Ianthodon schultzei Kissel and Reisz (2004)

Spindler, F., Scott, D. & Reisz, R. R., 2015, New information on the cranial and postcranial anatomy of the early synapsid Ianthodon schultzei (Sphenacomorpha: Sphenacodontia), and its evolutionary significance, Fossil Record 18 (1), pp. 17-30 : 18-25

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 basal sphenacodonts. The narrow ventral portion that would have touched the base of the quadrate is covered by a vertebra, but the dorsal process of the quadratojugal is exposed next to the coracoid. It is sheetlike above the ventral quadrate buttress and is slightly bifurcate at its thin dorsal end. It lacks an anterior process. Overall, its preserved portion is similar to that seen in Haptodus garnettensis and other sphenacodonts.

Both vomers can be identified, partially covered by other elements and exposed in partial dorsal view. Anteriorly, its narrow tip has a short indentation for contact with the premaxilla. This element broadens posteriorly, but is covered by the mandible. Very small teeth are observed below the lateral margin, where the vomer would have formed the medial edge of the choana.

The rough dimensions of the palatine are discernible on the central block ( Fig. 5 ), near a series of neural arches. The choanal notch of the palatine is also exposed, although the outline of the anterior region is incomplete.

Both pterygoids are preserved. Measured from the level of the thickened transverse flange, the anterior process is about twice the length of the quadrate ramus. The dorsal blade emerging from the medial rim of the palatine ramus is as low as in Haptodus garnettensis ( ROM 43606) but with a steeper posterior edge. The transverse flange bears at least three teeth. Regarding the structural similarity with the pterygoid of Haptodus garnettensis , there are most likely more tooth positions, particularly since the teeth of this row are proportionately smaller in Ianthodon . The high quadrate ramus equals the proportions seen in other basal synapsids. As is typical for basal sphenacodontians, the basipterygoid articulation lies more ventrally on the body of the pterygoid than in Dimetrodon and other sphenacodontids but more dorsally than in varanopids and ophiacodontid synapsids (compare to Romer and Price, 1940; Reisz, 1986). A peg-like process marks the area where the epipterygoid articulates with a subvertical notch in the anterior edge of the quadrate ramus, as seen also in most stem sphenacodonts (not in Palaeohatteria ), Sphenacodon ( Eberth, 1985, Fig. 21) and Ianthasaurus ( ROM 59933, contra Mazierski and Reisz, 2010).

As with the palatine, no significant difference could be found in the parasphenoid–basisphenoid complex of Ianthodon schultzei and Haptodus garnettensis . The basipterygoid processes point anterolaterally and have flat articular facets. Compared to Haptodus garnettensis ( ROM 43602, 43604), the cultriform process is proportionally longer.

The supraoccipital is subrectangular in outline, with a shallow embayment for the foramen magnum in its central margin. There is a modest vertical median ridge that widens toward the embayment, where the supraoccipital articulated with the exoccipitals on either side of the foramen magnum. Other elements of the braincase may be preserved, but are difficult to identify with confidence because of poor preservation.

2.1.2 Mandible

Both mandibles are preserved, with the better-articulated left ramus measuring approximately 84 mm in total length. In lateral view, the mandible is only slightly bent in its tooth-bearing anterior half. The posterior end of the dentary, together with the dislocated surangular, forms a moderate bulge of the coronoid region, resembling that of the Haptodus garnettensis specimen ROM 30099 . The anterior part of the mandible is dorsoventrally slenderer than in all known specimens of Haptodus garnettensis , including the youngest known mandible in ROM 29872 . This juvenile is of similar size to the Ianthodon holotype but is much more robust and shows an initial dorsoventral thickening of the symphyseal area, a region that is strong in the largest specimen ROM 43604 .

The dentary is very slender in dorsoventral view, with a thickened alveolar shelf that occupies almost half the height on the lingual side. The slight concavity of the tooth margin corresponds to the moderate convexity of the maxilla. There are 21 teeth preserved, with one certain gap for an unpreserved and maybe enlarged third tooth near the tip of the left dentary. Applying the pattern of the tooth density seen in the middle region, there might be even more positions, for a possible maximum of 25. The tooth bases are broad, but often appear to overlap each other, partly due to shape and partly due to compaction. The tooth morphology is readily distinguishable from that of Haptodus garnettensis both by size of the base and the narrowness of the tip (see Laurin, 1993, Fig. 4b ) as well as by the overall delicate morphology of the crown. The level of compaction of the teeth and the overall reduced thickness of the dentine suggest that the teeth in Ianthodon reflect trophic specializations that are different from Haptodus . The teeth in the posterior region of the dentary increase in broadness as they become smaller posteriorly.

Dislocated from the articulated mandibles, there is a single splenial next to the premaxillae. Both the anterior and posterior tips are delicate and poorly preserved. The dorsoventral extension is high, suggesting that the splenial had a thin exposure in lateral aspect. Its posterior end has a slight dorsal expansion, possibly contributing to the coronoid eminence.

The angular of Ianthodon schultzei is slender and obviously not as tall or as massive as in Haptodus garnettensis , especially relative to the surangular height; it has a flat ventral lamina that does not extend as far below the prearticular level as in Haptodus garnettensis (compare to Laurin, 1993, Fig. 10). Both genera share the gently convex ventral edge of the angular, which is thin in cross section but not developed as a reflected lamina. Its ventral edge is marked by slight rugosities. On the medial side, the angular has a strong longitudinal bar for the prearticular contact.

The right surangular is exposed medially, showing the posterior articulation for the articular and the anterodorsal groove to host the dentary. A gently developed ridge runs along its dorsal edge where an aponeurosis would have attached for the adductor muscles. The posterior border of the adductor fossa is marked by a small triangular process for the anterodorsal process of the articular. There are no clear differences from the surangular of Haptodus garnettensis , apart from being slightly narrower dorsoventrally ( Laurin, 1993, Fig. 10).

The posterior coronoid is a slender, elongate element with a narrow anterior process and a bifurcate posterior region. The body of the left posterior coronoid, exposed in medial view, is covered by more than 20 small teeth. Another denticulate coronoid element is preserved, but it is largely covered by other elements, making precise identification difficult.

Both articulars are preserved, one in articulation with the rest of the left mandible, the other isolated and displaced in front of the right dentary. No detailed observations on the presence of a pterygoideus or a retroarticular process can be made. There are extremely elongate, slender prearticulars preserved in place on the left mandible and separately from the right mandible, the latter displaced posteriorly to lie under the humerus of Petrolacosaurus and a rib. Its posterior process is slightly bifurcate. Both seem untwisted, contrasting with all edaphosaurids and basal sphenacodonts. Based on our experience with the compaction properties of bones in the Garnett fossil beds, we can determine that this is not a diagenetic artifact. Unfortunately, the juvenile Haptodus garnettensis specimen ROM 29872 is not exposed for direct comparison, but the rather adult ROM 30099 has a twisted prearticular. Thus, the Ianthodon prearticular appears to preserve the plesiomorphic condition. Along with the articular, the main body of the prearticular is strongly built, and, despite its juvenile stage, the articular is well ossified and suggests that it had large quadrate condyles, although not preserved on the slab.

2.1.3 Postcranial skeleton

Several elements of the postcranial axial skeleton, mostly from the dorsal region, have been recovered ( Fig. 6 ). The average centrum length is about 10 mm. A blunt ridge seems to be present on the ventral side of each centrum. Dorsal neural arches measure approximately 24 mm from articulation with the centrum to the dorsal edge of the spine. All arches are delicate, but do not differ significantly from those of Haptodus garnettensis . Laterally, they are not, or only shallowly, excavated; this also depends on the level of compression. The zygapophyses are short. Unlike in Haptodus garnettensis , the postzygapohyses are widely spaced and single, not showing the broad double-lobed plate as in ROM 43604 or ROM 29872. All diapophyses are moderate lateral extensions and have sub-rounded cross sections. The diapophysial laminae are unreduced, connecting the central edge of the neural arch to the very tip of the diapophysis by a convex blade.

Relatively large intercentra appear to be present between the supraoccipital and one of the posterior coronoids, indicating that Ianthodon has the plesiomorphic tetrapod condition, in contrast to the tendency of reduction of these vertebral elements in advanced pelycosaur-grade sphenacodonts.

No elements posterior to the middle trunk are known, except for a single neural arch with a shortened spine that might belong to the proximal caudal series. The anterior zygapophyses are shortened compared to average dorsal vertebrae, and the spine is half as tall as in the dorsals.

All dorsal ribs are long and weakly curved, with the strongest flexion in the proximal portion. At least one cervical rib is identified with confidence, bearing a straight, stout shaft that flares distally. An anterior process is present on the cervical rib, presumably a plesiomorphic character. In dorsal ribs, the tuberculum is prominent, but not separated from the capitulum by a notch. Like in other basal synapsids, a lamina is present in the dichocephalous rib head, allowing no aperture between the diapophysial lamina and the rib.

Between the cranial elements described by Kissel and Reisz (2004, Figs. 2 and 3 ), there is one labeled with a question mark. Given the postcranial proportions, it matches the shape and size one would expect for the cleithrum. A flat and broadened dorsal tip instantly turns into a rounder cross section, which is again gradually flattened towards its presumed contact with the clavicle. Similar cleithra are present in Edaphosaurus and Dimetrodon ( Romer and Price, 1940, pl. 28 B and 38 C) as well as in Pantelosaurus . It most closely resembles the cleithrum of an undescribed sphenacodont from Garnett, as well as that of the holotype of Haptodus garnettensis RM 14,156.

The replacement bones of the pectoral girdle are delicately constructed and remain unfused in this juvenile specimen. The scapula is 44 mm tall and 19 mm wide in midlongitudinal extension. The bone is well-ossified, as is typical for even very young individuals in basal amniotes. As seen in Palaeohatteria , the scapula and anterior coracoid ossify earlier in ontogeny than the posterior coracoid, possibly to support the glenoid fossa. The ontogenetic stage of the holotype of Ianthodon schultzei is between the average condition found in Palaeohatteria longicaudata and that of Haptodus garnettensis , as the scapula and anterior coracoid are still unfused, but the ventral margin of the scapula is fully differentiated and is not as simple as the dorsal end like in Palaeohatteria juveniles. The lateral foramen (supraglenoid foramen) is located above the glenoid fossa, at about one quarter of the total scapular height, and slightly posterior to the edge of the supraglenoid ridge. There is no indication of a notch interrupting the proximal anterior margin. In I. schultzei , the dorsal end of the scapula flares anteroposteriorly, in strong contrast to the condition in Haptodus garnettensis (compare to Laurin, 1993, Fig. 19). Unfortunately, the distal portion of the juvenile holotype of H. garnettensis is not preserved ( Currie, 1977, and pers. obs. by F. Spindler). No basal sphenacodontian shows a distally flaring scapula in early ontogeny that is compensated for by further ossification to form a more rectangular outline. Therefore, we assume that the flared scapular blade is an age-independent character of I. schultzei .

The anterior coracoid is a simple oval disc with delicate radiating texture, measuring 32 × 21. 5 mm along the orthogonal axes.

A left humerus is the only known appendicular element. It measures 53 mm in length, 8 mm in mid-diaphysial width and 32 mm in width at the distal epiphysis. As the specimen is a juvenile, it resembles juvenile specimens of Palaeohatteria . Its proximal head is strongly distorted by crushing, revealing the broken edge of the tubercle for the M. latissimus dorsi and the beginning of the articulation area. The distal end is relatively well-preserved. In typical sphenacodontian fashion, the distal dorsal surface of the bone has an elongate groove on the entepicondyle for the entepicondylar foramen. The ectepidondylar ridge on the dorsal surface of the bone is modestly developed. The supinator process, as in Palaeohatteria and other basal sphenacodontians, is a blade-like structure that extends distally without flaring significantly anteriorly. In effect, its anterior edge is nearly parallel to that of the ectepicondylar ridge. In overall shape, it resembles Dimetrodon kempae , Lupeosaurus and Casea (see Romer and Price, 1940, Figs. 31 and 32). There is no evidence that the distal end of the supinator process enclosed an ectepicondylar foramen.

Figures

Figure 1. Ianthodon schultzei holotype KUVP 133735, combined slab, with outlines of skull bones (outlines by D. Scott, made prior to Kissel and Reisz, 2004, Fig. 2; now exposed as embedded counter slab) and a disarticulated skeleton of the diapsid reptile Petrolacosaurus kansensis (shaded areas); a – angular; ar – articular; c – vertebra centrum; cau – caudal neural arch; cle – cleithrum; co – anterior coracoid; cp – cultriform process of parasphenoid–basisphenoid complex; cr – cervical rib; d – dentary; dr – dorsal rib; h – humerus; n – nasal; na – presacral neural arch; pal – palatine; pc – posterior coronoid; pm – premaxilla; pra – prearticular; ps – parasphenoid–basisphenoid complex; pt – pterygoid; qj – quadratojugal; sa – surangular; sc – scapula; soc – supraoccipital; sp – splenial; v – vomer.

Figure 2. Ianthodon schultzei holotype KUVP 133735, slab in present condition, combined with photograph of skull area (lower left) before its removal.

Figure 3. Ianthodon schultzei holotype KUVP 133735. (a) Redocumented skull; (b) dislocated premaxillae. Fr – frontal; j – jugal; l – lacrimal; m – maxilla; p – parietal; po – postorbital; pof – postfrontal; pp – postparietal; prf – prefrontal; sq – squamosal; st – supratemporal; t – tabular.

Figure 4. Ianthodon schultzei. (a) Referred left maxilla KUVP 133736; (b) referred right maxilla FO 176.

Figure 5. Ianthodon schultzei holotype KUVP 133735. (a) Close-up of central block; (b) detail of right posterior coronoid with eroded denticles; (c) detail of right pterygoid transverse flange dentition in dorsolateral aspect. Ic – intercentrum; pt-a – pterygoid anterior ramus; pt-q – quadrate ramus of pterygoid.

Figure 6. Ianthodon schultzei cranial and skeletal reconstruction. Three-dimensional arrangement and projections based on a wax maquette. Skull in dorsal, ventral and lateral view; mandible in lateral and medial view.