Cosmoselachus mehlingi, Bronson & Pradel & Denton & Maisey, 2024

Bronson, Allison W., Pradel, Alan, Denton, John S. S. & Maisey, John G., 2024, A new operculate symmoriiform chondrichthyan from the Late Mississippian Fayetteville Shale (Arkansas, United States), Geodiversitas 46 (4), pp. 101-117 : 104-106

Cosmoselachus mehlingi n. gen., n. sp.

( Figs 1-6 )

urn:lsid:zoobank.org:act:4F449FFA-E9C8-4190-8478-7BCC0ADA88BA

H OLOTYPE. — Partial cranium, jaws, gill arches, partial pectoral girdle and fins, and lower teeth, AMNH FF 20509 .

ETYMOLOGY. — Cosmoselachus mehlingi n. gen., n. sp. is named in honor of American Museum of Natural History Senior Museum Specialist Carl Mehling, nickname “Cosm”, therefore “ Cosm ” -oselachus, in recognition of his contributions toward the acquisition and identification of numerous fossil chondrichthyans, as well as his indefatigable enthusiasm for all unusual vertebrates and many years of service to paleontology.

GEOLOGICAL RANGE. — Fayetteville Shale (Upper Mississippian, Middle Chesterian) of northwest Arkansas, United States.

TYPE LOCALITY. — River bed of Cove Creek, near the town of Leslie, Searcy County, Arkansas, United States, Middle Chesterian (Upper Mississippian), Carboniferous.

DIAGNOSIS. — Teeth small and cladodont type, with unfused bases and three lingually gently recurved cusps, set in elongate, scalloped Meckel’s cartilages that house widely spaced tooth families. Denticles present between tooth families. Hyoid and branchial arches possess elongate rays, which are fused together to form a corrugated structure covering the gill openings.

The presence of ten upper and lower families of small cladodont teeth, widely spaced and housed in large alveoli, which, combined with phylogenetic analysis places the taxon within the family Falcatidae ( Zangerl 1981; Ginter et al. 2010).

The presence of buccopharyngeal denticles that may be compound or single (insufficient resolution in CT scan to determine their type), scalloped jaw margins, and tentatively a single canal for the dorsal aorta entering the basicranium, combined with symplesiomorphic characters (such as possessing 15-18 distal fin radials and cladodont teeth, the shape of the basicranium [see Description], and elongate jaws), as well as the results of a phylogenetic analysis, place this taxon within Order Symmoriiformes ( Coates & Sequeira 2001; Maisey 2007).

DESCRIPTION

Our description is based on the holotype AMNH FF 20509, which has been part of the Mapes Fossil Collection since the 1970s. The specimen was recognized as a chondrichthyan by Royal Mapes and brought to the attention of Rainer Zangerl in a photograph dated 1979. Since this early photograph (Supplementary Material, Fig. S1 ; Appendix 1) was taken, the specimen was damaged in shipping, and later experienced significant pyrite decay; it was previously a continuous slab of matrix, but now the pectoral girdle is separated from the pharyngeal region ( Fig. 1 ). Much of the morphology is recognizable on the surface: An acuminate rostral end of elongate lower jaws, posterior to which are a collapsed set of gill arches that support elongate cartilaginous rays which extend posteriorly for most of the length of the pharynx, as well as an exceptionally preserved pair of pectoral fins with three-dimensional fin radials and impressions of the ceratotrichia and fin margins. Computed tomography reveals teeth, pharyngeal denticles, a partial basicranium, and fragments of additional branchial arches within the largely phosphatic matrix.

Teeth

AMNH FF 20509 ( Fig. 1 ) has an apparently complete lower dentition ( Figs 2 , 3 ), with cladodont teeth that have a lingually recurved central cusp and a reniform tooth base ( Fig. 3 ) ( Ginter et al. 2010). No teeth from the upper jaw are identifiable in the specimen, and all teeth are hidden within the matrix and made visible only by CT scanning. Like Ozarcus ( Pradel et al. 2014) and all other falcatid symmoriiforms (e.g., Falcatus , Damocles [ Lund 1985, 1986; Maisey 2009]), the teeth are small and housed in widely-spaced “pockets,” with tooth families an average of 4.4 mm apart (ranging from 1.6 to 6.42 mm apart, excluding areas where the fossil is broken). Tooth family spacing is most regular at the middle of Meckel’s cartilages; tooth families are slightly closer together at the anterior of the jaw, and spacing is difficult to determine more posteriorly, where teeth are clumped and disorganized, possibly because of taphonomic processes. The fossil has uniformly sized teeth within tooth families, indicating relatively rapid tooth replacement in a linguo-labial sequence. There are generally seven teeth per family, though this can be hard to see in rows that are less organized, and there are at least 25 tooth families (12 clear families on one side, and 13 on the other, plus a jumble of teeth at the back of that row), as well as a single symphysial tooth family ( Fig. 2 ).

Based on the shape of the teeth, with a large central cusp and subtle lateral cusps, they were likely used for clutching prey ( Ginter et al. 2010; Maisey et al. 2014). Their appearance is somewhat like that of Symmorium or Cladoselache Dean, 1894 , but no features are identifiable that might warrant placement in a previously described genus ( Fig. 3 ). Due to the scan’s resolution, no basal canal opening can be identified. No large orolingual buttons are obvious, though a lingual torus is present, much like the morphology of teeth in Cladoselache and related taxa ( Ginter et al. 2010).

Denticles

Denticles cover the space between tooth families ( Fig. 3 ), and pharyngeal denticles are visible both as bumps just barely under the matrix surface, and in more detail through CT imaging ( Fig. 4 ). The location of these denticles in the specimen, between the posterior ends of the Meckel’s cartilages, indicates they are buccopharyngeal denticles. The pharyngeal denticles are difficult to see in detail in the CT scan, due to low resolution, but they are shaped like grains of rice. The denticles between tooth rows are less regular in shape. Higher resolution scanning would confirm whether or not the pharyngeal denticles are stellate ( Williams 1979; Ivanov 2005), as well as determine whether they are simple or compound (as in Akmonistion [ Coates & Sequeira 2001]). No external dermal denticles have been observed in this specimen, though they may have been lost to taphonomy or preparation early in the specimen’s history.

Cartilage structure

Scan resolution was insufficient to discern many details of cartilage structure, apart from there being evidence of only single-monolayered tessellated calcified cartilage ( Maisey et al. 2021) in much of the skeleton (i.e., it seems to lack multiple-monolayer tesserae like that of forms such as Tamiobatis, Cladodus Agassiz, 1843 , etc.). The tessellated layer of the hyoid and branchial rays appears to be thinner than elsewhere, suggesting that the onset of biomineralization here was delayed relative to other parts of the skeleton (the size of individual tesserae has been shown to be related to their age in Urobatis Garman, 1913 [ Seidel et al. 2016]).

Visceral arches

The jaws are long and gracile, extending nearly 5 centimeters behind the posterior margin of the cranium. The mandibular joint is located posterior to the braincase, as is the case in symmoriiform taxa such as Falcatus and Ozarcus , as well as in Notorynchus , and xenacanths like Orthacanthus Agassiz, 1843 ( Lund 1985; Wilga 2002; Ginter & Maisey 2007; Pradel et al. 2014). This is in contrast to the condition in Tristychius Agassiz, 1837 , Chondrenchelys Traquair, 1888 , Iniopera Zangerl & Case, 1973 , and other Paleozoic crownchondrichthyans with elongate otico-occipital regions, in which this joint is located anteroventral to the braincase ( Coates et al. 2017). The Meckel’s cartilages have a moderate depression along the ventral surface, running along the concave curvature of the jaw from roughly five to ten centimeters anterior to the jaw joint, which does not appear to be an artifact of taphonomy and may have housed an attachment for the adductor musculature in life ( Fig. 2 ). A similar ventral depression is present in the jaws of Ozarcus ( Pradel et al. 2014) .

The visceral arches are partially preserved, but the branchial elements have collapsed. The flattened condition of the branchial arches makes it difficult to identify individual elements with certainty, though components of at least three pharyngeal arches are recognizable ( Fig. 2 ). Each of these three pharyngeal arches is represented by paired ventral (ceratal) elements ( Fig. 2 ). The dorsal (epal) elements of the pharyngeal arches are mostly absent, though the dorsal portion of the mandibular arch is preserved as a small remnant that is likely part of the palatoquadrate. A single basihyal may be present based on an irregular chunk of cartilage present in the correct location; however, its identity as a basihyal is uncertain because it is poorly preserved, and as such is not rendered in the reconstruction in Fig. 2 . Two pairs of ceratobranchials are preserved, presumably representing the first and second branchial arches, with a fragment of a third arch just posterior to them. The ceratohyals ( Fig. 2B ) have a slight flange or ridge on their ventromedial edge; however, the ceratobranchials do not have a similar flange and are more rounded in cross section than the flattened ceratohyals.

Gill cover

One unique feature of AMNH FF 20509 is a structure interpreted as an operculum, which extends posteriorly from the level of the hyoid and first branchial arch. The hyoid rays extend to almost the same point as the rays from the first branchial arch. Though the gill arches are collapsed so we cannot confirm that the hyoid and first branchial arch rays covered all the posterior successive gill slits in life, the rays are certainly long enough to close off all the gill openings. The cartilaginous opercular rays attached to the hyoid arch appear to have a ventral and at least partly dorsal component ( Fig. 5 ). Possibly because of weathering, or events postpreparation, the rays lift off from the rest of the specimen in cohesive layers. The flaps formed by the cartilaginous rays have a corrugated appearance, indicating the rays are more adhered to one another than they are to any other part of the specimen, unlike the hyoid rays of other chondrichthyans. A similar adherence of other small structures (denticles, cartilaginous fin radials) is not observed in the specimen, so we interpret the adherence of these hyoid rays as a true feature of the fossil rather than purely a taphonomic artifact. This close adhesion of adjacent rays is visible directly on the surface of the fossil and is consistent in tomographic sections. At least two other branchial arches possess thin and elongate rays, which do not seem to be fused together quite so closely as the rays from the hyoid arch, but they do appear to be fused as they also produce a corrugated appearance. It is difficult to determine the length of the branchial rays from the oblique sectional view provided by CT scans, but the hyoid arch rays are over 10 cm long, covering much of branchial opercular rays and therefore likely covering the branchial openings in life.

Fins

Two pectoral fins and partial pectoral girdle cartilages are preserved with the specimen (Supplementary Material, Fig. S1 ; Appendix 1); however, they are no longer articulated with the rest of the specimen and were not CT scanned alongside the cranial skeleton. The connection between the fins and cartilages is degraded due to pyrite decay, leaving little preserved of the pectoral cartilages, however the preservation of the fin radials is exceptional; they are three dimensionally preserved without any evidence of becoming crushed taphonomically. Joints appear to be present between the proximal and distal radials (Supplementary Material, Fig. S2 ; Appendix 1). Impressions of the fins are also preserved, including impressions of ceratotrichia (Supplementary Material, Fig. S1 ; Appendix 1). Length of the fin impressions indicates the fins were at least 24 cm in length. In this specimen, there are 15-18 distal fin radials (differing from right to left, due to preservation), with no interradials (smaller, sometimes distally forked elements found between fin radials). Among Paleozoic chondrichthyans, only Cladoselache is known to have interradials ( Maisey 1989).

Partial basicranium

The most posterior portion of the cranium is partially preserved, and it is not dramatically crushed by taphonomic processes ( Figs 2 ; 6 ). However, little information can be inferred about the structure and size of the orbits, or about the internal structure of the braincase. Compared with the braincase of ‘ Cobelodus ’ (FMNH PF 13242, see http:// zenodo.org/record/10110242), also from the Fayetteville Shale ( Maisey 2007), as well as in comparison with Phoebodus saidselachus (Frey, 2019), this specimen has a more elongate, more tapered occipital region, terminating in a rounded occipital surface. Unlike ‘ Cobelodus ’ or Maiseyacanthus Bronson, 2021 ( Bronson 2021), the dorsal surface of the cranium is not domed; unfortunately, this region is so degraded that no precerebral fontanelle can be discerned. There is no evidence of subcranial ridges or a spiracular groove, the lateral commissure appears chondrified, and there is no evidence of a subotic occipital fossa. A crack runs through the right ventral side of the cranium just anterolateral to the hypotic lamina; however, this opening is an artifact of taphonomy. There appears to be a single opening or canal for the dorsal aorta ( Fig. 6 ), but due to the resolution of the scan, it is challenging to reconstruct any smaller foramina.

Figures

FIG. 1. — Cosmoselachus mehlingi n. gen., n. sp., specimen AMNH FF 20509, anterior to right: A, entire specimen, ventral view. Abbreviations: mc, Meckel’s cartilage; op, opercular flap; B, detail of opercular flap. Scale bars: A, 5 cm; B, 2 cm. Photographs by Lorraine Meeker.

FIG. 2. — Cosmoselachus mehlingi n. gen.,n. sp., three-dimensional renderings of specimen AMNH FF 20509,produced by computed tomographic (CT) scanning: A, ventral view of posterior half of specimen, anterior to top; B, ventral view of anterior part of the specimen, anterior to right, with opercular elements removed; C, dorsal view of anterior half of specimen,anterior to right.Note loss of anterior portion of cranium due to taphonomy.Abbreviations:add, depression potentially for adductor musculature; c, cranium; cb, ceratobranchials; ch, ceratohyals; d, denticles; mc, Meckel’s cartilage; op, opercular cartilage; pa f, pharyngeal arch fragments; pq, palatoquadrate fragment; t, teeth. Scale bars: 5 cm.

FIG. 3. — A, Tooth families of Cosmoselachus n. gen. on the Meckel’s cartilages, with denticles between tooth families. Teeth make contact but have no fused bases, dorsal oblique labial view; B, detail of tooth batteries in axial section, anterior to right, showing denticles between each tooth family, on the scalloped Meckel’s cartilage. Abbreviations: d, denticles; mc, Meckel’s cartilage; t, teeth. Scale bars: A, B, 4 cm; C, 2 mm.

FIG. 4. — Cosmoselachus mehlingi n. gen., n. sp., specimen AMNH FF 20509,pharyngeal denticles (arrowhead) revealed by CT scanning.Anterior to top, ventral view in frontal section. Scale bar: 2 cm.

FIG. 5. — Cosmoselachus mehlingi n. gen., n. sp., specimen AMNH FF 20509: A, cross section through the posterior half of the specimen, at the point indicated by the inset 3D reconstruction, showing multilayered opercular structure made of cartilaginous branchial rays, fused to one another. Layer of rays from the first branchial arch (behind the hyoid arch) indicated by the second-from-left arrowhead. The right-most arrowhead marks the second row of pharyngeal rays (first branchial arch); another layer (second branchial arch) can be found just beneath it (second arrowhead from the right). Hyoid operculum appears to fold at the arrowhead furthest to the left;B, longitudinal section of posterior half of AMNH FF 20509 showing hyoid operculum (right-most arrowhead) and additional elongate branchial rays (left two arrowheads).Anterior to left, slice location within the specimen indicated by 3D reconstruction in the upper right corner. Scale bars: 5 cm.

FIG. 6. — Cosmoselachus mehlingi n. gen., n. sp., partial basicranium of specimen AMNH FF 20509: A, dorsal view;B, ventral view.Anterior to top. Abbreviations: da?, possible opening for the dorsal aorta; occ, occiput; ot cap, otic capsule. Scale bar: 3 cm.