Chaoceratodus, Apesteguia, Agnolin, and Claeson, 2007

Genus CHAOCERATODUS Apesteguía, Agnolin, and Claeson, 2007

Type species. Chaoceratodus portezuelensis Apesteguía, Agnolin and Claeson, 2007

Holotype. MCF-PVPH-373, prearticular tooth plate with the homonymous bone fused.

Type locality. Portezuelo Formation (locality Sierra de Portezuelo).

Referred material. MCF-PVPH-425 to 440: 425, pterygopalatine tooth plate; 426, pterygopalatine tooth plate; 427, pterygopalatine tooth plate; 428, pterygopalatine tooth plate; 429, prearticular tooth plate; 430, prearticular tooth plate; 431, pterygopalatine tooth plate; 432, fragmentary tooth plate; 433, pterygopalatine tooth plate (in Apesteguía et al., 2007, numbered as 453); 434, pterygopalatine tooth plate; 435, fragmentary tooth plate; 436, prearticular tooth plate; 437, pterygopalatine tooth plate; 438, pterygopalatine tooth plate; 439, pterygopalatine tooth plate; 440, prearticular tooth plate; and others without numbering; MCF-PVPH-542, three tooth plates; MCF-PVPH-500, prearticular tooth plate; MCF-PVPH-506, fragmented tooth plate; MCF-PVPH-374, indeterminate pterygopalatine bone; MCF-PVPH-424, prearticular tooth plate; MCF-PVPH-572, pterygopalatine tooth plate; MCF-PVPH-915A, B: sections of tooth plates, 915A: horizontal cross-section; 915B: two vertical cross-sections.

Diagnosis. Pterygopalatine tooth plates display five denticulations, with the inner angle positioned slightly behind the second denticulation. Prearticular tooth plates display four denticulations, and the inner angle is slightly ahead of the second denticulation. Superficial clefts, with the first being the deepest. Medial and lingual edges generally maintain straightness, with a curvature in the inner angle. In pterygopalatine tooth plates, the lingual edge forms a slightly obtuse angle with the sagittal plane. The first denticulation of pterygopalatine tooth plates is short and has a wide base, which is posteriorly curved at the end, with a small anterior facet. The first denticulation of prearticular tooth plates is straight, with a narrower base and a marked facet toward the inner angle. The second denticulation in prearticular tooth plates may exhibit a posterior wear facet. The fourth and fifth denticulations in pterygopalatine tooth plates are involved in the formation of the step. The fourth denticulation is involved in the formation of a bilobed spur in prearticular tooth plates. The ascending process is evident at the level of the second denticulation. The interdenteonal dentine forms bands with monorefringent centers coinciding with the ridges, externally evidenced as a reticulated wear pattern with pronounced ridges.

Morphological Description

The tooth plates of Chaoceratodus portezuelensis display varying sizes. The larger tooth plates measure 2.794 cm along the medial edge and 2.404 cm along the lingual edge, while the dimensions of the smaller fragments cannot be precisely determined. The clefts are subtly deep, and the denticulations feature sharp ridges with rounded or acute crests. The inner angle measures 85.91° in pterygopalatine tooth plates and 85.56° in prearticular tooth plates.

Pterygopalatine tooth plates and associated bones. The pterygopalatine tooth plates are intricately fused to the pterygopalatine bone ( Figure 3 View FIGURE 3 A-C). The external surface of the pterygopalatine bone displays a textured anastomosed pattern, creating small, delineated areas and featuring foramina ( Figure 3A View FIGURE 3 ). Generally, these surfaces bear lichen marks, seamlessly blending with the bone texture. None of the specimens have a fully preserved pterygopalatine bone, and only the posterior portion of the symphysis is known. Dorsally, at the level of the second denticulation, the pterygopalatine process is evident, characterized by a large and circular base ( Figure 3 View FIGURE 3 A-C).

The tooth plates exhibit five denticulations, and the punctuations are randomly distributed ( Figure 3D View FIGURE 3 ). While the first denticulation is the longest, it does not differ significantly from the others. Its base is wide, and its tip abruptly directs posteriorly towards the end ( Figure 3D View FIGURE 3 ). The remaining denticulations are of similar sizes, with the fifth being the shortest ( Figure 3D View FIGURE 3 ). The ridges and crests tend to curve posteriorly, and the clefts have a comparable depth, although the first is slightly deeper ( Figure 3 View FIGURE 3 E-H). The inner angle of the pterygopalatine tooth plates is rounded and positioned slightly posterior to the second denticulation ( Figure 3D, E View FIGURE 3 ).

The medial edge surpasses the lingual edge in length, both displaying a subtle curvature but tending more towards straightness. Conversely, the area surrounding the inner angle is highly curved ( Figure 3D View FIGURE 3 , I-J). Toward their free portions, both the medial and lingual edges exhibit a projection that corresponds to the curvature of the first and the base of the fifth denticulations ( Figure 3 View FIGURE 3 HI). Enamel is arranged along the mediolingual and labial edges, forming distinct bands.

Wear resulting from the occlusion of the jaws is evident on the plateau area, the posterior facets of the denticulations, and the last denticulations ( Figure 3G View FIGURE 3 , K-M). The plateau area may either be flat or concave ( Figure 3 View FIGURE 3 K-L). The posterior facets of the denticulations experience wear and enamel loss ( Figure 3M View FIGURE 3 ). The furrows deepen, and the last denticulation, involving the fourth, forms a step ( Figure 3G View FIGURE 3 ). On the occlusal surface, impacted by the action of the jaw, a wear pattern, characterized by clustered pits and pronounced acute ridges, emerges ( Figure 3E View FIGURE 3 , I-J). This pattern is masked in the larger tooth plate ( Figure 3D View FIGURE 3 ). Only one specimen displays cusps on the ridges of the occlusal surface ( Figure 3I View FIGURE 3 ), while others have cusps on the crests.

Prearticular tooth plates and associated bones. The prearticular bone is not fully preserved in any of the specimens and exhibits a texture the same as that of the pterygopalatine bone ( Figure 4 View FIGURE 4 A-C). On the ventral and labial side, the Ruge’s canal is divided by a ridge positioned at the level of the first cleft ( Figure 4D View FIGURE 4 ). Unfortunately, the symphysis remains unpreserved in all specimens. The prearticular tooth plate is fused dorsally to the prearticular bone. It has four denticulations, and its surface is adorned with punctuations ( Figure 4A, E View FIGURE 4 ). Both the denticulations and the clefts share similar lengths and depths, with the first denticulation being slightly longer and straight ( Figure 4A View FIGURE 4 , FG). All the ridges tend to curve anteriorly ( Figure 4A, F View FIGURE 4 ). The inner angle is positioned slightly anterior to the second denticulation ( Figure 4A View FIGURE 4 , G-H).

Both the medial and lingual edges tend to be straight; however, they curve at the level of the inner angle. Abrupt changes in direction occur towards the free portions of the medial and lingual edges, one corresponding to the curvature of the first denticulation and the other to the base of the fourth denticulation ( Figure 4A View FIGURE 4 , F-H).

Similar to pterygopalatine tooth plates, wear occurs in the same regions due to jaw action. The plateau area becomes convex (occluding in the concavity of pterygopalatine tooth plates, Figure 4I View FIGURE 4 ), the furrows deepen, and the fourth denticulation forms a bilobed spur ( Figure 4A, F, J View FIGURE 4 ). On the anterior side of the denticulations, a wear facet is generated due to jaw action ( Figure 4 View FIGURE 4 F-G). In certain cases, a posterior wear facet is present ( Figure 4 View FIGURE 4 K-M). The wear pattern is the same as in pterygopalatine tooth plates. Some prearticular tooth plates have cusps on the ridges and crests ( Figure 4 View FIGURE 4 L-M).

Histological Description

Horizontal cross-sections: section made in the middle part of the tooth plate ( Figure 5A View FIGURE 5 ). A discernible disorganization characterizes the pattern of interdenteonal dentine ( Figure 5 View FIGURE 5 B-D). The birefringent interdenteonal dentine surrounds the denteons, deviating from the formation of a crosspattern. A row-like structure of monorefringent interdenteonal dentine is found within this disarray ( Figure 5 View FIGURE 5 D-F). Originating from the center of each denticulation ( Figure 5 View FIGURE 5 E-F), these rows of monorefringent interdenteonal dentine dendritically extend towards the plateau area, positioning themselves between the denteons and the surrounding birefringent interdenteonal dentine ( Figure 5 View FIGURE 5 B-C).

In the center of the denticulation, the monorefringent row of interdenteonal dentine is thicker, gradually thinning towards the sides and in the plateau area ( Figure 5E View FIGURE 5 ). The denteons in the plateau area assume a non-rounded shape, exhibiting a kidney-shaped form. Typically, they are surrounded by dentinal tubules with a granular texture ( Figure 5 View FIGURE 5 G-I) and are smaller in diameter in the interdenteonal dentine rows. Surrounding the denteons is the circumdenteonal dentine, appearing birefringent under the lambda filter at certain points ( Figure 5G View FIGURE 5 ). In other regions, the circumdenteonal dentine lacks birefringence under the lambda filter and assumes a silver hue with a polarized light ( Figure 5 View FIGURE 5 H-I). Despite this variation, the concentric arrangement of fibers remains evident.

Transitional denteons emerge towards the region close to the inner angle and in the clefts, forming dense clusters ( Figure 5 View FIGURE 5 J-K). Externally, a row of birefringent enamel is observed, and there is no evidence of mantle dentine ( Figure 5L View FIGURE 5 ). Towards the denticulations, the transitional denteons attached to the edge extend to the central area of the denticulation ( Figure 5E View FIGURE 5 ). In this area, it is not possible to distinguish between mantle dentine and interdenteonal dentine.

Vertical cross-sections. The denteons emerge in an occluso-pulp direction from a specific point. In one of the cross-sections, they exhibit some branching ( Figure 5 View FIGURE 5 A-B). On the other, they are discontinuous and linear ( Figure 6C View FIGURE 6 ). Between the denteons, birefringent interdenteonal dentine is arranged in two bands with diffuse boundaries ( Figure 6D View FIGURE 6 ). A wider monorefringent interdenteonal dentine band is positioned between these bands. Towards the sides of the denteons, the circumdenteonal dentine is located ( Figure 6 View FIGURE 6 D-F). The latter appears transparent in the vertical cross-sections and is crossed by numerous dentinal tubules. At their base, the dentinal tubules contain granules. Very little bone tissue is observed in all sections, with only one primary osteon observed. A total of approximately 9/10 incremental lines were counted ( Figure 6A View FIGURE 6 ).