Agassizodus St. John & Worthen, 1875

Jiachun, Li, Zuoyu, Sun, Cuny, Gilles, Siyan, Zhao, Mingtao, Yao & Lijun, Song, 2025, New ichthyofaunal microfossils from the Late Permian at Chaotian, Sichuan, Southwest China, Swiss Journal of Palaeontology (29) 144 (1), pp. 1-17 : 4-7

Genus Agassizodus St. John & Worthen, 1875 .

Type species— Lophodus variabilis Newberry & Worthen, 1870

aff. Agassizodus sp.

Figures 2A–J , 3

Material —Two lateral teeth: one is complete (GMPKU- P-3805) and the other is incomplete (GMPKU-P-3807).

Description —the complete tooth, GMPKU-P-3805, measures 1.6 mm mesio-distally, 0.9 mm labio-lingually and 1.2 mm in height. Te flattened crown lacks cusplets. Tere is a moderately sharp, quite irregular longitudinal crest that is curved lingually at one extremity. It separates the crown surface into labial and lingual parts

( Fig. 2C ). Te labial crown surface displays four pronounced buttresses. Tey are dumbbell-shaped, labially protruding, and formed by both crown and root tissues

( Fig. 2A ). Te adjacent buttresses are separated by a deep notch. Oblique and vertical ridges are irregularly distributed on the labial crown surface. Tey originate from the longitudinal crest and extend into the upper part of the labial buttresses and notches. Te ridges on the lingual crown surface are arranged in a way like those on the labial crown surface. Some of them bifurcate into two in the middle part of the crown and then reach the crown/root boundary ( Fig. 2B ). A deep groove separates the crown from the root lingually and labially, but the labial one is partially covered by the four strong buttresses. In comparison, the other specimen, GMPKU-P-3807( Fig. 2 F-J), is preserved as a fragmentary crown with two labial buttresses.

Te root equals the crown in height. Its outline is slightly convex labially, trilobed lingually, and rounded mesially and distally ( Fig. 2D ). Tere are two large foramina that open at the baso-lingual edge of the root

( Fig. 2B ). Tey form two apico-basal crevices situated between the lobes ( Fig. 2B, D ). Te labial root surface is vertically flattened, but its upper part is occupied by the four labial buttresses mentioned above ( Fig. 2A, E ). Foramina are absent on both labial and basal root surfaces.

Dentinal histology —Te inner core of dentine displays many vascular canals ( Fig. 3A–R ). It comprises three main pulp cavities, nine pulp canals, seven ascending canals and two transverse canals. Two larger pulp cavities (pca1and pca2) run into two prominent transverse canals (tc1 and tc2), and open at the lingual rim of the root ( Fig. 3G, J ). Seven ascending canals originate from both the lingual and labial parts of the base and rise to the middle level of the root where they are connected to the pulp cavities ( Fig. 3J, P ). Tese canals stretch downwards but do not open at the root surface ( Fig. 3O ). Tere are six slender and three thick pulp canals projecting upwards from the pulp cavities. Te slender pulp canals bifurcate into smaller branching tubules ( Fig. 3J, L, P ) and reach the middle level of the crown ( Fig. 3G, I, M ). Te thick pulp canals terminate near the crown-root junction and one of them opens as a foramen at the distal or mesial extremity ( Fig. 3M, N ). Te upper pulp canals are located on the lingual side of the pulp cavities, whereas the lower ascending canals are mostly located on the labial side ( Fig. 3H, K, N, Q ).

Remarks—Te relatively low crown with strong labial buttresses is reminiscent of the lateral teeth of several eugeneodontid sharks ( Ginter et al., 2010; Zangerl, 1981): e.g., Agassizodus , Arpagodus , Bobbodus , Caseodus and Eugeneodus . Te material described herein shares the following characters with Agassizodus , Arpagodus and Caseodus : a deep root, moderately sharp longitudinal crest, both labial and lingual ridges on crown surfaces and the absence of lingual buttresses. It is more closely allied to Agassizodus than to Arpagodus and Caseodus due to its labial buttresses extending over both crown and root. However, the root of GMPKU-P-3805 has a trilobed outline, two large lingual foramina and smooth labial and basal surfaces, which differs from the root of Agassizodus that is moderately pitted, roughened and sometimes ornamented with a lamellar layer on its bottom (Newberry & Worthen, 1870, pl. 4, figs. 4, 5, 11; St- John & Worthen, 1875, pl, 8, figs. 2–21; Zangerl, 1981, fig. 85; Ginter et al., 2010, fig. 119). Te unusual trilobed root makes the material described herein difficult to attribute with confidence to Agassizodus and other eugeneodontid taxa. Furthermore, a complicated controversy developed over the relationship between Agassizodus and Campodus (see Supp. Table 2). In 1875, St. John and Worthen designated Lophodus variabilis from the Coal Measures (Pennsylvanian) of La Salle, Illinois, USA, described by Newberry and Worthen (1870), as the holotype specimens of the genus Agassizodus they proposed. Tey also added a mandibular dentition from the Coal Measures (Pennsylvanian) of Osage County, Kansas, USA (St. John and Worthen, 1875 pl.8:1) into Agassizodus variabilis . Afterwards, Eastman (1902a, 1902b, 1903) associated his newly founded symphyseal dentition from the Coal Measures of Cedar Creek, Nebraska with the mandibular dentition from Osage and placed them in a new taxonomic combination ‘ Campodus variabilis ’. Te genus Agassizodus was thus considered as a synonym of

(See figure on next page.)

Fig. 3 Tomographic images of aff. Agassizodus sp. , GMPKU-P-3805. A–C, surface rendering; D–F, virtual sections in D, frontal view (line b); E, sagittal view (line a); F, axial view (line c). G-I, M–O, transparent tooth with vascularization system; J–L, P– R, reconstruction of vascular canals. pc: pulp canal;pca: pulp cavity;tc: transversal canal; ac:ascending canal;uc: unidentified canal. A, G and J in lingual views; C, I and L in apical views; M and P in labial views; B, H, K, N and Q in lateral views; O and R in basal views. All scales equal 200 μm

Campodus ( Eastman, 1902a, 1902b, 1903; Eaton, 1962; Lohest, 1884). However, Zangerl (1981), Ginter et al. (2010) and Ginter (2018) disagreed because they thought St. John and Worthen (1875) incorrectly connected the mandible from Osage with the type specimens of Lophodus variabilis Newberry & Worthen, 1870 , and it should be excluded from Agassizodus variabilis ( Ginter, 2018; Ginter et al., 2010). In that case, Agassizodus is not conspecific with Campodus . In fact, lateral teeth of Agassizodus display strong buttresses on the labial crown surface and the lingual side is almost flat, which distinguishes it from those of Campodus that bear both labial and lingual transverse ridges symmetrically separated by a longitudinal crest ( Ginter, 2018). We follow the view of Zangerl (1981), Ginter et al. (2010) and Ginter (2018) and tentatively attribute the herein material to aff. Agassizodus based on their crown similarities, pending better material and further taxonomic investigation.

Eugeneodontiformes indet.

Figure 2 K-T

Material —Two incomplete teeth, GMPKU-P-3802, 3806.

Description —Te fragmentary teeth measure 0.4– 0.5 mm labio-lingually, 0.8–1.3 mm in height and tooth (GMPKU-P-3802) is 1.5 mm long mesio-distally, whereas the other (GMPKU-P-3806) measures 3.1 mm mesio-distally. Te teeth are flat and mesio-distally elongated, lacking cusplets on the crown. An irregular crest traverses the crown mesio-distally and asymmetrically separates the relatively wider lingual face from the narrower labial face. Te bar-shaped crown is ornamented by vertical ridges that originate from the longitudinal crest and reach to the base of the crown. Ridges may sometimes bifurcate in the middle part of the crown. Te ridges are sharp, parallel to each other, and are densely distributed on the crown surface ( Fig. 2K ). Te ornamentation of the labial crown surface is less developed than that on the lingual face. A shallow groove is present at the crown/root junction lingually, whereas such a groove is absent labially. Te specimen GMPKU-P-3806 displays a fragmentary crown and root. Tere is a small cusp-like protrusion on the longitudinal crest ( Fig. 2P ). It shares a similar crown ornamentation pattern with the specimen GMPKU-P-3802, but the ridges ornamenting the crown are more oblique.

Te root of the specimen GMPKU-P-3802 is partially preserved. Two rounded foramina are present on the labial face: they differ in size and the larger one lies above the small one ( Fig. 2K ). Tere is a row of shallow indentations below the labial crown/root junction. A crevice-like foramen extends apico-basally on the lingual face of the root ( Fig. 2L ).

Remarks —Te material described herein closely resembles the posterior teeth of Caseodus varidentis recovered from the Early Triassic of Western Canada (Mutter & Neuman, 2008). Tey share a bar-shaped slender crown with a conspicuous longitudinal crest. Te material also resembles the posterior teeth of Caseodontidae gen. et sp. indet., which was associated with Caseodus varidentis in the same site (Mutter & Neuman, 2008, fig. 18). Besides, the material differs from the above-described aff. Agassizodus sp. in having a rather slender crown and the absence of a pronounced labial buttress. Tere is, however, a possibility that these differences may be caused by the heterodonty pattern observed in most eugeneodontiform sharks ( Ginter et al., 2010; Mutter & Neuman, 2008). In addition, the lateral and posterior teeth were usually considered questionable, unknown or reportedly equivocal for taxonomic identification (Mutter & Neuman, 2008). For example, the lateral teeth pavement of some eugeneodontids, such as Agassizodus , Caseodus and Bobbodus , are generally very similar in morphology (see Zangerl, 1981). It further blurs a possible link between the lateral teeth of aff. Agassizodus described above and the potential posterior teeth described herein. We therefore tentatively assign the material to Eugeneodontiformes indet.

Figures

Fig. 2 A–J, aff. Agassizodus sp., A–E, GMPKU-P-3805; F–J, GMPKU-P-3807;K–T, Eugeneodontiformes indet., K–O, GMPKU-P-3802;P–T, GMPKU-P-3806. A, F, K and P in labial views; B, G, L and Q in lingual views; C, H, M and R in apical views; D, I, N and S in basal views; E and T in lateral views; J and O in mesio/disto-apical views. All scales equal 200 μm. Black and white images (letter followed by 1) were taken with a JEOL JCM-6000 Plus SEM, whereas corresponding colour images (letter followed by 2) were taken with a KEYENCE VHX-7000 Digital Microscope

Fig. 3 (See legend on previous page.)