Caseidae Williston, 1912

Ronchi, Ausonio, Sacchi, Eva, Romano, Marco & Nicosia, Umberto, 2011, A huge caseid pelycosaur from north-western Sardinia and its bearing on European Permian stratigraphy and palaeobiogeography, Acta Palaeontologica Polonica 56 (4), pp. 723-738 : 731-734

Caseidae Williston, 1912

? Cotylorhynchus sp. Stovall, 1937 Taxonomic background of Caseidae .—The Caseidae , mono−

phyletic according to Maddin et al. (2008), includes a compact, small group of genera of pelycosaur−grade synapsids. Members of this group were first described by Williston (1910, 1913) and Stovall (1937), and subsequently studied by Olson (1954, 1955, 1962, 1968), Olson and Beerbower (1953), Olson and Barghusen (1962), Stovall et al. (1966) and Reisz (1986, 2005). As many as nine genera (and 16 species) have been placed in the family, namely: Casea (the family name−bearing genus), Cotylorhynchus , Angelosaurus , Ennatosaurus , Oromycter , Caseoides , Phreatophasma , Trichasaurus , and Caseopsis .

Caseoides was later considered as a caseid of uncertain position, because “nothing is known of the skull and the few postcranial elements are poorly preserved” ( Reisz 1986: 62).

Phreatophasma was considered a therapsid “incertae sedis” in Romer (1956) and ascribed to Caseidae by Olson (1962, 1968); based on a single femur, it was assigned only tentatively to the family by Reisz (1986) as well as by Carroll (1988). Subsequently it was excluded from caseids and moved to an indeterminate family by Ivakhnenko et al. (1997). Maddin et al. (2008) retained the classification of Ivakhnenko et al. (1997).

Trichasaurus has been considered “probably a caseid” by Romer and Price (1940) and a caseid by Romer (1956). Subsequently it was transferred to Labidosauridae by Stovall et al. (1966) and finally was considered among the “pelycosaurs incertae sedis” by Reisz (1986), Carroll (1988), and Maddin et al. (2008).

Maddin et al. (2008) considered Caseopsis a doubtful taxon, and for this reason excluded it from their phylogenetic analysis, even though it was not formally excluded from the family.

At present the family groups six genera (and one uncertain taxon) including 13 formalized species; moreover three as−yet undescribed taxa are known (see below).

Material.—At present the material includes more than 180 complete bones or fragments, ranging in size from more than 40 cm to a few mm−long minute fragments. Some bones, still articulated, were moulded in place before removing to pre−

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serve their original relative positions, and some fragmented and/or incomplete bones have been restored. Every complete or fragmentary bone has been labelled with the acronym MPUR NS followed by 151 (specimen number) and by a slash and a serial number starting at 1 (for simplicity below will be reported only MPUR 151/ and the serial number).

Among the recovered material, a number of elements have been identified as follows: 15 well preserved caudal vertebrae, and numerous large fragments clearly referable to at least eight other vertebrae; seven proximal portions of haemal arches; three proximal segments of dorsal ribs and ten undetermined fragmentary ribs; distal portion of a left ulna; right scapula and badly crushed right coracoid plate; 12 pedal elements. Comparisons were made with material housed at the FMNH.

Description.—The available, taxonomically meaningful elements are few: the skull of the Sardinian specimen was not found, so the most diagnostic elements are lacking. This made difficult the attribution of the specimen mostly taking into consideration the absolutely prevailing “craniocentric” taxonomy, widespread in the concerned literature. So we were obliged to base our study on the few available elements that, nevertheless, early steered our attention to particular members of the Caseidae .

The vertebrae are amphicoelous, with the centrum as long as wide ( Fig. 9 ). Their maximum length ranges from 25 to 50 mm. The centra are half as high as the complete vertebrae and show the typical pelycosaurian sub−cylindrical shape (“spool−shaped” according to Romer and Price 1940). Preservation does not allow us to observe if the notochord was really interrupted (discontinuous). In the ventral part the bevellings for the insertion of the proximal portion of the haemal arches (intercentra) are apparent. Bevellings are small and more apparent on the posterior margin of the centra. On some centra, a narrow groove is present ventrally. The neural arch width never exceeds the centrum width and thus the vertebrae can be considered “narrow” (sensu Sumida and Modesto 2001). Zygapophyseals facets are gently sloping (ca. 30 °). The neural spines are posteriorly inclined and gradually decrease in height posteriorly. The neural canals show a circular section, with a diameter of around 7–9 mm ( Fig. 9B, C ). The shape and proportions of the vertebrae of the Sardinian specimen are typical of caseid pelycosaurs. Vertebra MPUR 151/28 is very similar to the caudal FMNH UR 894, labeled as Cotylorhynchus hancocki , particularly in having a well preserved longitudinal groove on the ventral surface of the centrum. The Sardinian vertebrae are comparable in size and morphology to those described by Olson (1962) for Cotylorhynchus hancocki , and can be tentatively referred to the postsacral vertebrae starting at the 12 th.

The ribs are massive and elliptical to concavo−convex in cross−section. Some of them show a shallow, longitudinal groove. Among the recovered material, three well−preserved proximal portions (MPUR 151/13, MPUR 151/62, MPUR 151/69), show a well−developed capitulum and a tuberculum reduced to an oval area, facing upward and inward typical of the “rounded bodied, barrel shaped edaphosaurs” ( Romer 1956: 292) ( Fig. 10 ). Specimen MPUR 151/13 is closely comparable to ribs of Cotylorhynchus romeri and is particularly similar to one illustrated as rib 13–14 by Stovall et al. (1966) in their fig. 5; specimen MPUR 151/62 corresponds to their rib illustrated as no. 17 while MPUR 151/69 is closely similar to rib 19 of the same published illustration. Specimen MPUR 151/62 is comparable in known details to FMNH UR 266, ascribed to C. hancocki . These ribs also permit us to estimate the dimensions of the corresponding presacral vertebrae at around 10 cm in length, well corresponding to or slightly longer than pre−sacral vertebrae 9–12 of C. hancocki (Olson 1962: 42, table 19), or dimensionally comparable to the vertebra FMNH UR 566 labeled as C. hancocki .

Two scapulocoracoid portions were found close each other lying flat in the same bedding plane. One of the two portions is a right scapular blade, a sheet of bone thin and high. The bone is characterized by the typical narrowing just above the supraglenoid buttress while the dorsal termination is antero−posteriorly expanded, with an unfinished edge. It probably continued with a cartilaginous suprascapula, as generally found in early amniotes (early reptile sensu Romer 1956). The expanded upper portion of the scapular blade has a gently hollow surface on the lateral side and the bone axis shows, dorsally, the typical gently inward bending. Below the maximum narrowing of the scapular blade, the bone shows a thickened posterior edge that probably represents the dorsal portion of the supraglenoid buttress. There is no supraglenoid foramen, as in all known caseid scapulocoracoid in which this portion is preserved ( Olson 1968). The second portion of the scapulocoracoid is a badly crushed partial coracoid plate. In the upper part of the bone there is a long and low depression which could be the glenoid fossa. The preserved portion of the coracoid plate is somewhat deformed, showing a concave surface instead of the typical convex structure. The ventral portion of coracoid plate, below the glenoid cavity, is badly crushed and hard to prepare so at the moment is not possible to determine the presence of a supracoracoid foramen. This foramen, carrying nerve and vessels in living forms ( Romer 1956), is present in all known scapulocoracoids of caseids ( Olson 1968) and probably is a familial plesiomorphy. Because of poor preservation the suture between the scapula and the coracoid plate can not be observed. Moreover it is impossible to check for the presence of one or two coracoids. Nevertheless, well−preserved caseid specimens lack sutures between the elements of the scapulocoracoid. The presence of two coracoids is only hypothetical, as already recognized by Olson (1968: 258) who stated “the existence of the two coracoids, thus, is merely conjectural, based on the condition of other pelycosaurs”.

Concerning the partial left ulna, only its distal head and the distal portion of the axis are preserved. The head, with a sub−crescentic distal outline, is somewhat flattened and wide in a typical pelycosaurian fashion ( Reisz 1986). The dorsal surface is better preserved, compared to the ventral one, and shows rugosity and strong tubercles that probably indicate the origins of ligamentous connections to the forefoot elements ( Olson 1968). The articular surface for the pisisform, ulnare and intermedium is wide and deep, with only the intermedium’s portion of the surface visible in dorsal view. The preserved bone shows the greatest thickness at the level of the articular surface and thins proximally. The lateral margin of the preserved axis is nearly straight while the medial margin is gently concave in outline to face inwardly the left radius.

The preserved foot bones are: one mesopodial fragment, three metapodials, five non−ungual and three ungual phalanges. Taken as a whole, the foot was very large and formed by bulky elements. The mesopodial is flat sub−discoid in shape and shows a particularly cancellous internal structure. Metapodials, distally and proximally expanded, show large and sloping articular surfaces. One phalanx (the one found articulated with the metapodial) is wider proximally than distally, where it is characterized by a crescentic termination. The other non−ungual phalanges are as long as wide and bear, proximally and distally, strongly developed accessory scars for the tendon insertion ( Fig. 11 ). All the non−ungual phalanges are characterized by sloping articular surfaces. The best preserved ungual (MPUR 151/41) is triangular in outline and claw−shaped ( Fig. 12 ), gently curved downward, with a strong flexor tubercle and a longitudinal groove for blood vessel on each side. The massive construction, the sloping articular surfaces and the strong tubercle on the ungual phalanges are typical of caseids ( Reisz 2005). The protruding and massive flexor tubercle on the ventral surface of the ungual phalanx permits good morphological comparison with Cotylorhynchus ( Maddin and Reisz 2007) , Casea and, to a lesser extent, with Oromycter . In particular, ungual MPUR 151/41 is strikingly similar in shape to FMNH PR 272 (catalogued as Cotylorhynchus romeri ), even though their dimensions are quite different, and the non−ungual MPUR 151/2 is comparable to IV 1 and III 1 of FMNH UR 836 (catalogued as Cotylorhynchus bransoni ), although the latter are a little more slender.

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Metapodial MPUR 151/1 resembles Mt IV of FMNH UR 988, UR 836 and UR 905 catalogued as Cotylorhynchus bransoni , and also resembles that of FMNH PR 272 (catalogued as C. romeri ) even if more medially compressed, while is completely unlike that of the other bones of the carpus and tarsus. If MPUR 151/1 actually is a Mt IV, the observed differences suggest unexpectedly high variability of the genus or a still unknown new taxon.

In conclusion, preserved characteristics of specimen MPUR 151, in particular the unique morphology of the pedal bones, are completely sufficient for its identification as a caseid. Caseids phalangeal elements are highly apomorphic, differing radically from those observed in other pelycosaurs ( Romer and Price 1940). The massive and short phalanges in MPUR 151, their proximal and distal expansion, the presence of distal accessories scars for tendon insertion and the strongly tilted articular surfaces, not perpendicular to the bone axis, represent certain exclusive synapomorphies of Caseidae (see Reisz 2005: 909). The very large and bulky skeletal elements of the Sardinian specimen preclude reference to the small and lightly built caseids Casea , Caseoides , Oromycter , and Ennatosaurus , which are less than half as large in linear dimensions. Among Caseidae , only Cotylorhynchus and Angelosaurus are morphologically and dimensionally comparable to the specimen under study. However, the short, wide and smooth unguals of Angelosaurus , which are hoof−like rather than claw−shaped, suggest that Angelosaurus can be excluded as well.

Based on foregoing consideration, the Sardinian specimen, a gigantic pelycosaur with a total extimated length of nearly 6 m, could be ascribed either to one of the known species of Cotylorhynchus or to a closely related new taxon.

Figures

Fig. 9. Caseid pelycosaur cf. Cotylorhynchus sp. from Cala del vino Formation, Permian: Late Kungurian–Roadian; NW Sardinia, Italy. Caudal vertebra (MPUR 151/27), in lateral (A), anterior (B), and posterior (C) views.

Fig. 10. Caseid pelycosaur cf. Cotylorhynchus sp. from Cala del vino Formation, Permian: Late Kungurian–Roadian; NW Sardinia, Italy. Proximal portion of a dorsal rib (MPUR 151/62).

Fig. 11. Caseid pelycosaur cf. Cotylorhynchus sp. from Cala del vino Formation, Permian: Late Kungurian–Roadian; NW Sardinia, Italy. Nonungual phalanx (MPUR 151/4), in dorsal (A) and left lateral (B) views.

Fig. 12. Caseid pelycosaur cf. Cotylorhynchus sp. from Cala del vino Formation, Permian: Late Kungurian–Roadian; NW Sardinia, Italy. Clawshaped phalanx (MPUR 151/41), in dorsal (A) and right lateral (B) views.