Pachynolophus (Remy, 1972)

Remy, Jean A., Krasovec, Gabriel, Lopez, Éric, Marandat, Bernard & Lihoreau, Fabrice, 2019, The Palaeotheriidae (Equoidea, Perissodactyla, Mammalia) from the Eocene fauna of Aumelas (Hérault department, France), Geodiversitas 41 (13), pp. 525-585 : 544-547

Pachynolophus sp.

( Fig. 10 )

MATERIAL. — AUM 199, right maxillary fragment with P3/-M3/; AUM 213, left M3/; AUM 1586, right maxillary fragment with (alv. P1/)-P2/-(M2/); AUM 1596, left fragment with P3/-P4/; AUM 1648, 1649, left P4/. AUM 162, left mandible fragment with M/2, (M/3); AUM 227, left M/3 ( Pr. sudrei in Remy et al. 2016 ); AUM 323, symphysis with both /C and sockets of incisors; AUM 332, right DP/4; AUM 1557, right M/3; AUM 1599 left mandible fragment with (P/3)-(M/3); AUM 1600, right mandibular fragment with M/2-M/3; AUM 1602, left mandible fragment with M/2-M/3; AUM 1615, right mandible with P/4-M/3; AUM 1616, left fragment with M/3; AUM 1617, mandible with right /C-(alv. DP/1)- DP/2-M/3, and (alv. six I and left /C); AUM 1620, left mandible with DP/2-M/2-(M/3); AUM 1627 left mandible with M/2-M/3; AUM 1654, right M/3.

DESCRIPTION

The two featured upper cheek teeth series, AUM 199 and AUM 1586, are preserved in a very bad condition. AUM 199 which pertains to an old individual, is affected by a deep longitudinal fissure at the level of P3/-M2/ and the loss of ectoloph of M3/. The teeth of AUM 1586 are even more worn and molars partly broken.

These specimens and other associated upper cheek teeth present rather conspicuous differences compared to Pachynolophus ruscassierensis n. sp. They are roughly 20% larger than average for most parameters. That would correspond to an estimated length of the row P2/-M3/ of at least 60 mm, and to an estimated weight of 28 kg (instead of 52 mm, 21 kg for Pa. ruscassierensis n. sp.) ( Table 4; Appendix 8).

As far as we can see, compared to Pa. ruscassierensis n. sp., the molars appear slightly more lophodont. The labial side of ectolophs is more slanted and indicates an IH index slightly lower. Thus the M3/ (AUM 213), although barely worn, has an IH of 0.40, near the lower variation limit of Pa. ruscassierensis n. sp. (Appendix 12). The metaloph of molars, more forwardly oriented, is directed to the distal side of paracone. The cingula are on the whole thinner and likely lower. The distal cingulum of M3/ is nevertheless fairly prominent, and it can give rise to a small hypostyle (AUM 213). The occlusal faces of P3/ and P4/ offer a more defined V shape. P3/ is more squared and has a wider lingual outline and a broader distal basin.

In addition, the surface ratio of the premolar sector appears to be slightly lower than that of Pa. ruscassierensis n. sp. Thus, P4/ is somewhat smaller (SP 4/ SM3 = 49 instead of 52-59) and the ratio SP / SM (by combination of two specimens, AUM 199 and AUM 1586), could have reached only 38 to 42 instead of 44-49 (Appendix 21). P2/ is very unlike that of Pa. ruscassierensis n. sp. as it is almost rectangular and possesses two distinct labial cusps.

Some mandibular fragments and lower teeth have been assigned to the same taxon on the basis of molar size. The crescents of the molars are rather sharp, not very rounded. The metastylid is not highly developed and poorly separated from metaconid. The ectocingulum is generally thin, variable in height (high on AUM 1599, lower on AUM 1617), and interrupted on cuspids except on AUM 1615. The P/4 (AUM 1599) is molariform, even if it bears only a small knob instead of a true entoconid. Its relative area is low compared to Pa. ruscassierensis n. sp. (Appendix 22). Despite P/3 (AUM 1599) is in a bad condition, we can see that the trigonid is long and tapers mesially. Its prominent protoconid overhangs a somewhat flattened crescent. The talonid is short and low, crossed by a weak ridge that ends in a small hypoconid.

DP/4 is fully molariform. On AUM 332, the metastylid is clearly separated from metaconid but slightly lowered. The DP/3 outline slightly tapers mesially and the paralophid bears a tiny lingual accessory cuspid (AUM 1620). DP/2 is simple, with a prominent cusp, overhanging small mesial and distal knobs, with a very short talonid. DP/1 was bi-rooted.

Other anatomical data

The upper DPC is not preserved. On the mandible AUM 1617, the DPC (considered in that instance as the length /C-DP/2) measures 19.5 mm. With a teeth row length of about 63 mm, the DPC represents 31% of this value. But this specimen is a juvenile and is likely not representative of the adult condition. Due to its size, the mandibular fragment AUM 323 could perhaps be assigned to the same taxon. It shows a symphysal region not reaching alveoli of the P/1, but with both /C and the sockets of incisors. The preserved part of diastema is 24 mm long; assuming at least 6 mm for P/1 length (the DPC being the distance /C-P/2), with a tooth row length of 60 mm (estimation for AUM 199) to 63 mm (AUM 1617), this DPC (24 + 6) should have reached 48 to 50% of LRDJ.

The mandibular body is more robust than that of Pa. ruscassierensis n. sp., high from 16 mm under the DPC, to 28 mm under M/3, with a minimal width of 12 to 16 mm at the DPC. On AUM 1617, a single foramen mentale is observed under DP/2. The vertical ramus is unknown.

At maxillary level, the anterior opening of the infra-orbital foramen (FIO) is located above anterior half of P3/ (AUM 1586).

COMMENTS

The generic status of this assemblage needs some comments. It can be a priori assigned to Pachynolophus due to lack of mesostyle on upper molars, lack of hypocone or entoconid on premolars, rather sharp crescents on lower cheek teeth, long DPC. But, one might wonder whether these specimens should not rather refer to Propalaeotherium cf. gaudryi from Aumelas. Indeed, their size is identical, with variation limits overlapping almost exactly. Moreover, we know that some specimens lack mesostyle in Propalaeotherium gaudryi ( Remy 2017) . On the other hand several features clearly differentiate this cluster. First, the relative surface area of the premolar series seems smaller than in Pr. cf. gaudryi from Aumelas, (actually more alike than data observed in the true Pr. gaudryi !). Thus P4/ is rather small (SP 4/ SM3 = 49 instead of 50-61 in Pr. cf. gaudryi ; SP 4/ SM = 18 instead of 19-22). Then, we speculate that the total surface of the premolars represents only 38 to 42% of the surface of the molars instead of 41 to 50% in Pr. cf. gaudryi (Appendices 9; 21 [caption]).

Pachynolophus sp. from Aumelas

P2/ P3/ P4/ M1/ M2/ M3/

L W L W L W L W D d L W D d L WD d

N 1 1 3 3 5 5 2 1 1 2 1 1 1 1 2 2 2 1 mean 7.3 7.4 7.7 10.1 8.8 11.2 10.5 14.2 14.8 13.8 13.6 16.2 17.9 16.1 14. 16.5 17.2 16.3 variation – – 7.4-8.2 9.3-10.8 8.2-9.4 10.7- 10.1- – – 13.2- – – – – 14.0- 16.1- 17.1- – range 11.8 10.8 14.3 14.1 16.8 17.3 standard – – 0.436 0.764 0.537 0.527 – – – – – – – – – – – – deviation

coeff. of – – 5.7 7.5 6.1 4.7 – – – – – – – – – – – – variation

upper series LP2-M3 LP2-P4 LM1-M3 PMI

N 1 1 1 1

mean 60.6 24.3 (1) 36.3 67.1

P/2 P/3 P/4 M/1 M/2 M/3

L W1 W2 L W1 W2 L W1 W2 L W1 W2 L W1 W2 L W1 W2 W3 N 1 1 1 1 – – 2 2 2 4 4 4 8 7 8 9 9 9 9 mean 6.7 3.8 3.8 8.6 – – 9.0 6.6 6.6 9.9 7.3 7.7 11.8 8.3 8.3 17.2 8.7 7.7 5.9 variation – – – – – – 8.6- 6.5- 6.3- 9.6- 6.7- 7.2- 11.5- 7.9- 7.9- 16.2- 8.5- 7.1- 5.6- range 9.3 6.7 6.8 10.2 7.8 8.1 12.1 9.1 8.7 18.8 9.2 8.3 6.6 standard – – – – – – – – – 0.258 0.457 0.392 0.217 0.416 0.251 0.893 0.201 0.387 0.310 deviation

coeff. of – – – – – – – – – 2.6 6.3 5.1 1.8 5.0 3.0 5.2 2.3 5.0 5.2 variation

DP/2 DP/3 DP/4

L W1 W2 L W1 W2 L W1 W2

N 2 2 2 2 2 2 3 3 3

mean 7.4 3.1 3.8 8.9 4.7 5.7 9.6 5.9 6.4

variation 7.3-7.4 3.1-3.1 3.7-3.9 8.4-9.3 4.6-4.7 5.6-5.7 9.1-10.3 5.5-6.2 5.9-6.8

range

standard – – – – – – 0.643 0.361 0.458

deviation

coeff. of – – – – – – 6.7 6.1 7.2

variation

Some morphological differences are also conspicuous between the upper cheek teeth and those of Pr. cf. gaudryi from Aumelas as well as with those of Pr. gaudryi from the “Ageian fauna”. The parastyle of molars is a slightly less prominent. P3/ and P4/ have a more mesially shifted protocone with a broader distal basin. P2/ shows a more rectangular outline, with two more separated labial cusps. The lingual cingula are less developed.

The associated mandibular specimens confirm this differentiation with respect to the lineage of Pr. gaudryi . Indeed, the crescents of the lobes of lower cheek teeth are less rounded and more acute. The DPC is likely longer than in Pr. gaudryi where it should not exceed 30 to 32% of LRDJ ( Remy 2017). Conversely, we have seen that on the mandible AUM 323, it could have reached 50%.

It seems that this set of specimens from Aumelas should not finally be brought close to a Propalaeotherium , and must rather be assigned to the genus Pachynolophus .

Compared to Pa. ruscassierensis n. sp., conspicuous differences have been previously highlighted, particularly in size. The size difference with the genotypic species Pa. duvali , which is one of the smaller species of the genus, is even larger and around + 40%. Furthermore, despite some similarities that lead to bring the two forms closer together in a cladistic analysis (see below), the Aumelas specimens are quite distinct from this latter species by their P2/ even more square, their P3/ P4/ more transversely elongated. So, the Aumelas form can not be considered as affine to Pa. duvali .

In fact, we are dealing with one of the largest Pachynolophus . Assuming that FSL-2977 could be a good representative of Pa. cesserasicus (see above), the specimens of Aumelas are about 15% larger for most parameters. Conversely, they are 15% smaller than the type and single specimen of Pa. cayluxi. Besides, compared to both species, the upper molars and mostly the P4/ of Aumelas are less bunodont, with ectolophs more flattened. All cingula are thinner and less continuous. P2/ is more quadrangular. Therefore, this Aumelas material probably represents an original new species, but it is too damaged and too poorly known to define a new taxon in the current state.

Figures

FIG. 10. — Pachynolophus sp. from Aumelas: A, AUM 199, right maxillary fragment with P3/-M3/; A1, labial view; A2, occlusal view; B, AUM 1586, right maxillary fragment with (alv. P1/)-P2/-(M2/); B1, labial view; B2, occlusal view; C, AUM 213, left M3/ (reversed), occlusal view; D, AUM 1596, left fragment with P3/-P4/ (reversed),occlusal view;E, AUM 332, right DP/4,occlusal view;F, AUM 1600,right mandible with M/2-M/3 (pars); F1, occlusal view;F2, labial view;G, AUM 323, symphysis with both /C and sockets of incisors, dorsal view; H, AUM 1599 left mandible fragment with (P/3)-(M/3) (reversed); H1, occlusal view; H2, labial view; I, AUM 1620, left mandible with DP/2-M/2-(M/3), occlusal view; J, AUM 1617, mandible with right /C-(alv. DP/1)-DP/2-M/3, sockets of incisors and left /C); J1, occlusal view; J2, labial view. Scale bars: 2 cm.