Ianthodon schultzei Kissel and Reisz 2004

Ianthodon schultzei Kissel and Reisz (2004) Holotype. KUVP 133735 consists of a nearly complete skull including both mandibles and an anterior postcranial skeleton including vertebrae, ribs, right scapula and coracoid as well as left humerus (Fig. 1). The specimen is incompletely ossified, indicating a juvenile individual. On the same slab, a partial skeleton of Petrolacosaurus is preserved. Palatal, mandibular, occipital and postcranial elements are newly assigned to the holotype. The skull was removed from the slab during preparation (Fig. 2). After documenting the originally exposed side (Kissel and Reisz, 2004, Fig. 2), the skull was embedded and prepared from the other side, now exposing the labial surface of the maxilla (Fig. 3). Referred specimens. KUVP 133736, left maxilla; FO 176, right maxilla. These referrals are based not only on the shape of the maxilla but also on tooth shape and the reduced number of precaniniform teeth relative to that in Haptodus garnettensis. Revised diagnosis: Small sphenacodontian characterized by the presence of three premaxillary tooth positions and with conical marginal teeth that overlap each other at the base. It differs from Haptodus garnettensis in having fewer marginal teeth, with spaces for up to 20 teeth in the maxilla, rather than 23. There are at least 21 tooth positions in the dentary, rather than 24. Distal marginal teeth are slenderer and more distinctly recurved than in Haptodus, with narrow, rather than bulbous, tips. It differs from H. garnettensis in having four or fewer maxillary teeth anterior to the enlarged pair of teeth on this element rather than six. The teeth on the transverse flange of the pterygoid are smaller than in Haptodus. It differs from other sphenacodonts in having the pineal foramen located at the midpoint of parietal length. 2.1 Description Ianthodon schultzei is known from a single juvenile skeleton with delicate bones, differing from contemporary specimens of Haptodus garnettensis even when of similar size. The reconstructed skull length of Ianthodon is slightly less than 10 cm, similar in length to the juvenile and smallest specimens RM 14,156, RM 14,157 and ROM 29872 of Haptodus garnettensis (see Currie, 1977; Laurin, 1993). As reconstructed, the skull of Ianthodon is slenderer than H. garnettensis specimens of the same size in the region of the snout and the anterior mandible. As far as can be discerned, the postcranial proportions of Ianthodon are nearly the same as in RM 14,156, while the higher number of precaniniform maxillary teeth and the more rectangular shape of the humerus entepicondyle distinguish the holotype of H. garnettensis from that of Ianthodon. The holotype skeleton is preserved on the same block with some skeletal elements of Petrolacosaurus, from which it can easily be distinguished on anatomical grounds. The humeri, although of similar size, show clear differences such as proportions and the positions of the foramen and supinator process. 2.1.1 Skull The dorsal skull roof of KUVP 133735 has already been described in detail by Kissel and Reisz (2004), and this need not be repeated here. The skull elements are spread across the slabs that form the fossil block (Fig. 1), with several elements trending off the edges. Most of the skull roof is preserved on the original small block, mostly disarticulated. The nasals, quadratojugal and premaxillae have clearly moved away from the other skull elements. A concentration of palatal, occipital and mandibular elements is found in close proximity to each other on one of the other blocks, together with other skeletal elements. Partly because of the juvenile condition of the specimen, it is somewhat difficult to interpret the exact edges of some of the elements. We are, however, confident that these bones belonged to one individual because of the consistency in anatomy, level of ossification and lack of duplication. The premaxilla is an unusually slender element, especially in view of the large size of the first tooth. All three rami of the premaxilla are slender. The nasal process is slightly broader transversely than the maxillary process, whereas the vomerine process is the smallest and most slender of the three rami. The slenderness of the maxillary process and of the body of the bone suggests that the teeth were not deeply rooted, a condition similar to that seen in the maxilla, despite the unusually large size of the first tooth. The right premaxilla, the better preserved of the pair, bears three marginal teeth. The first tooth is broad at the base, although slightly exaggerated by compression, and much taller than the second tooth. These teeth are only slightly curved posteriorly, and this curvature is restricted to the tip. The upper crowns are densely striated on the lingual side, and moderately striated on the labial side. The maxilla of the holotype KUVP 133735 is poorly preserved, but two referred specimens provide valuable additional information about the anatomy of this element. These specimens are identified with confidence as belonging to Ianthodon because of the unique dentition that they share with the holotype. Overall, the shape of the teeth is quite unusual, not seen in other coeval amniotes. The teeth have a broad base, but very slender crowns, tapering rapidly crownward without any bulbous thickening. The slender crowns are recurved. This is in strong contrast to the teeth of Haptodus garnettensis, which are characterized by their overall robustness, as well as a slight bulbousness below the crown. These maxillae also differ from those of Haptodus in the outline of the dorsal edge and in the presence of fewer precaniniform teeth. The dorsal blade of the maxilla is low. Best seen in the medially exposed referred specimens, it increases in height gradually along the anterior one-third of the bone and above the alveolar shelf, reaching its maximum above the enlarged pair of teeth. Its dorsal edge extends posteriorly along four or five tooth positions before it starts to slope ventrally, reaching the alveolar shelf by the nineteenth tooth position. Anteriorly, the sutural contact with the premaxilla covers most of the alveolar shelf of the first maxillary tooth. Between the second and sixth tooth positions, the alveolar shelf is smooth, defining the maxillary contribution to the elongate internal naris. There is little or no dorsal expansion of the alveolar shelf in the region of the enlarged teeth. The medioventral part of the dorsal blade is slightly swollen above the alveolar shelf but less so than in Haptodus garnettensis (RM 14,157). The maxillary tooth count can be calculated for Ianthodon because the two referred maxillae are nearly complete. There are at least 18 positions preserved in KUVP 133736, and it is likely that there would have been up to two more tooth positions in the maxilla posteriorly. As seen in the referred specimens (Fig. 4), either three (FO 176) or four (KUVP 133736) anterior teeth increase in size towards the pair of enlarged teeth. As in other sphenacodonts, there are two tooth positions for the largest teeth of the maxilla, and, generally, their location is indicated by the center of growth on the maxilla. Posterior to this region, the teeth gradually decrease in size posteriorly. All teeth are broad at their bases, remaining largely uncurved through most of the crown, with tips curving posteriorly (Laurin, 1993). Striations are present in the presumed crown portions of the maxillary teeth. As seen in the spectrum of juvenile to adult specimens of Haptodus garnettensis, as well as other non-therapsid synapsids, significant ontogenetic changes of the tooth crown type are not to be expected. When compared to the known maxillae of Haptodus garnettensis where a higher tooth count is present both in adult and in juvenile stages (see Currie, 1977), the reduced dentition of Ianthodon is of taxonomic significance. This reduced number of maxillary teeth is related to differences in the precaniniform tooth count, a feature that appears to correspond to the reduced premaxillary tooth number in this taxon relative to the condition in H. garnettensis (Laurin, 1993). The tall lacrimal of Ianthodon schultzei, despite its relatively poor preservation, is sufficiently complete for determining its outline, showing it to be as tall as the maxilla or even slightly taller. This is consistent with the lacrimal height of other basal sphenacodontians (Laurin, 1993, Fig. 3) but contrasts with the still taller lacrimal of Palaeohatteria (Credner, 1888, pl. 25, Fig. 4) and the slender elements in ophiacodontids and eothyridids. The latter basal synapsids have distinct, elongate maxilla–lacrimal sutures, and their lacrimals tend to be slender anteriorly. As in edaphosaurids and basal sphenacodontians, I. schultzei has a tall lacrimal, indicating that the snout was proportionately taller than in eothyridids or ophiacodontids. Nevertheless, details of the lacrimal foramina and its precise sutural contacts are not available for description or evaluation. In the original description (Kissel and Reisz, 2004), the left prefrontal was identified as an element covering the maxillary tooth row, and the right was located between the lacrimal and frontal. The supraorbital bar of the “left” currently shows a ridge that may indicate the dorsal surface of the skull, and therefore this element more likely represents the right prefrontal. The anterior tips of the two prefrontals point toward one other and are exposed in medial view. A prefrontal pocket is now clearly observed as the depression in the innermost prefrontal in Kissel and Reisz (2004, Fig. 2). As with many other elements, the squamosal is already well-described. However, the element identified as the left squamosal by Kissel and Reisz (2004) bears an ascending rim that might mark the inflection towards the occipital plane. The quadratojugal is nearly complete and quite similar to those in edaphosaurids and

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