Were there Cretaceous anthropoids? Part 2: What is an anthropoid?

This is the second post in a series, “Were there Cretaceous anthropoids?”

Before I go too far, I think I’d better make sure everybody knows what an anthropoid is. The living anthropoids include Old World and New World monkeys (the superfamilies Cercopithecoidea and Ceboidea) and apes and humans (superfamily Hominoidea). To give a more technical definition, applicable to fossil as well as living taxa, I can hardly improve on this passage from Williams et al. (2010):

By definition, crown Anthropoidea includes all species, living and fossil, descended from the last common ancestor of extant anthropoids. Stem Anthropoidea includes all fossil taxa that are more closely related to crown anthropoids than they are to tarsiers, but are outside the anthropoid crown group.

The concepts of “crown” and “stem” groups are very important to paleontological systematics. The concepts recognize that the most recent common ancestor of the living anthropoids (the crown ancestor) lived after the most recent common ancestor of anthropoids and any other primates (the stem ancestor). Williams and colleagues (2010) assume that tarsiers are the closest living relatives of anthropoids, and this is the most widespread hypothesis today despite some detractors. A systematist tries to distinguish crown and stem groups based on derived features. A fossil that shares one or more derived features with a particular group of living anthropoids (and not some others) would be interpreted as a member of the crown clade. By contrast, a fossil that shares some anthropoid derived features, but none of the derived traits of any particular group of living anthropoids, would be a stem anthropoid.

Williams and colleagues went on to list a number of features by which fossils might be recognized as stem anthropoids. These aren’t always the features that would come to mind if you’re thinking of living anthropoids.

Many features distinctive to living monkeys and apes are soft tissue characters. These traits may be useful evidence about our relationship to other living primate superfamilies, because some of them are shared with tarsiers. Nowadays, many derived genetic markers are known to characterize anthropoids. Again, these are useful for ascertaining the relationships of anthropoids to other living primates, but not so useful for fossils.

So I’ve extracted from Williams et al. (2010:4798) a list of those characters that comprise bony anthropoid derived characters:

Most anthropoids have orbits that are relatively small, forward facing, and convergent (20). A bony lamina posterior to the orbit completely separates the eyes from the chewing muscles in the temporal fossa (21).
Anthropoid features of the auditory region include a distinctive configuration of the internal carotid arterial system that supplies the orbit and much of the cerebrum (23). The middle ear cavity of the temporal bone extends forward into an air-filled accessory chamber containing a network of bony trabeculae (24). The tympanic bone that supports the eardrum is fused to the bony sidewall of the middle ear.
Early fusion occurs in both the frontal metopic suture and the mandibular symphysis (25). The body of the mandible is relatively deep (26).
Other anthropoid dental features include small, vertically implanted and spatulate lower incisors, simplified molar trigonids, and lower third molars with short heels (8).

To these dental traits can be added another feature important to the identification of Eosimias as a stem anthropoid: the lower premolars are implanted obliquely in the jaw, rather than parallel to the mesiodistal axis of the mandibular corpus.

And then there’s the foot:

The bony anatomy of the crown anthropoid foot is distinctive: the facet between the talus and fibula is steep-sided, and the groove for the tendon of the flexor fibularis muscle is in a midtrochlear position (6). The calcaneus is wide with a shortened heel and a distinctive calcaneocuboid joint shape (27). The peroneal tubercle on the first metatarsal that receives the tendon of the peroneus longus muscle is reduced in size (28).

The ankle and foot are among the biggest contrasts between living tarsiers and anthropoids, as the tarsiers are highly derived in support of their leaping. Their fibulae are fused distally with the tibia and their foot bones are elongated and specialized.

With this list of bony characters, we’re relatively well-equipped to recognize an anthropoid skeleton. But fossils of potential stem anthropoids still present some obstacles. Of course, fossils are fragmentary, so a specimen may only preserve a small part of the anatomy. If it happens to be a partial mandible with the symphysis and a molar or two, we’re on the right track. That gives us the opportunity to look at most of the dental and mandibular features that are derived in anthropoids. Or a partial skull that preserves the temporal features together with the orbit. That’s really a rich environment for anthropoid-derived traits.

The lack of associations among specimens can be a big problem. If we have a mandible with some anthropoid traits, and then we find a tibia, what are we to make of it? It may not be safe to assume that they come from the same kind of animal, even if both are anthropoid-like. If one has only one or two anthropoid-like features, and we assume that they come from the same species, it will influence our phylogenetic interpretation of other fossil lineages.

The biggest obstacle is that the characters of crown anthropoids didn’t all evolve simultaneously. A stem anthropoid may have some of them, but not others. It may also have its own derived traits not present in any crown anthropoids. In a complete specimen, this mixture of traits may be the best evidence for the pattern of evolution of the derived traits in the crown group. But in a fragmentary fossil, such a mixture may easily cause confusion.

Next: Ghost lineages.

References:

Williams BA, Kay RF, Kirk EC. 2010. New perspectives on anthropoid origins. Proc Nat Acad Sci USA 107:4797-4804. doi:10.1073/pnas.0908320107