This is the first installment of my primate evolution research ’series’. Because I understand that not everybody knows a great deal about primates, especially their evolution, and even more generally the vocabulary of evolutionary biologists, I will attempt to make this as digestible as I can (and hopefully interesting!).

The article I’m focusing on in this post is one recently published in The Anatomical Record by Alfred Rosenberger @ City University New York and Anthony Pagano @ the American Museum of Natural History. In the article the authors are asking the important question of whether or not a fused metopic (often called ‘frontal’) suture is a synapomorphic (shared derived) character of anthropoid primates. Already the big words, right? To help I will use as many visual aids as possible.

Figure 1 (National Institute of Health)

This is where the metopic suture (frontal suture) is located (Figure 1). It separates the two halves of the frontal bone and in humans usually disappears by the age of six. The reason this character is viewed as important to primate evolution is that, up until this article, it had been considered a synapomorphic feature of higher primates. Synapomorphic, as I wrote above, means a ’shared derived’ character. In other words, it is a character that was derived (apomorphic) in the last common ancestor of the individual species being studied, and has been maintained by all of the descendants (being analyzed) of that ancestor.

In this case that ancestor would be the most basal anthropoid, an individual in which the metopic suture was thought to be fused. Historically this assumption has rarely been questioned, and when it has the conclusions have not been… well… conclusive. Either bad methodology or small sample sizes in previous attempts have led Rosenberger and Pagano (2008) to take this task on themselves. Using a wide variety of archontan specimens from the Department of Mammology at the AMNH and the Natural History Museum of London (Figure 2) and a fairly simple scoring system the researchers were able to evaluate the condition of fusion among a wide variety of relevant taxa, both extant and extinct.

Figure 2. List of species analyzed and the status of their frontal suture fusion. (Rosenberger & Pagano, 2008)

The following image (Figure 3) is the histogram of the sample means of interfrontal suture fusion among adults analyzed in the study:

The resultant data show that a fused state has a high frequency in a number of genera, while a completely unfused state was very rare. In addition, all of the Eocene fossil strepsirrhines examined (Notharctus, Smilodectes, Adapis, and Leptadapis) exhibited either full or partial frontal fusion. In fact, even the non-primate dermopteran, tupaiid, and chiropteran specimens have fused interfrontals. From this high incidence of fusion in many close relatives of Primates, as well as non-anthropoid primates, it is not only apparent that frontal fusion is present in non-anthropoids, but that it may be more common among them than in the author’s total sample of strepsirrhines.

There is no prosimian-anthropoid, anthropoid-nonanthropoid, or even strepsirrhine-haplorrhine dichotomy in this feature. Fused frontal bones are widespread among primates living and extinct, refuting the hypothesis that the anthropoid morphology is unique and lessening the likelihood that it evolved independently as a derived condition in higher primates.

Only the strepsirrhines, of all primates, appear to go either way – manifesting both fused and unfused metopic sutures – while it is all other euprimates which appear to develop the very character that was attributed to anthropoids. Therefore, combining both the extinct fossil and extant primate evidence, they conclude that the closure of the interfrontral suture is more ancestral than the origin of anthropoids. Siting an article on small pigs, Rosenberger and Pagano (2008) speculate that frontal fusion may be most likely a “preadaptation to the evolution of the anthropoid postorbital septum” (Herring and Teng, 2000).

This finding, thus, provides the most convincing argument that, until there is evidence to the contrary it is more parsimonious to accept the null hypothesis that fusion of the metopic suture in all haplorrhines is homologous (similar structures due to shared ancestry)

(References)

Rosenberger, A & Pagano, A, 2008, 'Frontal Fusion: Collapse of Another Anthropoid Synapomorphy', The Anatomical Record: Advances in Integrative Anatomy and Evolutionary Biology, vol. 291, no. 3, pp. 308-317. 10.1002/ar.20647