2D X-radiographs of E.

heinrichi: FMNH PF8046, paired Meckel‟s with lower and partial upper blade; FMNH PF5871 palatine, distal Meckel‟s with upper and lower blades; Mecca Quarry Shale, Linton

Formation. Edestus minor specimens include: Xrays of FMNH PF8047, anterior part of Meckel‟s with lower and upper blades, Logan Quarry Shale; Xrays and CT of USNM 7255, anteriormost Meckel‟s and palatine with upper and lower blade, unknown provenance.


Mechanical advantage (MA) and force output was estimated for jaw closing based on our reconstruction of FMNH PF2204. MA (in-lever: out-lever) was measured from the

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quadratomandibular joint to the estimated center of the jaw adductor muscle (in-lever) and to each tooth tip (out-levers). Values closer to 1.0 provide a greater force advantage (cusps closer to the joint transmit more force than cusps further from the joint), whereas values closer to 0.1 confer a greater velocity advantage (cusps further from the joint move faster than cusps closer to the joint). Force output was estimated for three divisions of the quadratomandibular muscles based on potential insertional area using Helicoprion (Tapanila et al., 2013) and Hydrolagus collei (Didier, 1995) as models: QMV, ventral division from Meckel‟s to quadrate; QMA, anterior division from posterior half of palatine to Meckel‟s; and QMP, posterior division from Meckels‟ to the anterior third of the chondrocranium (between eye and nasal capsule).

Sinohelicoprion CT scanning of USNM 235393 using NSI scanner (University of Texas High-Resolution

X-ray CT Facility). The specimen was scanned at 180 kV with an aluminum filter at 2 fps. A total of 1903 slices were collected with a final voxel resolution of 55.70 µm.

Institutional abbreviations: AMNH, American Museum of Natural History, New York, U.S.A.; ANSP, Academy of Natural Sciences Museum, Philadelphia, U.S.A.; FMNH, Field Museum of Natural History,


Chicago, U.S.A.; IMNH, Idaho Museum of Natural History, Pocatello, U.S.A.; MCZ, Museum of Comparative Zoology, Harvard University, Cambridge, U.S.A.; NMMNH, New Mexico

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Museum of Natural History, Albuquerque, U.S.A.; TUGC, Tufts University Geology Collection, Medford, U.S.A.; UCM, University of Colorado Natural History Museum, Boulder, U.S.A.; UCMP, University of California Museum of Paleontology, Berkeley, U.S.A.; USNM, National Museum of Natural History, U.S.A.

RESULTS AND DISCUSSION

Toxoprion (Agassizodontidae)

Description: Toxoprion specimens show a series of 17–22 tooth crowns on a continuous root having a

spiral comprising two-thirds of a volution (Figs. 5 and 6). The shape of individual tooth crowns

is distinct in having a tall occlusal edge (upper part) and an equally tall lower part of the crown, which includes the anteriorly-curved projection of enameloid. The middle part of the tooth crown is much shorter by comparison to Helicoprion. Crowns are finely serrated on both edges. Projections of the lower enameloid extend almost to the base of the root, making the shaft only 1/18th of the volution height (base of root to apex of crown). CT scans of USNM 330003 and 330004 provide images of the earliest part of the whorl. The angle of spiraling is approximately 60°, and begins with a hook-shaped juvenile tooth arch. The root is deeply concave opposite the crowns. The surface of the root between crowns is deeply pitted (Fig. 5H).


Interpretations: Originally named Edestus lecontei (Dean 1897), the type specimen, UCMP 32015, was

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later defined within a new genus, Toxoprion (Hay 1909). In this and most subsequent discussions of Toxoprion (e.g., Zangerl, 1981; Ginter et al., 2010; Itano and Lucas, 2014), sketches of the fossil illustrate short, knobby crowns at the narrow end of the curved root, abruptly changing to

long triangular shapes at the thickest part of the root. Although CT-scanning was not an option for this type specimen, examination of the photograph presented in Figure 5E shows that the „knobby‟ to „triangular‟ transition in appearance coincides with a fracture in the specimen that gives an oblique cross-sectional view of the crowns, and not an accurate representation of the full

lateral view of the crown. We interpret the series of crowns to all be triangular in form, and that

the blunt appearance of some crowns is simply an artefact of specimen preservation. We therefore reject previous interpretations founded on the „triangular‟ to „knobby‟ appearance of Toxoprion crowns, such as ontogenetic change in crown morphology or abrasion through usage. Itano and Lucas (2018) recently came to a similar conclusion regarding the exposure of

crowns in the type Toxoprion lecontei, however they misinterpret the left side of the specimen as the anterior part of the whorl. It is evident in UCMP 32015 (their text-figure 18 and here in Fig. 5E), and from previous illustrations, that the basal projections of the crowns point to the right. Furthermore, the leftmost crown and root on the specimen terminates with an unbroken, rounded, non-tapering form, which is typical for the posterior region for other well-studied edestoid whorls, e.g., Helicoprion and Edestus. We therefore interpret the UCMP 32015 specimen as oriented in the rock with its anterior end pointing to the right.


Both USNM specimens are nearly identical to the type UCMP 32015, thus providing three examples of Toxoprion, and rejecting previous interpretations of the USNM specimens as

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Campyloprion (Itano and Lucas, 2018). All three are approximately the same diminutive size. The initial hook-shaped juvenile tooth arch and general shape of crowns resemble the early ontogenetic expression of the whorl seen in Helicoprion. Although it has an open spiral, Toxoprion is morphologically similar to Helicoprion and its symphyseal dentition achieves the greatest volution (~240°) among agassizodontids.

North American occurrences: The type specimen, UCMP 32015, is from near Eureka, Nevada. It was collected near a

mine in the 19th Century and its precise locality is unknown. In a recent article, Itano and Lucas (2018, p. 415) assert that UCMP 32015 “must have come from the lower Permian Carbon Ridge Formation.” Unfortunately this assertion is made without having conducting field work in the geologically complicated region of Eureka, nor analyzing the lithology of the matrix to form an

argument based on comparison with regional lithologies. Given the importance of provenance and its propagation in the literature as objective data, we reject this non-conservative approach to

assigning certainty to the age of a fossil. Based on limestone rocks in the region of the town of Eureka, Nevada and the Eureka mining district, UCMP 32015 cannot currently be dated more precisely than Late Paleozoic in age (Carboniferous to Permian). Two additional specimens presented here from the Smithsonian, USNM PAL 330003 and

USNM PAL 330004, come from Jack County, Texas. These two concretionary specimens were collected in the Jacksboro Limestone Member of the Graham Formation of Pennsylvanian age.


Campyloprion (Agassizodontidae)

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Description: The symphyseal whorl of Campyloprion forms a shallow open spiral of very tall crowns

dominated by long middle and lower parts of the crown (Fig. 7). The upper cutting surface of the

crown is relatively short and serrated on both edges. Long projections of enameloid angle below three adjacent crowns. The projections leave only a narrow lateral band of shaft, 1/15th to 1/25th the height of volution. In all examined specimens, the root appears to be continuous and has a shallow concave slot opposite the crowns.

Interpretation: All North American specimens are incomplete large-crowned whorls of Campyloprion.

The most complete specimens are from Russia and approach 180° in total volution, though none include the smallest, most juvenile series of teeth. Therefore determining spiral angle is challenging because the earliest part of the whorl, needed to define the axis of spiraling, is absent in available specimens. Itano and Lucas (2018) applied an algorithm to fit a logarithmic spiral to Campyloprion specimens and attained values between 60–67°, though at least some of these

values are based on a circular method of fitting a spiral to broken pieces in order to reconstruct

the curvature of the same broken specimens (e.g., their text-figure 13B and explanation on page 411–412).


North American occurrences: Campyloprion annectans, MCZ 2039 (and cast TUGC 1293), type specimen, locality

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speculative, ?Iowa, of ?Pennsylvanian age. Eastman 1902; NMMNH P-68551, Campyloprion cf. C. ivanovi, New Mexico, Pennsylvanian (Hodnett and Lucas, 2015; Itano and Lucas, 2018); UCM 109521, two fragmented, incomplete crowns assigned to Campyloprion sp., Texas, Pennsylvanian (Itano and Lucas, 2018); USNM PAL 443547 from Locality 68 ATR-153 on the Kivalina River, NW Alaska (Fig. 7D, E). Unpublished notes in 1968 from paleoichthyologist

D.H. Dunkle identified the specimen as Campyloprion, though it has remained unfigured and undescribed until now. Based on regional geology for Locality 68 ATR-153, the specimen is likely from Pennsylvanian age rocks. We refer IMNH 14095 described by Tapanila and Pruitt

(2013:fig. 2.9), as Campyloprion sp. from the Phosphoria Formation of Idaho, middle Permian.

Agassizodus (Agassizodontidae) Description: The CT scan of FMNH PF8517 shows a tight spiral of four, very wide crowns

characterizing the symphyseal whorl of Agassizodus (Fig. 8). The cutting apex of the crowns is

laterally compressed and forms a thin ridge along both medial edges. The base of the crown projects laterally and anteriorly. Each crown abuts posteriorly above the root of one adjacent crown. Lateral teeth are associated with the two specimens of this genus, and form spiral tooth rows of crushing dentitions on at least the lower jaw.


Interpretation: The symphyseal whorl of Agassizodus is most similar to the crushing whorls of Paleozoic

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caseodontids. The association with lateral crushing toothplates leads to an interpreted lower jaw depicted in Fig. 8E.

North American occurrences: Only two specimens of the genus are known based on the symphyseal whorl. FMNH

PF8517 is from the Queen Hill shale, Nebraska, upper Pennsylvanian (Zangerl, 1981:fig. 85); type specimen, Lophodus variabilis Newberry and Worthen 1870 (their fig. 119), Upper Pennsylvanian, Coal Measures, La Salle, Illinois.

Edestus (Edestidae): A new type of extreme dentition Edestus (Leidy, 1856) has opposing curved whorls of tooth blades that formed in the

symphysis of the upper and lower jaw. Each blade comprises a single tooth family having up to 12 serrated triangular tooth crowns at the end of elongate V-shaped roots. Teeth vary in size and proportion along the length of the whorl. Crowns are slightly smaller toward the anterior tip of the whorl, whereas the length of the tooth roots increases anteriorly. These observations suggest that continuous addition of new, larger teeth is offset by shedding of teeth at the front end of the blade (Hay, 1912; Zangerl, 1981). The roots are stacked en echelon to the front of the blade such that the anteriormost tooth root is exposed for shedding, whereas roots to the posterior are supported between roots of adjacent teeth (Zangerl, 1981; Zangerl and Jeremiah, 2004). A


prominent bulge on the base of the blade marks the site of jaw attachment, beyond which the anteriormost tooth was shed.

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Dozens of blades and hundreds of ejected Edestus teeth have been collected from Pennsylvanian age (330 million years ago) marine shale deposits of midwestern United States and Britain, but cranial material is exceedingly rare. In the absence of anatomical context, functional models of Edestus propose comparisons to slashing sawfish rostrums (Eastman, 1902; Hay, 1909), shearing scissor jaws (Peyer, 1968; Zangerl and Jeremiah, 2004), or fixed vertical slashing weapons (Itano, 2014, 2015, 2018). Hay (1912) described a specimen of Edestus mirus having blades positioned at the jaw symphysis, though only cartilages at the anteriormost site of attachment were preserved. Recently, three Edestus specimens with teeth and associated crania were described, but not fully reconstructed (Zangerl and Jeremiah, 2004). These specimens were reexamined, but one in particular (FMNH PF2204) forms the core of this description.

Description: FMNH PF2204 is a crushed specimen measuring 25 x14 x1 cm and contains a complete

upper and lower tooth blade consistent with Edestus heinrichi (Fig. 9). The individual is subadult based on blade length roughly 1/3 of the largest examples of this species (Zangerl and Jeremiah, 2004). The chondrocranium and jaws are remarkably intact and confined. The chondrocranium rests on the side with the left and right jaws collapsed on one another. The

posterior regions of the chondrocranium are rotated to face dorsally. The posterior half of left Meckel‟s cartilage lies 3 cm away from the rest of the elements.


Meckel‟s cartilage (m) is approximately 1.5 times the length of the lower blade. The lower blade (lb, 8cm long) curves ventrally between the left and right sides with four crowns

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extending anteriorly beyond the symphysis. The symphysis at the anteroventral end supports the posteroventral aspect of the blade. The anterior end of the Meckel‟s cartilage flares dorsally to produce a triangular prominence over the posterior end of the blade. A low density region occurs just posterior to the blade at the ventral portion of the symphysis, which may be the site for tooth development (i.e., tooth pit). The posterior end narrows to form the quadratomandibular joint (QMJ). The jaw joint has two prominent processes on both sides of the socket, with the ventral

process extending far posterior. Xrays of the PF5871 specimen reveal a high-density curved

region in the socket that suggests the presence of a medial longitudinal ridge. Articulating with Meckel‟s cartilage is a slender quadrate element that extends dorsally to

articulate with the otic process of the chondrocranium. The distal end of the quadrate contains a process that articulates with the socket on Meckel‟s cartilage. The proximal end of the quadrate is poorly preserved and terminates at the otic region. The upper blade (10.4 cm long) curves dorsally between paired cartilages that we

interpret as the palatines. Three tooth crowns extend anteriorly beyond the symphysis. The anterior region of the palatine cartilage is lunate and matches the profile of the tooth blade. Posteriorly, an arm-like extension abuts the ethmoid region of the chondrocranium. Most chondrocranial regions are distinguishable. The ventral surface of the otic and

occipital regions form a T-shape with the quadrate articular processes terminating laterally. An ethmoid plate extends anteriorly, and the chondrocranium is capped by a shield-like dorsal plate.


Paired elements that resemble suborbital bars or ceratohyal/branchial elements were in the center

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of the cranial material. Nasal and rostral elements lie dorsal to the palatines.

Interpreted Jaw Function: Anatomical relationships described here contradict earlier functional models that rely on:

(1) loose attachment of the blades to the jaw and shearing passage of opposing blades (scissor: Zangerl and Jeremiah, 2004); (2) large rotations of fixed jaw joints (vertical slashing: Itano, 2014, 2015, 2018); or (3) greatly protruding blades (sawfish: Eastman, 1902). In stark contrast to previous depictions of Edestus, the tooth blades curve inward toward the throat presenting an

externally streamlined profile of the jaws (Fig. 1B). Four upper and three lower tooth crowns extend beyond the symphysis, and were likely embedded in mucosal tissue beyond the symphysis. Tooth ejection could be accommodated by sliding of the anterior tooth forward along the V-shaped root beyond the anterior margin of the symphysis. The quadratomandibular joint has dual perpendicular articulations characteristic of

Chondrichthyes (Hotton, 1952; Zangerl, 1981), which prohibits lateral motion and dislocation (Motta and Wilga, 1995). The large processes on Meckel‟s cartilage associated with the QMJ restrict dorsoventral motion to 24°. The unique quadrate-cranial joint (QCJ), which also functions as the jaw suspension, has more freedom of motion at an estimated 33° and allows the distinctive slicing motion. Edestus tooth blades rigidly held between palatine cartilages functioned as effective

grasping and slicing tools for disabling soft-bodied prey. After prey capture by grasping, jaw adductors pull Meckel‟s cartilage anterodorsally to close the mouth and cause the lower tooth


blade to bite into the prey. At this time, each tooth crown slices into the prey roughly two to three crown lengths while pushing the prey against the teeth on the fixed upper blade to further

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slice the prey. Subsequent mouth opening can further slice the prey as jaw depressor muscles pull Meckel‟s cartilage posteroventrally (Fig. 9F-H). The occlusal surfaces of the tooth blades would be nearly parallel when biting to full mouth closure (Fig.9J). Repeated cycles of mouth opening and closing, with each tooth crown cutting up to three crown lengths, would have functioned to effectively slice and cut through prey (Fig. 10). Estimation of mechanical advantage suggests that Edestus heinrichi has a two gear

system that operates the jaws. The QMA and QMV muscles appear to have similar MA values (0.33-0.20 for proximal-distal teeth) when the mouth is closed. However, when the mouth is

maximally opened, MA increases (0.46-0.28 for the proximal-distal teeth) transferring more

force for slicing through prey. Greater force can be transmitted through the QMP muscle (0.520.32 MA for proximal-distal teeth) regardless of the state of mouth opening. The QMP (425-265 N proximal-distal) and QMV (375-234 N) have better leverage for transmitting force to the teeth compared to the QMA (134-83 N). Perhaps the QMA functions to close the mouth faster while the QMP transmits more bite force. QMV appears to function during ventilation in modern sharks. Total estimated force output for all muscle divisions is estimated at 1907 N, half that

estimated for Helicoprion, which only has a single 23cm diameter tooth whorl (Ramsay et al., 2015). The quadrate double joint mechanism (QMJ and QCJ) described here for Edestus is

analogous to and predates the Permian (Grogan and Lund, 2000) evolution of streptostyly in terrestrial lizards, snakes and marine reptiles. Streptostyly is a form of cranial kinesis where the distal and proximal quadrate joints are mobile (Schwenk, 1999). Several functions for


streptostyly have been proposed in lizards such as allowing anteroposterior movements of the lower jaw for tooth alignment or shearing action, and amplifying the mechanical advantage of

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the posterior jaw adductor muscle while potentially decreasing joint reaction force (Schwenk, 1999). The quadrate slides as it rotates on the cranium during food shearing in Uromastix acanthinurus allowing a larger gape (DeBeer, 1985). Thus, the proposed functions for streptostyly are also present in Edestus: the lower jaw moves anteroposteriorly during the gape cycle to cut through prey, and the posterior quadratomandibularis muscle has a higher mechanical advantage than the anterior jaw adductors. Curvature of Edestus tooth blades of various species range from approximately 80–174

degrees (half a volution to virtually straight) along the ventral edge with the central tooth at the apex (Zangerl, 1981). Jaw function would be similar for blades that are relatively straight (E. heinrichi) or moderately curved (E. mirus). However, as blade curvature nears 100 degrees, jaw function may approach that of Helicoprion (Tapanila et al., 2013; Ramsay et al., 2015). Edestus heinrichi has a modified autodiastylic jaw suspension with two upper jaw-cranial

articulations (Zangerl, 1981; Grogan and Lund, 2000; Wilga, 2002). The quadrate-otic articulation is analogous to the modern selachian hyomandibular-otic joint, which is lacking in

Edestus. Whether the anterior craniopalatine connection was retained depends on the state of the palatine cartilage. If the palatoquadrate split during development into discrete quadrate and palatine elements and became widely separated, then the ethmoid articulation was retained. Embryological evidence supports the splitting theory: quadrate and palatine elements condense and chondrify separately during development, with fusion occurring later (DeBeer, 1985). The palatine and quadrate cartilages are separate elements in at least one selachian species, cookie-

cutter shark Isistius brasiliensis (Shirai and Nakaya, 1992). In contrast to Edestus, the two


elements remain in the same location as other species with intact palatoquadrates, with a closely abutting articulation where they meet (Shirai and Nakaya, 1992). The palatine and quadrate

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cartilages in Isistius have very reduced teeth that are not well supported and appear to play a role in maintaining labial contact by suction with the prey rather than disabling prey (Shirai and Nakaya, 1992). Alternatively, the palatine element of Edestus might have become reduced and

ultimately lost, then new palatine-like cartilages would have developed and separated from the

chondrocranium. Several taxa within Eugeneodontida have reduced or completely lost the palatoquadrate cartilages, however none have a novel palatine-like cartilage supporting the upper

teeth, which are all reduced in size (Zangerl, 1981). The most parsimonious hypothesis suggests that the palatine remained separated from the quadrate and remained a laterally supporting

structure for the upper tooth blade. If so, retaining separated palatine and quadrate elements through development suggests that the characteristic arose by paedomorphosis.

North American occurrences: FMNH PF2204 was collected by R. Zangerl (Zangerl and Jeremiah, 2004) at Level J in

the Logan Quarry Shale, Staunton Formation, Indiana, Pennsylvanian age. A comprehensive review of North American Edestus (Itano et al., 2012) documents their occurrence in Pennsylvanian age rocks from Michigan, Illinois, Kentucky, Kansas, Missouri, South Dakota, Oklahoma, Texas, Colorado, and Wyoming.


Sinohelicoprion (Edestidae)

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Description: A CT scan of USNM 235393 shows an open spiral of triangular crowns with fine

serrations (Fig. 11). The crown is composed mostly of the upper part, with a short middle, and almost no projection in the lower part. The projection slightly underlies the adjacent tooth in a posterior direction. The shaft is moderate, being 1/8th the height of volution. The curved root appears to be segmented. The base of the tooth root is concave.

Interpretation: Previously known only from Chinese deposits (Liu and Chang, 1963; Zhang, 1976; Lei,

1983; Liu and Wang, 1994), USNM 235393 compares well to the Asian examples. By comparison to Edestus, the strong segmentation of tooth units makes it likely that

Sinohelicoprion also shed its teeth. The trait of having projections directed posteriorly and shedding teeth unite this genus with Edestus, however, the concave base in Sinohelicoprion differs significantly from the convex base in Edestus. The lack of associated material makes it difficult to relate Sinohelicoprion to other edestids. It is uncertain if the whorl presented here is from the upper or lower jaw.

North American occurrences: We assign two specimens from Wyoming to Sinohelicoprion: USNM 235393 from the Teton Range and an unnumbered specimen upper part of a crown from the Wind River Range


(Branson, 1933:fig 1.15). Both are from the Phosphoria Formation of Permian age, which also preserves Campyloprion and Helicoprion. This is the first report of Sinohelicoprion outside of

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China.

The Function Of Whorl Curvature Helicoprion and Edestus employ their symphyseal dentitions very differently, and this

difference may extend to other members of their clades. The single whorl of Helicoprion is highly curved, supported by a short lower jaw, and does not have an opposing whorl in the upper jaw. By contrast, two opposing open-spiraled whorls (blades) of Edestus are supported at the end

of long jaws. Unfortunately, cranial fossils are rare among edestoids, but there are some general statements that can be made regarding symphyseal whorls. In relating dental anatomy to jaw anatomy, the key variable of the symphyseal whorl is its curvature. A whorl with a closed spiral, as in Helicoprion, differentiates the functional role of

crowns in a biting cycle, giving it a saw-like slicing motion through prey. This curvature

provides for continual addition of new crowns that function for feeding with minimal jaw elongation, while older crowns have a new function in supporting the whorl. The diameter of the whorl is approximately equal to the lower jaw length in Helicoprion. Adding one volution of crowns increases the diameter of the whorl by half as compared to a „straight‟ whorl. Morphologically, Helicoprion employs the same spiral geometry found in many other spiral

forming animals, such as gastropods and cephalopods (Tapanila and Pruitt, 2013). The closed spiral angle of ~80° aligns the overlap of each volution to maximize strength and minimize the use of materials and space (Raup, 1966, 1967). In open spiraled agassizodontid genera that do


not appear to have shed teeth, such as Toxoprion and Campyloprion, addition of new crowns

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requires commensurate elongation of the jaw.

Reconciling An Extreme Dentition Edestoids and their unique dentitions contributed some of the Paleozoic Era‟s most

spectacular and large predatory fish. Estimation of body size (Tapanila et al., 2013) suggests that, at over 25-foot in length, Helicoprion was the largest animal species on the planet during its time. Both Campyloprion and Edestus have species of very large size (Newberry 1889;

Obruchev 1964), based on tooth crowns exceeding 10 cm in height, though body length estimates await better preserved specimens with jaws. Edestoid genera also appear to have been

long-lived. Species of Helicoprion are found globally in marine deposits spanning 8 million years and Edestus are known from estuary deposits for over four million years. It seems reasonable to question why this slicing dentition arose in euchondrocephalans but not in other sharks. The slicing symphyseal dentition in Edestoidae is descended from the more common

crushing toothplates found in euchondrocephalans, and Agassizodus is perhaps the best bridge between sister clades, the caseodontids (crushing) and edestoids (slicing). The Paleobiology Database (searched January 2018) documents Late Paleozoic cephalopod diversity on the rise among ammonoids and nautiloids, which would have supplied fast-swimming sources of food. The symmetrical tail lobes and fusiform body of Eugeneodontida (which includes the Edestoidae) suggest that they cruised long distances searching for prey and were capable of making high speed dashes to capture them (Zangerl 1981; Compagno, 1990; Maia et al., 2012).


These species included durophagous predators like Caseodus, which had crushing teeth, as well as Helicoprion and Edestus, which specialize on slicing softer tissues (Compagno, 1990). Both

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of these predator types could have fed on the pelagic shelled cephalopods, such as ammonoids, that were abundant in upper Paleozoic waters (Korn et al., 2015), but used divergent strategies to capture and disable them. Other chondrichthyans were unlikely to develop symphyseal dentitions. The lack of

protrusible upper jaws and a mobile jaw suspension, as in the lineage leading to modern sharks (Schaeffer, 1967), may preclude the development of symphyseal teeth. Tooth whorls require restricted mobility (thus an immobile jaw suspension) and rigid support to function effectively to grasp and slice elusive prey. The foremost branchial arch, i.e. hyoid arch, in euchondrocephalans has not developed into a suspensory structure for the jaws. The close connection of the upper jaw to the cranium in euchondrocephalans was apparently sufficient to capture and process the prey available in Paleozoic seas.

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ACKNOWLEDGMENTS

We thank R. Troll and J. Ramsay for artistic renderings. CT scanning by M. Hill (AMNH), P. Holroyd (UCMP), A. Neander (Chicago), J. Maisano (UTexas). Collections access granted by A. Stroup, W. Simpson (FMNH), M. Carrano, D. Bohaska (USNM), T. Daeschler (ANS). Digital


reconstruction for Agassizodus assisted by Evelyn Bitikofer and Miles Bloom (IMNH). This research was supported by NSF IOS 1631165 to CW and ISU College of Science and

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Engineering to LT.

FIGURE CAPTIONS

Fig. 1. Head reconstruction of Helicoprion (A) and Edestus (B). Art by Troll and Ramsay from

Tapanila et al., 2013 (A) and Pruitt and Troll (B).

Fig. 2. Terminology for symphyseal dentition and whorls of edestoid chondrichthyans. (A) Crown and root and (B) whorl of agassizodontid; (C) whorl with shed tooth in Edestus. Adapted from Tapanila and Pruitt, 2013.

Fig. 3. Helicoprion reconstruction. (A) IMNH 37899 fossil, arrow points anteriorly; (B-C) CT

scan model with superimposed whorl (D) showing palatoquadrate (green), Meckel‟s cartilage (blue) and labial cartilage (red); Model reconstruction in dorsal (E) and lateral views showing

bite cycle (F-I). bf, basitrabecular fossa; bp, basal process; c, cup-shaped portion of labial cartilage; ep, ethmoid process; lj, labial joint with base of root; pf, lateral palatine fossa; pp, process limiting jaw closure; qf, lateral quadrate fossa; qmf, quadratomandibular fossa; qp, quadrate process. Adapted from Tapanila et al., 2013.


Fig. 4. Proposed prey capture behavior in Helicoprion sp. Prey is approached with the head raised and the lower jaw fully depressed with teeth aligned to catch the soft parts. The strike

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occurs when the soft parts of the prey is impaled on the tooth cusps and pulled into the oral cavity by elevation of the lower jaw and depression of the head. The hard parts are pinched out of the oral cavity between the jaws. Transport of the prey occurs when the lower jaw continues to elevate, pushing the prey further into the oral cavity where suction is then used to move the prey to the esophagus. CR, cranium; MC, Meckel‟s cartilage (lower jaw); OC, oral cavity; PQ, palatoquadrate (upper jaw); RO, rostrum. Blue arrows show direction of jaw movement. Purple

dashed arrows show direction of force of individual tooth cusps. Used with permission from Ramsay et al., 2015.

Fig. 5. Toxoprion fossils. (A-B) USNM 330004 part and counterpart; (C-D) USNM 330003 part

and counterpart; (E) UCMP 32015; (F) Detail of USNM 330004 showing serrated crowns; (G) Detail of (D), USNM 330003, showing nutrient canals in bisected whorl that converge posteriorly, though crown projections point anteriorly; (H) Detail of USNM 330004 showing pitted region between crowns. Arrows points anteriorly.

Fig. 6. CT scan of Toxoprion in lateral and oblique views, (A-B) USNM 330004, (C-D) USNM 330003, and idealized model based on CT scans (E-F). Arrow points anteriorly.


Fig. 7. Campyloprion, USNM PAL 443547, in lateral (A) and transverse (B) views. Arrow

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points anteriorly.

Fig. 8. Agassizodus, FMNH PF8517, front (A) and back (B) view of fossil; Modeled teeth with CT scanned tooth at posterior, in lateral view (C) and occlusal view (D); (E) Reconstruction of lateral and symphyseal dentition. Arrows points anteriorly.

Fig. 9. Edestus reconstruction, FMNH PF2204. (A) Xray and (B-C) CT scans show multielement chondrocranium (D); Model reconstruction in dorsal (E) and lateral views showing bite cycle (F-

J). c, ventral surface of otic and occipital regions of chondrocranium; dc, dorsal plate of chondrocranium; e, ethmoid plate; q, quadrate; lb, lower blade; m, Meckel‟s cartilage; n, nasal;

p, palatine; pe, paired elements; r, rostral; ub, upper blade. Arrow points anteriorly.

Fig. 10. Proposed prey capture behavior in Edestus sp. The prey is approached with the head raised and the lower jaw fully depressed and protracted forward by the quadrate with teeth aligned to catch the soft parts. The strike occurs when the head is depressed and the lower jaw retracted and elevated by the quadrate and jaw adductor muscles (not shown). This causes the

lower tooth whorl to slice through the prey that is held against the stationary upper tooth whorl. Transport of the prey occurs when the lower jaw continues to retract, while continuing to slice the prey to pull it further into the oral cavity. This cycle of protruding and retracting the lower jaw to slice through the prey while held against the stationary upper jaw could be repeated to


disable and reduce the prey (blue arrow sequence). Suction is then used to transport the prey to the esophagus. CR, cranium; MC, Meckel‟s cartilage (lower jaw); OC, oral cavity; PA, palatine

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(anterior part of upper jaw); QU, quadrate (posterior part of upper jaw with modified function for jaw suspension); RO, rostrum. Blue arrows show direction of jaw movement. Purple dashed arrows show the direction of force of individual tooth cusps.

Fig. 11. Sinohelicoprion, USNM 235393, fossil (A) and CT scan in lateral view (B) and beneath the root (C). Arrows points anteriorly.


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