HYBODONTIFORMES Maisey, 1975
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publication ID |
https://doi.org/ 10.26879/730 |
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persistent identifier |
https://treatment.plazi.org/id/039C8791-FF8C-426E-FCA2-FA06FE05FCD2 |
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treatment provided by |
Felipe |
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scientific name |
HYBODONTIFORMES Maisey, 1975 |
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Order HYBODONTIFORMES Maisey, 1975 Hybodontiformes Gen. et sp. indet.
Figure 2.1 View FIGURE 2 -5
Material. One tooth of indeterminate jaw position, AMPG 550.
Description. The crushing-type tooth is characterized by a well-preserved crown and a somewhat damaged root. The isolated nature of the tooth allows only a tentative attribution of one of the two broad lateral surfaces to labial, based on the combination of a well-defined root sulcus accommodating a single row of foramina, as well as the lingual inclination of the underlying root surface.
The crown bears a single, low and rounded main cusp. In occlusal view ( Figure 2.1 View FIGURE 2 ) the crown is triangular with truncated mesial and distal edges. It overhangs the root almost completely and forms a distinct presumably lingual projection. The crown bears a distinct labiolingual crest and a less distinct mesiodistal crest, the two meeting at an almost right angle, on the apex of the main cusp. The mesiodistal crest fades before reaching the edges of the crown. Most delicate secondary, crests radiate from the junction point of the two primary crests, while few originate from the labiolingual and mesiodistal crests, near their junction. Few secondary crests run continuously until the base of the crown where they might bifurcate. Others fade midway to reappear near the base of the crown. When viewed lingually or labially, the crown is boomerang-shaped. The lingual surface is convex ( Figure 2.3 View FIGURE 2 ), bearing a well-developed, median, laterobasally directed protuberance. The presumed concave labial surface bears fewer secondary crests and is characterized by a socket-like elliptical hollow ( Figure 2.5 View FIGURE 2 ), presumably for accommodating the lingual protuberance of the neighboring tooth of the same file, indicating some weak imbrication of the dentition.
The anaulacorhize root is apicobasally, mesiodistally, and labiolingually shorter than the crown and conforms to its contour. In lingual view ( Figure 2.3 View FIGURE 2 ), the root is damaged, but is populated by randomly arranged, apicobasally elongate foramina. In labial view ( Figure 2.5 View FIGURE 2 ), the root is markedly shallow, less than one fourth of the crown height. The labial face of root bears a weak sulcus along the crown-root margin, populated by a single row of well-arranged, apicobasally elongate foramina, smaller and more numerous (~20) than those of the other lateral face. The basal half of the root is slanted lingually, bearing larger, sparsely arranged, enlarged foramina. The base of the root, although damaged, appears flat and sub-rectangular, without a distinct lingual protuberance. In profile view ( Figure 2.4 View FIGURE 2 ), the crown clearly overhangs the root.
Remarks. Several Paleozoic and Mesozoic chondrichthyans have convergently evolved low crowned, crushing-type teeth. However, the presence of a single cusp, the ridged crown ornamentation and the anaulacorhize root anatomy, which includes a distinct sulcus with specialized foramina along the crown–root boundary, compare favorably to features seen in durophagous euselachians (e.g., Ginter et al., 2010; Cappetta, 2012). Isolated teeth of Paleozoic and early Mesozoic stem euselachians, hybodontiforms and stem neoselachians are often difficult to distinguish and attribute to less inclusive groups, due to their generalized and/or often homoplasic morphologies ( Ginter et al., 2010; Cappetta, 2012). Despite this fact, a review of dental anatomy of Paleozoic–early Mesozoic forms can provide some information about the systematic affinities of the Hydriot tooth.
Macroscopic teeth of Paleozoic–early Mesozoic stem neoselachians exhibit crowns with well-defined median cusps and, when present, accessory cusplets and/or a median cutting edge ( Ginter et al., 2010; Koot et al., 2013). Their roots are either hemiaulacorhize or pseudo-polyaulacorhize; they typically bear fewer, enlarged foramina (the median ones in particular) than other euselachians; and are somewhat arcuate in basal view, due to the presence of a lingual protuberance ( Ginter et al., 2010). None of the above is seen in the examined specimen, rendering a neoselachian attribution unfavorable. Low-crowned crushing teeth without lateral cusplets, but with anaulacorhize, multiforaminate roots, which often include a labial sulcus accommodating a single row of specialized foramina, are seen in members of the Hybodontiformes (e.g., Acrodus ; Lissodus ; Omanoselache ; Onychoselache ) as well as in the stem euselachian (sensu Maisey, 2011) Acronemus ( Johnson, 1981; Rieppel, 1982; Ginter et al., 2010; Cappetta, 2012; Koot et al., 2013, 2015).
Hybodontiformes can exhibit very disparate dental features, and are formally united as a group by means of cranial and postcranial anatomy ( Maisey, 1975, 1982; Ginter et al., 2010; Cappetta, 2012). Within the Paleozoic–Triassic Hybodontiformes , dental anatomy somewhat comparable to that of our specimen occurs in Acrodus , Lissodus , Hamiltonichthys , and Onychoselache (e.g., Ginter et al., 2010; Cappetta, 2012). Despite the uncertain affinities of Acronemus within euselachians, its tooth morphology is hybodontiform-like ( Rieppel, 1982; Maisey, 2011; Cappetta, 2012), and resemblant to that of AMPG 550. One of the most conspicuous differences among the abovementioned genera is the occurrence of a lingual crown protuberance in Hamiltonichthys ( Maisey, 1989) , Onychoselache ( Coates and Gess, 2007) and Acronemus ( Rieppel, 1982) , rather than a labial one as in most other Hybodontiformes .
The single, blunt main cusp and the ridged ornamentation are common features of Acrodontidae (sensu Cappetta, 2012) . Acrodus (s.l.) is the only member of the family that shows resemblances to our specimen and has Paleozoic occurrences (as? Acrodus ) ( Johnson, 1981; Hodnett et al., 2011; Hampe et al., 2013). It is otherwise prominently known from Triassic (e.g., Rieppel, 1981; Mutter, 1998a, 1998b; Cappetta, 2012) and younger Mesozoic deposits ( Cappetta, 2012). Acrodus teeth exhibit marked monognathic heterodonty, with symphyseal, parasymphyseal and posterior teeth being mesiodistally narrower and more apicobasally arcuate than lateral teeth ( Mutter, 1998a; Ginter et al., 2010; Cappetta, 2012), resembling the Hydriot tooth. However, Acrodus teeth bear a distinct mesiodistal crest and a less distinct or absent labiolingual crest ( Johnson, 1981; Rieppel, 1981; Mutter, 1998a, 1998b; Ginter et al., 2010; Hodnett et al., 2011; Cappetta, 2012; Hampe et al., 2013). In addition, secondary crests initiate all along the horizontal crest, are tightly packed and exhibit strong bifurcation patterns, whereas a socket for tooth interlocking is absent in most species ( Johnson, 1981; Rieppel, 1981; Mutter, 1998a, 1998b; Ginter et al., 2010; Hodnett et al., 2011; Cappetta, 2012; Hampe et al., 2013), except in A. georgii , where it is situated lingually ( Mutter, 1998b). The abovementioned differences preclude the inclusion of the Hydra chondrichthyan in Acrodus .
Teeth of the Pennsylvanian genus Hamiltonichthys resemble the Hydriot tooth in terms of occlusal crown ornamentation, while they also bear a lingual protuberance and a labial scar ( Maisey, 1989). Our specimen differs from Hamiltonichthys in exhibiting more rounded corners at the mesial and distal end of the crown in occlusal view. Onychoselache ( Coates and Gess, 2007) exhibits teeth of more subtle crown ornamentation and higher roots than AMPG 550. Finally, unicuspidate Lissodus teeth have strongly lingually bent roots, whereas mesiodistal and labiolingual occlusal crests form more pronounced, sharp, and often jagged, cutting edges ( Rees and Underwood, 2002; Duncan, 2004; Ginter et al., 2010). The marked labiolingual crest on the crown of AMPG 550, the lingual bulbous crown projection, along with the wider spacing between the secondary ridges are also reminiscent of characteristics of medial teeth of the?Pennsylvanian–Middle Triassic Acronemus (Euselachii incertae sedis) ( Rieppel, 1982; Rees and Underwood, 2002; Maisey, 2011). Despite the presence of a distinct lingual protuberance, Acronemus teeth do not possess a labial socket, differing in that regard from the Hydriot tooth. Acronemus teeth are further differentiated by their height and their shorter, strongly saddle-shaped crown ( Rieppel, 1982). Unfortunately, little is known about root vascularization in Acronemus teeth.
A close relationship between AMPG 550 and the Triassic Palaeobates or Homalodontus (= “ Wapitiodus ”), both possessing flat-crowned teeth, is excluded based on the general tooth morphology and ornamentation ( Mutter et al., 2007, 2008; Romano and Brinkmann, 2010). The potential Permian stem euselachian Wodnika possess a smooth crown, markedly dissimilar to that of the Hydriot specimen ( Haubold and Schaumberg, 1985; Hampe in Cappetta, 2012). Finally, the unicuspidate teeth of the hybodontiform Omanoselache differ in ornamentation, shape, and direction of crown protuberance and exhibit fewer but larger root foramina ( Koot et al., 2013, 2015).
In summary, AMPG 550 shows moderate to strong morphological affinities with Hamiltonichthys , Acronemus and moderate affinities with Paleozoic? Acrodus teeth of Johnson (1981). However, conspicuous differences in crown shape and ornamentation, interlocking process and root development preclude its assignment to any of the aforementioned genera. Our small sample size does not permit the erection of a new genus and, on the basis of dental characteristics alone, we prefer to leave it in open nomenclature within Hybodontiformes until additional fossil material becomes available.
Cohort EUSELACHII Hay, 1902 Euselachii indet.
Figure 2.6 View FIGURE 2
Material. One fragmented dermal denticle, AMPG 551.
Description. The relatively well-preserved crown is lanceolate and curved posteriorly. It possesses a well-developed, tricuspid distal crown, a neck and a base. Three keels can be seen on the anterodistal part of the crown. The median keel bears a shallow groove along its basal half and is distinctly higher than the two lateral keels. Its proximal end continues as a gentle ridge on the anterior surface of the neck. The lateral keels are grooved along their length and splay dorsolaterally, in anterior view. The neck is slightly narrower than the crown. The base is poorly preserved, but must have had a triangular outline and is wider than the crown, in proximal view.
Remarks. The presence of a slender crown with three keels on its anterior surface and a narrow neck are common features in scales of Paleozoic– Mesozoic ctenacanthids, but are also common in euselachian chondrichthyans ( Reif, 1978; Hansen, 1986; Rieppel et al., 1996; Johns et al., 1997; Derycke-Khatir et al., 2005; Ivanov et al., 2013). The Hydriot denticle compares favorably to the paragenus Moreyella ( Gunnell, 1933; Hansen, 1986), which has been tentatively affiliated with Carboniferous–Permian hybodontiform chondrichthyans (e.g., Derycke-Khatir et al., 2005). The Triassic paragenera Fragilicorona and Labascicorona ( Johns et al., 1997) also display very similar, tricuspid distal crowns like the denticle in question, but their systematic affinities beyond the euselachian level have not been discussed ( Ivanov et al., 2013). Hybodontiform dermal denticles can exhibit disparous morphologies, even in the same individual, ranging from somewhat stockier and shorter types with more keels and stout or undeveloped necks ( Reif, 1978), to more delicate and elongate ones like AMPG 551. Denticles of the latter type cannot be effectively distinguished from those of other euselachians ( Rieppel et al., 1996; Ivanov et al., 2013). Thus, it is unclear whether the Hydra denticle comes from the same genus or individual as the tooth AMPG 550.
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