Postcranial anatomy of the early notoungulate Allalmeia atalaensis from the Eocene of Argentina

Abstract

We redescribe and interpret the postcranial elements of Allalmeia atalaensis Rusconi, found in the Divisadero Largo Formation, Mendoza, Argentina. Allalmeia is one of the few basal notoungulates with preserved postcranial remains. Rusconi initially documented a skull, several vertebrae and part of the appendicular skeleton but these specimens were subsequently lost. Recently though, distal components of the holotype humeri, the proximal portion of the right femur, several broken diaphyses and most of the left and right pes (MCNAM-PV 507), were relocated. Preparation of this material has revealed new details, especially from the plantar side of the feet. The calcaneum has a distal peroneal process with marked rugosity in the lateral end, a well-developed calcaneal plantar tubercle and a large sustentaculum tali. The naviculars exhibit a well-developed medial tuberosity and slightly contact the calcaneum on the dorsal side, a condition called a ‘reverse alternating tarsus’. The disposition and morphology of the cuneiforms are similar to those of more recent typotheres and the archaic ungulate Tetraclaenodon puercensis. The phalangeal rows are oblique to each other, as in some extant digitigrade mammals. The ungual phalanges are claw-like suggesting that they might have borne claws. Long bones are gracile in comparison with other notoungulates, but with well-developed muscle origins and insertions. The use of body-mass allometric equations and a comparison with extant analogues suggests that Allalmeia was a small, generalized, digitigrade animal. An estimated body mass of approximately 3 kg is consistent with oldfieldthomasiids and archaic ungulates

我们重新描述和解释了Allalmeia atalaensis Rusconi 的颅后骨骼，这些骨骼发现于阿根廷Mendoza的Divisadero Largo组。Allalmeia是为数不多的带有颅后骨骼的基部南方有蹄类之一。 Rusconi最初记录了一个头骨、若干脊椎骨和附肢骨骼的一部分，但这些标本后来丢失了。然而，近来肱骨模式标本的远端骨骼、右股骨的近侧部分、若干破碎的骨干和最左和右的PE（MCNAM-PV 507）被重新找到。这些材料的制备揭示了新的信息，特别是脚的跖侧。跟骨具有远端腓突起（其侧端明显粗糙）、一个发达的跟骨结节跖和大型sustentaculum大里骨。舟状骨表现出发达的中结节，在背侧与跟骨微微相触，这是一种被称为“反向交替跗节'的症状。契形骨的配置和形态类似于较近代的型兽和古老的有蹄类Tetraclaenodon puercensis。指骨行互相倾斜，这和一些现生有趾哺乳动物一样。爪状指骨暗示他们可能具有爪子。长骨与其它南方有蹄类相比呈细长，但具有发达肌肉的起源和插入。使用体重异速生长方程，并与现存类似物的比较,表明Allalmeia是一个小的、广义的有趾动物。据估计，它的体重约三公斤，这与oldfieldthomasiids及古老的有蹄类动物相一致。