The petrosal of Omomys carteri and the evolution of the primate basicranium

New insights into the ear region anatomy and cranial blood supply of advanced stem Strepsirhini: evidence from three primate petrosals from the Eocene of Chambi, Tunisia

We report the discovery of three isolated primate petrosal fragments from the fossiliferous locality of Chambi (Tunisia), a primate-bearing locality dating from the late early to the early middle Eocene. These fossils display a suite of anatomical characteristics otherwise found only in strepsirhines, and as such might be attributed either to Djebelemur or/and cf. Algeripithecus, the two diminutive stem strepsirhine primates recorded from this locality. Although damaged, the petrosals provide substantial information regarding the ear anatomy of these advanced stem strepsirhines (or pre-tooth-combed primates), notably the patterns of the pathway of the arterial blood supply. Using μCT-scanning techniques and digital segmentation of the structures, we show that the transpromontorial and stapedial branches of the internal carotid artery (ICA) were present (presence of bony tubes), but seemingly too small to supply enough blood to the cranium alone. This suggests that the ICA was not the main cranial blood supply in stem strepsirhines, but that the pharyngeal or vertebral artery primitively ensured a great part of this role instead, an arterial pattern that is reminiscent of modern cheirogaleid, lepilemurid lemuriforms and lorisiforms. This could explain parallel loss of the ICA functionality among these families. Specific measurements made on the cochlea indicate that the small strepsirhine primate(s) from Chambi was (were) highly sensitive to high frequencies and poorly sensitive to low frequencies. Finally, variance from orthogonality of the plane of the semicircular canals (SCs) calculated on one petrosal (CBI-1-569) suggests that Djebelemur or cf. Algeripithecus likely moved (at least its head) in a way similar to that of modern mouse lemurs.

First record of the genus Microchoerus (Omomyidae, Primates) in the western Iberian Peninsula and its palaeobiogeographic implications

In this paper we describe new material of Microchoerus (Microchoerinae, Omomyidae, Primates) from Zambrana (Miranda-Trebiño Basin, northern Iberian Peninsula, Spain), a locality assigned to Reference Level MP18 (middle Headonian, Late Eocene). The specimens studied consist of two mandibular fragments, bearing p3-m3 and p4-m3. The teeth resemble in size and morphology those of Microchoerus erinaceus from Hordle Cliff, England, although some differences prevent us from making a definitive ascription to this species. We therefore refer the material from Zambrana to Microchoerus aff. erinaceus. Some traits, such as the development of the mesoconid and hypoconulid in the m1 and m2, and the shape of the hypoconulid lobe in the m3, are intermediate between those of M. erinaceus and Microchoerus edwardsi. Thus, the material from Zambrana is very similar to other species of Microchoerus present in Europe, representing a transitional form between M. erinaceus and M. edwardsi. The described material represents the first discovery of a primate from the Miranda-Trebiño Basin, and also the westernmost record of the genus Microchoerus in the Iberian Peninsula. Moreover, the identification of this microchoerine, with clear similarities to the representatives of this genus described from other European sites, reinforces the idea of the existence of connections between western Iberia and the rest of Europe in the Late Eocene, previously hypothesized after the discovery of typical European artiodactyls in the site of Zambrana.