Hystrix refossa Gervais, 1852

Mörs, Thomas & Hugueney, Marguerite, 2017, A Hystrix Linnaeus, 1758 incisor (Mammalia, Rodentia) from the Early Pleistocene of Senèze, France, Geodiversitas 39 (4), pp. 797-802 : 798-800

Hystrix refossa Gervais, 1852

( Fig. 1 A-C)

Castoridae indet. – Delson et al. 2006: 277, 286.

MATERIAL. — The incisor ( SEN 04-0139) described here was obtained in the 2004 excavation at “locus P233, couche H moyenne”, a site which is characterized by lacustrine near shore sediments with repetitive slope slides ( Pastre et al. 2015: text-figs 3-4).

MEASUREMENTS. — Antero-posterior diameter (DAP) = 8.74 mm × transverse diameter (DT) = 6.75 mm.

DESCRIPTION

Orientation

This very incomplete incisor is represented only by a 3 cm long fragment which lacks the anterior part as well as the posterior part. Despite the small size of the fragment, a noticeable bending of the tooth can be observed and allows to identify it as an upper incisor. Unfortunately the chisel-shaped anterior tip, which would make it possible to orientate the tooth and which bears several diagnostic characteristics, is not preserved. However, one of the extremities of the fragment is hollow, it represents the growing part of the incisor i.e., the posterior one. Consequently the opposite extremity, completely filled with dentine, is the anterior part. Therefore it is possible to determine this tooth as a right upper incisor.

Shape

The shaft has a DAP larger than the DT, its cross-sectional shape is therefore less than 0.8 (DT/DAP = 0.77; Rybczynski 2007: appendix 48). The shape of the cross-section of the incisor is triangular with rounded angles, the inner side being the more rectilinear.

Enamel band

The enamel cover extends on the inner side for roughly 2 mm, while it ends with a thin longitudinal fold, and a little lower on the more rounded outer side. The anterior face is a little convex and its enamel surface faintly grooved, with three folds separated by two flat furrows.

Enamel microstructure

The incisor enamel is two-layered, with a thick inner layer (portio interna, PI) making up at least 75% of the overall thickness of the enamel band, and with a thin outer layer (portio externa, PE) which forms maximal 25% of the overall thickness ( Fig. 3A, C, H). The PI is formed by multiserial Hunter-Schreger bands (HSB) with each band being four to six prisms thick ( Fig. 3D, F-G, I). The HSB are inclined with approximately 20°. The orientation of the interprismatic matrix (IPM) is parallel to the prisms. The transition zone between PI and PE is somewhat irregular and not very sharp. The PE is formed by radial enamel with steeply inclined prisms and IPM oriented almost vertically to the prisms.

COMPARISONS

Castor

The incisor of extant beaver is described byMiller (1912: 951) as follows: “Upper incisor heavy […]; shaft about as deep as wide, the anterior face slightly curved, a little longer as subequal outer and inner faces […]; enamel […] essentially smooth but with very obscure longitudinal wrinkles”. Rybczynski (2007: appendix 48 = upper incisor enamel) indicates that: “a small number of Castor canadensis Kuhl, 1820 individuals exhibit weakly grooved incisal enamel” and upper incisors of Early Pleistocene (MN 17) Castor fiber Linnaeus, 1758 from St-Vallier show faint longitudinal wrinkles ( Hugueney 2004). Moreover, a Steneofiber Geoffroy, 1833 individual from the Early Miocene (MN 2) of St-Gérand-le-Puy (Allier; FSL 98190) shows the same ornamentation as the Senèze incisor whereas the other incisors from St-Gérand-le-Puy are smooth, as all Steneofiber incisors from the Early Miocene (MN 5) of Hambach ( Mörs & Stefen 2010).

Trogontherium

In the two extinct castorid species Trogontherium minus Newton, 1890 and Trogontherium cuvieri Fischer von Waldheim, 1809 the upper incisor appears irregularly rounded in transverse section. The incisors of T. minus are a little smaller in size than the Senèze incisor and the incisors of T. cuvieri are much larger. The enamel cover ends abruptly at the beginning of the inner face but extends obliquely more largely on the outer one. The enamel band displays longitudinal raised and evident striation considered as characteristic of this genus. Schreuder (1929: 165, fig. 15) stated for T. cuvieri : “on the granular enamel the older specimens sometimes show distinct longitudinal ridges and grooves”. Mörs et al. (1998: 143) described incisor fragments of T. minus with “finely-wrinkled enamel […] with longitudinal furrows” from the Late Pliocene (MN 16) of Hambach.

Hystrix

For living Old World porcupine Miller (1912: 547) stated “Upper incisor robust […] the shaft scarcely compressed, but with antero-posterior diameter perceptibly greater than lateral diameter […] Anterior face slightly oblique, nearly flat though a little rounded off at edges; posterior surface narrow so that the outline of cross-section is nearly an isocele triangle with all angles rounded”. Upper incisors are smooth and literature does not mention furrows on the enamel band. However, some NRM-VE specimens from Africa, labelled either as Hystrix cristata / galeata or as Hystrix africaeaustralis Peters, 1852 show upper incisors that look very similar to the Senèze incisor. Although this feature is not consistent, a huge adult skull of H. cristata / galeata (A60/1143) has clearly grooved upper incisors, whereas most of the large adults do not exhibit this feature (e.g.A59/3984, A64/2071, A64/1450). Juveniles usually do not show grooved upper incisors, but one small juvenile (A59/3982) has clearly grooved upper incisors, and even the lower ones show a medial furrow.In H. africaeaustralis , e.g. one adult specimen (A58/3980) possesses clearly grooved upper incisors, as does a juvenile specimen (A59/3982) of similar size but with dp4 and not yet erupted m3. None of the available specimens of Hystrix indica Kerr, 1792 has grooved incisors (adult e.g. A59/1902, A59/5436, juvenile A59/3701)

In the Early Pleistocene (MN 17) locality Saint-Vallier all three genera are present and upper incisors documented ( Hugueney 2004). These specimens are of particular interest as the stratigraphical age of Senèze is close to that of St-Vallier and the material was available for direct comparison. The size of the Senèze incisor is small compared to the castorids of St-Vallier. But it is worth to notice that ontogenic differences in size in castorids are huge, as beavers grow almost all along their life and can reach 15-17 years old or even older ( Freye 1978). The St-Vallier Hystrix incisor is small but it corresponds to a young individual; porcupines show like castorids a huge enlargement of their incisors during life time. In contrast to the upper incisors of Castor and Trogontherium of the same locality, the enamel band of the Hystrix upper incisor is smooth. Compared to the fossils from St-Vallier, the cross-sectional ratio of the Senèze incisor is closer to that of Hystrix than to Castor or Trogontherium , in which these ratios approach 1 ( Fig. 2 ; Rybczynski 2007: appendix 47).

Enamel microstructure

The incisor enamel in both Hystricidae and Castoridae is two-layered with a PI made of HSB and a PE made of radial enamel. The main difference in the schmelzmuster of these two rodent families is the thickness of the HSB. In Hystricognathi the HSB are multiserial, with each band containing four to seven prisms ( Martin 1992). In contrast, Castorimorpha have uniserial, one prism thick HSB ( Mörs et al. 2016). Therefore porcupines and beavers are easy to identify and keep apart by their incisor enamel microstructure ( Koenigswald & Mörs 2001). Additional discriminating features described by Koenigswald & Mörs (2001) are the inclination of the HSB and the PI/PE ratio. Castoridae , especially large forms like Anchitheriomys Roger, 1898 , Castoroides Foster, 1838 , and Trogontherium show less inclined HSB, and a significantly thicker PE than Hystricidae . The longitudinal and transverse sections of the enamel of the Senèze incisor ( Fig. 3) demonstrate clearly that it belongs to a hystricid. For comparison, we have additionally figured longitudinal sections of Hystrix primigenia (Wagner, 1848) from the Late Miocene (Turolian, MN 13) of Polgárdi, Hungary, which show an almost identical schmelzmuster ( Fig. 3B, E).

Figures

FIG. 1. — Hystrix refossa Gervais,1852 from Senèze,right upper incisor:A, buccal side; B, medial side; C, upper side. Scale bar: 1 cm.

FIG. 2. — Cross-sections and measurements (in mm) of upper incisors of large rodents from Saint-Vallier and Senèze.Abbreviations: C, Castor fiber Linnaeus, 1758; H, Hystrix refossa Gervais, 1852; T, Trogontherium cuvieri Fischer von Waldheim, 1809; S, specimen from Senèze. Scale bar: 10 mm.