Ancistronychus paradoxus, Gonçalves & Sidor, 2022

Gonçalves, Gabriel S. & Sidor, Christian A., 2022, A new drepanosauromorph, Ancistronychus paradoxus n. gen. et sp., from the Chinle Formation of Petrified Forest National Park, Arizona, USA, PaleoBios 36, pp. 1-10 : 2-5

ANCISTRONYCHUS PARADOXUS N. GEN. ET SP.

FIGS. 1, 2C, 3

Diagnosis —Drepanosauromorph with autapomor- phic ungual phalanx of the second digit, characterized by a broad distal tip with lack of transverse tapering and a cleft, presence of midline furrow on distal exten- sor surface, a ridge along the flexor surface, and a distal tip recurving strongly ventrally toward the proximal end. Distinct from Vallesaurus , Dolabrosaurus , Megalancosaurus based on the relative size of the claw and the orientation of the flexor tuberosities. Distinct from Drepanosaurus unguicaudatus and the Hayden Quarry Drepanosaurus ( Pritchard et al. 2016) owing to its rela- tively short dorsoventral height proximally, asymmetrical flexor tubercle, and more medially located flexor pits. Similarities with both D. unguicaudatus and the Hayden Quarry Drepanosaurus ( Pritchard et al. 2016) are based on the large size of the ungual relative to the actual or inferred size of the penultimate element (i.e., the unguals have a relatively small proximal cotyle for articulation with the penultimate element, suggesting that the latter was of normal size).

Holotype — PEFO 42805/ UWBM 117331, an isolated manual ungual phalanx.

Paratypes — PEFO 39324/ UWBM 108325, PEFO 39325/ UWBM 108326, PEFO 39326/ UWBM 108327, PEFO 39379/ UWBM 108385, PEFO 39380/ UWBM 108386, PEFO 42806/ UWBM 117332, all isolated manual unguals.

Occurrence —All of the specimens come from the Kaye Quarry at Petrified Forest National Park, Arizona (locality PFV 410, locality UWBM C2226), which is pre- dominately composed of mottled purple mudstone with reduction halos commonly around fossils and occasional extra-basinal clasts. Stratigraphically, the quarry lies near the base of the Jim Camp Wash Beds (Sonsela Member, Chinle Formation) but its position relative to the "per- sistent red silcrete" approximating the Adamanian- Revueltian boundary has been difficult to establish. The vertebrate assemblage suggests that the site may be in the Adamanian land vertebrate holochronozone ( Martz and Parker 2010, 2017, Sidor et al. 2018).

Etymology —The genus name combines the Greek words for fishhook and claw and the species name is in reference to the unexpected nature of this taxon and other drepanosaurs, in general.

Description —We interpret PEFO 42805/UWBM 117331 as a left manual ungual phalanx from the second digit of a close relative of D. unguicaudatus . The specimen is morphologically distinct from other reptilian claws, with a more fishhook-like appearance that has laterally oriented flexor tuberosities ( Fig. 1 B, F), a cotyle located posteroventrally ( Fig. 1 A, D), two facets within the cotyle for a tight connection with the more proximal element ( Fig. 1 B, F), as well as the combination of both flexor pits and a flexor tubercle ( Fig. 1 B, F). In medial and lateral view ( Fig. 1A, D), the specimen has a flat dorsal (i.e., ex- tensor) surface proximally that then arcs nearly 180° to its tip. A straight line from the dorsal edge of the cotyle to the distal edge of the apex of the phalanx is 2.23 cm long. The same measurement in D. unguicaudautus (MCSNB 5728) is 2.88 cm and the Hayden Quarry Drepanosaurus (GR 712) is 2.07 cm ( Fig. 2). The ungual curves ventrally near the proximal margin of the lateral tuberosities 1.0 cm from the proximal end dorsally and 1.2 cm from the proximal end ventrally. This contrasts with the condition in the Hayden Quarry Drepanosaurus (GR 712) and D. unguicaudautus (MCSNB 5728), in which the curvature occurs throughout the entire length of the element or occurs much further distally, respectively.

The ventral (i.e., flexor) surface ( Fig. 1F) has a deep articular cotyle proximally and bilateral pairs of pits and tuberosities near the midline. In flexor view, the outline of the cotyle is hourglass shaped, with the lateral (i.e., postaxial) facet slightly larger ( Fig. 1F). The articular surface is concave and deep, with sharp-rimmed edges suggesting a tight articulation with the more proximal element. Near the middle of the articular surface is a low ridge that runs proximodistally and divides the cotyle into two facets. When compared to the ungual phalanges of most reptiles [e.g., Iguana Laurenti (1768) , Alligator Cuvier (1807) ], the articular surface in Ancistronychus paradoxus is displaced ventrally on the proximal end of the bone and faces more proximoventrally than proxi- mally ( Renesto 1994). These characteristics are shared with the enlarged manual unguals in D. unguicaudatus and the Hayden Quarry Drepanosaurus . As is found in the other manual unguals of Megalancosaurus , Vallesaurus , and Dolabrosaurus , and except for the second ungual in D. unguicaudatus , all the manual ungual phalanges have the articular surface located proximally rather than proximoventrally ( Spielmann et al. 2005, Renesto et al. 2010). Distal to the cotyle, the ventral surface of the specimen bears a pronounced ridge along its midline that connects to the proximal edge of the flexor tubercle.

In ventral view, the center of the ungual features a complicated array of pits, grooves, and tuberosities ( Fig. 1F) that have not been previously reported in a drepanosauromorph. The midline ridge is paralleled by shallow troughs on either side, which each end distally at a small pit. It is unclear if these pits represent foram- ina, but as currently prepared the preaxial pit is slightly deeper. Three rounded tuberosities surround the flexor pits: one distally and one to either side. The preaxial and postaxial tuberosities bulge laterally as opposed to the relatively straight sides of the ungual ( Fig. 1). A prominent postaxial tuberosity that is larger than the preaxial tuberosity is also present in GR 712 and PEFO 42805/UWBM 117331, which identifies each specimen as a left second manual ungual ( Pritchard 2015). Slightly distal to the flexor pits and centered on the ungual is the flexor tubercle. This structure is suboval in outline and oriented transversely. It is smooth and rounded, likely as the result of the insertion of muscles and tendons. In the Hayden Quarry Drepanosaurus , the flexor tubercle of the second manual ungual (GR 712) is hemispherical. This difference in morphology could be owing to taphonomic processes that distorted the original morphology of the tubercle in the holotype of A. paradoxus , but we believe it more likely indicates a somewhat different function of the ungual in each taxon because in some of the paratypes the flexor tubercle is present ( Fig. 3A, B, E, F) and is similar in morphology to the holotype (see Discussion).

Distal to the flexor tubercle, the flexor surface of the ungual curves strongly ventrally, such that the tip of the ungual points nearly proximally. Uniquely among drepanosaurs, the distal half of the holotype retains a wide transverse breadth in both flexor and extensor views. This contrasts with what is seen in GR 712, which is more similar to a typical reptilian ungual in which the bone tapers distally in both mediolateral and dor- soventral planes. Also, uniquely among drepanosaurs, a furrow is present on the extensor surface of the holotype that gradually becomes deeper distally and a narrow ridge along the midline becomes more prominent ( Fig. 1C). Eventually, the ventral furrow divides the tip of the ungual to form a V-shaped cleft, with the ridge present within the cleft terminating 3 mm before the distal end of specimen ( Fig. 1F). Within the cleft, both sides of the ridge are marked by deep oval fossae. The distal extrem- ity of the holotypic ungual of A. paradoxus is very broad and flat in proximal and distal view (resembling a shovel), which contrasts with most reptilian claws (including drepanosaurs) in which the ungual tapers distally to a sharply pointed apex.