Confused By A Talus: The Talus of Darwinius masillae

The tarsal is part of the ankel. It sits on the calcaneus and in turn articulates with the tibia (medial) and fibula (lateral). It is the main weight bearing portion of the ankle.

It looks like this:

From <a href="">here</a>

From here

There are some differences between some of the primates – and humans – in the shape of the talus. Two of the differences concern the position of the groove for the flexor hallucis longus (helps plantarflex the ankle) and the talofibular facet. In lemurs, and such, the groove for the flexor hallucis longus is angled or offset from the midline, whereas in tarsiers and anthropoids the groove runs down the midline of the talus. A picture should make this clear:

From <em>Functional Morphology of the Foot in Primates</em> by Daniel Gebo

From Functional Morphology of the Foot in Primates by Daniel Gebo

Also illustrated in the above picture is the differences in talofibular facet. In lemurs, the facet slopes outward, while in tarsiers and anthropoids (and humans) it is steep.

Which brings us to Darwinius masillae. The talus has figured in a lot of the hype (or misguided overkill) surrounding the find. here is an example:

Evidence in the talus bone links Ida to us. The bone has the same shape as in humans today. Only the human talus is obviously bigger.

As with everything connected with the find, this is kind of right, but kind of wrong. from the article:

The tarsus is exposed in laterocranial view (Fig. 10), with the processus coracoideus situated dorsally from behind the middle of the calcaneum as in Lemur and Europolemur kelleri [13: 70–71]. This differs considerably from omomyids and even more so from Tarsius, in which the part of the calcaneum distal to the processus coracoideus is extremely elongated, while it is much shorter in anthropoids. Hence it appears plesiomorphic for prosimians. Except for its smaller size, the tarsus of Darwinius, seen in lateral view, resembles that of Adapis parisiensis figured by Decker & Szalay [33: fig. 3]. The talofibular facet is steep and the peroneal tubercle is rather small and sharply angled [33] which is unlike that seen in adapids, Lemur, Hapalemur, and other lemuriforms, and is more like that in haplorhines (see [34], [35] for discussion). Unfortunately, the groove for the flexor fibularis cannot be seen while only a small part of the talotibial facet is exposed. These two characters, together with the shape of the talofibular facet, form the talar morphology shared by known Eocene adapiforms with lemuriforms and lorisiforms [33], [35]. The steep fibular facet on the talus or astragalus alone is not a synapomorphy for anthropoids because it also occurs in outgroups such as Scandentia, Dermoptera and Plesiadapiformes. Among primates it is, however, a haplorhine apomorphy[35], [37], and its presence in Darwinius supports taxonomic and phylogenetic classification with haplorhines rather than strepsirrhines (Table 3).

So, far from being an exact duplicate, in miniature, of the human talus what we have is a talus that displays a trait that is either:

  • A trait shared by all haplorhines
  • A trait shared by a wide variety of mammals
  • So what is the fuss about? Call me confused…

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    3 Responses

    1. Thanks for sharing this, Afarensis. I have noticed that in the past week the media reports have de-emphasized the toilet claw/tooth comb claim and ramped up the connection via the talus. If everything is hanging by one trait, then we’ve got a problem.

    2. One does wonder how an arboreal quadruped can have the same talus morphology as a biped?

    3. Especially if it seems to be shared by close relatives of primates among the scandentia, plesiadapids, etc.

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