Hexanus vitulus, Springer & Waller, 1969

Adnet, Sylvain, 2006, Biometric analysis of the teeth of fossil and Recent hexanchid sharks and its taxonomic implications, Acta Palaeontologica Polonica 51 (3), pp. 477-488 : 482-485

Hexanus vitulus / H. nakamurai / H. griseu s debate

Lower symphysial teeth.— The presence of a median erect cusp on the crown of the lower symphysial tooth has been used to separate H. nakamurai from H. griseus (Welton 1978) . Despite the incorrect representation in Compagno (1984), living H. griseus always seems to possess a lower symphysial tooth lacking a median erect cusp (see Fig. 2B 3 ), as seen in the seven gill shark Notorynchus cepedianus . Erect cusps ( Fig. 2M 3 ) appear to characterise Heptranchias perlo and H. nakamurai (or H. vitulus and H. nakamurai sensu Herman et al. 1994 ), although their absence has been often seen in the last of these species (personal observations). If we consider only the genus Hexanchus , the presence of a lower median tooth with an erect cusp may indicate a vituliform group. However, its absence is not diagnostic.

Development and serration of the acrocone.—Although this dental character has never been quantified, since Ward’s work (1979) many fossil Hexanchus teeth have been placed in grisiform or vituliform groups according to the size of the acrocone. In fact, the greater development of the acrocone in comparison to the second distal cusp of the lower antero−lateral teeth has been used to separate the two living nominal species H. griseus and H. vitulus (see Fig. 2 ). Unfortunately, figures of the teeth from dried jaws (USNM 110900) in the original description of H. vitulus (Springer and Waller 1968: fig. 2) and from a paratype (USNM 200765) ( Herman et al. 1994: text−fig. 2) do not show this last character. Based on this contradiction, Herman et al. (1994) suggested in an addendum to their previous paper ( Herman et al. 1987) that two species could be recognised within the common small Hexanchus material: H. nakamurai with a low acrocone, and H. vitulus with a high acrocone. As discussed below, this point of view must be considered. The new results presented here suggest that a well developed acrocone is the first sign of maturity in species of Hexanchus . As with the presence of a serrated mesial edge on the acrocone and cusp number, the development of an acrocone may indicate only that the shark has reached a large size or a state of maturity (see Fig. 2A 3 , B 1 , B 2 , B 4 for H. griseus , and Fig. 2M 1 , M 2 for H. nakamurai ). For these reasons, the use of such characters by Herman et al (1994) to distinguish between H. vitulus and H. nakamurai must be questioned. In fact, in the work of these authors, H. vitulus is represented only by mature males (> 1.48 m) and H. nakamurai by young sub−adults (<1.17 m) and one adult female (1.57 m). Such sexual and ontogenetic differences between samples explain the reasons why Taniushi and Tachikawa (1991) considered H. vitulus to be a synonym of H. nakamurai .

Concerning serration of the acrocone and contrary to the odontological criteria to identify Hexanchus species by Herman et al. (1994: 151, 155: “mesial serration absent on lower lateral teeth of acrocone in H. griseus ”), acrocones with serrated mesial cutting edges have been often observed (e.g., Fig. 2A, B ) on lower teeth of H. griseus (Welton 1979) . This serration has been illustrated in the previous analysis ( Fig. 5 ). It appears, for example, that larger individuals of H. griseus show a mesial serration on the lower teeth (sometimes on the upper teeth too) which increases with growth (Welton 1979). The presence of serrations appears to be dependant on shark growth because they begin to appear on the first file when the shark has reached 160 cm in total body length (e.g., Fig. 2D 2 ). With an increase in size, serration of the acrocone extends to the lateral files. Sexual dimorphism has been sometimes thought to explain observed differences ( Cappetta 1980; Kent 1994) but this has not been tested because of a lack of samples. Despite this, as males reach maturity before females in terms of total length (3.25 m and 4.21 m, respectively, see Ebert 1986), the development of a serrated acrocone may begin earlier in terms of absolute tooth width for males.

The use of these last characters for identification allows an overview of the maturity states of all of the populations studied. Without body size (or maturity state) information, lower teeth of the two valid species, H. griseus and H. nakamurai , are hardly distinguishable, especially from the lower teeth of juvenile or sub−adult specimen of both species.

The main problem with small samples of isolated Recent or fossil teeth is that observed differences in terms of size and shape can be interpreted either as the presence of several species or as ontogenetic variations in a single species. Kemp (1978: 66) suggested that “allometric growth patterns would enable the differentiation of a tooth of a juvenile H. griseus from a tooth of an adult H. vitulus , for example there would be more crownlets on the H. vitulus tooth than on the similar−sized H. griseus specimen.” In reality, H. nakamurai and H. griseus seem to possess the same pattern of dental development, but with a delay in time produced by the different maturity ages in terms of size (different parameters of the L2/L3 regression equation). Such heterochronic development may lead to a persistence of a “juvenile shaped” tooth (unserrated acrocone, lower value of L2/L3) in H. griseus compared to H. nakamurai .

Implications for fossil determination and Palaeogene history

In 1976, Cappetta described a new fossil species of Hexanchus , H. agassizi , from material previously attributed to Notidanus (Woodward 1889) from the London Clay Formation. Ward (1979) reviewed the systematic affiliation of Hexanchus teeth from the London Clay based on new material and named two new species, H. collinsonae and H. hookeri . The three contemporaneous species, which occur at the same two localities in southern England (Burnham on Crouch and Sheppey), possess lower teeth with a similar width, a similar number of cusps (7 or 8) and an acrocone more or less serrated. However, H. agassizi and H. collinsonae have been placed in grisiform groups and have been distinguished from H. hookeri (vituliform group) by the less developed acrocone. In the grisiform group, H. agassizi has been separated from H. collinsonae by its lower teeth showing relatively wider and shallower roots and weaker mesial serrations on the acrocone.

Values of L2 and L3 for the holotypes and paratypes of H. agassizi , H. hookeri , and H. collinsonae teeth have been plotted long with values of fossil Hexanchus teeth from south−western France (Fig. 4 and Appendix 1). On the basis of acrocone shape, the three London Clay species cannot be differentiated from the range of dental morphologies seen in the new French material of Hexanchus . Indeed, the dental differences described between H. agassizi , H. hookeri , and H. collinsonae could be reinterpreted as three different maturity states within a single species belonging to the vituliform group of Ward (1979). There is no evidence from acrocone morphology that more than one species is present in the Ypresian of England or the late Ypresian/early Lutetian of France. The new material from south−western France is therefore referred to H. agassizi Cappetta 1976 (non pl. 1: 6). This fossil species belongs to the vituliform group and is only distinguishable from the living species H. nakamurai by its slightly smaller size (absolute tooth width and number of cusps per cm of tooth width) and more serrated acrocone. Considering tooth width, H. agassizi may be more advanced in terms of maturity than H. nakamurai . The inference that H. hookeri and H. collinsoni are ontogenetic variations of H. agassizi must be tested by additional analyses, at least for H. collinsoni which seems to possess lower teeth with noticeably deeper roots than the other species (David J. Ward personal communication, 2004). However, the results here lead to rejection of an Early Eocene divergence between the grisiform and vituliform lineages.

While the assignment of Hexanchus teeth from the Cretaceous–Palaeocene to the single species H. microdon has appeared suspect (Ward 1979; Cappetta 1987; Siverson 1995), most Eocene Hexanchus teeth have been placed in three species (and sometimes in other questionable new species) without careful consideration. From a brief examination of Palaeocene–Eocene Hexanchus teeth worldwide (Appendix 1), most of the Early and Middle Eocene fossils may belong to H. agassizi and not to several contemporaneous species with identical geographical distributions, and very similar sizes and probably diets.

According to calculations of body size (Appendix 1), species of Hexanchus species did not exceed 1 metre in total length during the Palaeocene and 1.9 metres during the Early to Middle Eocene, the maximum size also observed for the living H. nakamurai . The first large form comes from the Late Eocene of Antarctica ( Cione and Reguero 1994) with a size exceeding 2 metres (see Appendix 1). This size contrast, never observed in living H. nakamurai , is perhaps the first step in a trend towards larger forms of Hexanchus belonging to the grisiform group which are better known in later faunas. Size increase is perhaps a response to a global change in the environment and in food resources (e.g. marine mammals), as observed in the living H. griseus when individuals reach a threshold size of 2 metres in length ( Ebert 1994).

Figures

Fig. 2. Overview of dental variations in lower teeth of living species of Hexanchus. Labial view of some lower teeth of Hexanchus griseus (A–L) and Hexanchus nakamurai (M–O). A. ♀uncertain (UMC−REC204bM),>300 cm TL; 1st left file (A), 3rd (uncertain) left file (A), and lateral left 1 2 file (A). B. ♂ (UMC−REC204M), 300 cm TL; 3rd right file (B), 1st right 3 1 file (B), symphysial file (B), and 1st left file (B). C. ♂ (UMC− 2 3 4 REC162M), 223 cm TL; 1st left file (C), 2nd right file (C), and 3rd left 1 2 file (C). D. ♂ (UMC−REC175M), 195 cm TL; 1st left file (D), 3rd left file 3 1 with enlargement on the mesial serrated cutting edge of the first cusp (D2), 4th left file (D), and 5th left file (D). E. ♀ (UMC−REC161M), 191 cm 3 4 TL; 1st left file (E) and 3rd left file (E). F. ♂ (UMC−REC611M), 1.08 cm 1 2 TL; 1st left file (F), 4th left file (F). G. ♀ (UMC−REC163M), 114 cm TL, 1 2 1st right file. H. ♀ (UMC−REC164M), 117 cm TL, 3nd left file. I. Unsexued (UMC−REC201M), 58 cm TL 2nd right file. J. Unsexued (UMC− REC172M), 72 cm TL, 2nd left file. K. Unsexued (UMC−REC173M), 75 cm TL, 1st left file. L. ♂ (UMC−REC174M), 80 cm TL, 1st right file. M. ♀ uncertain (UMC−REC197M), 148 cm TL; 4th rigth file (M), 1st 1 right file with enlargement on the mesial serrated cutting edge of the first cusp (M2), symphysial file (M3). N. ♂ uncertain (UMC−REC192M), 145 cm TL; 3rd right file (N) and 1st left file (N). O. Sex undetermined 1 2 (UMC−REC196M), 1 m TL, 1st right file (O), 1st left file (O), 3rd lateral 1 2 file (O), and 4th lateral file (O). A–D and F–O are respectively at the 3 4 same magnification (vertical white bars). Abbreviation: TL, total length.

Fig. 3. Fossil Hexanchus sp. lower teeth from the Donzacq Formation of Saint−Géours−d’Auribat (late Ypresian/early Lutetian, south−western France). A. UMC−SG13, antero−lateral file, labial view. B. UMC−SG130, antero−lateral file, lingual view. C. UMC−SG15, lateral file, labial view. D. UMC− SG14, antero−lateral file, labial view. E. UMC−SG131, lateral file, lingual view. F. UMC−SG132, antero−lateral file, lingual view. G. UMC−SG17, lateral file, labial view. H. UMC−SG133, lateral file, labial view. I. UMC− SG134, broken lateral lower tooth with peculiar deep root, labial (I1) and lingual (I2) views. J. UMC−SG135, lateral file, labial view. K. UMC−SG12, symphysial file, lingual view.

Fig. 1. A. Simplified terminology of a Hexanchus lower tooth (Hexanchus griseus, 4th right file,>300 cm total length, UMC−REC204bM). B. Schematic drawing of measurements (homologuous points and intermediate distances) discussed in the text.

Fig. 5. Acrocone shape of lower teeth in Recent hexanchid species. Small pictures illustrate the changes in acrocone size according to the L2/L3 values. Interrupted lines are regression curves from data for the two living Hexanchus species. For H. griseus, different symbols have been used to indicate the sex of the sharks (m, male; f, female; ind, indeterminate).