New Primate Fossil Informs Us of the Ape-Monkey Split During the Oligocene

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ResearchBlogging.orgThe newly reported Saadanius hijazensis may or may not be a “missing link” but in order for this monkey to climb onto the primate family tree, a new branch had to be sprouted. So, not only is Saadanius hijazensis a new species, but it is a member of a new taxonomic Family, Saadaniidae, which in turn is a member of a new Superfamily, Saadanioidea. Why is this important? It’s complicated. But not too complicated.

The fossil was found while University of Michigan paleontologist Iyad Zalmout was busy looking for dinosaur fossils in western Saudi Arabia. He found the monkey, from a much later time period, instead. Ooops.

“I didn’t know whether to be disappointed or not, but I thought, well, maybe something interesting will pop up here, so I started looking around. Within minutes, I found teeth sticking out of the ground, and when I realized what they were I was shocked. I had worked with Phil [Gingerich] on terrestrial mammals in the Bighorn Basin, and my first look at the size and shape of these teeth told me I had found a primitive primate.”

Zalmout sent a photo to Philip Gingerich, top monkey fossil expert, who confirmed its primate status and potential importance.

Major early Oligocene to early middle Miocene Afro-Arabian catarrhine primate sites. Key: 1, Harrat Al Ujayfa, Saudi Arabia; 2, Thaytiniti, Oman; 3, Taqah, Oman; 4, Fayum, Egypt; 5, Gebel Zelten, Libya; 6, Lothidok, Kenya; 7, Meswa Bridge, Kenya; 8, Koru, Kenya; 9, Songhor, Kenya; 10, Buluk, Kenya; 11, Moroto, Uganda; 12, Napak, Uganda; 13, Kalodirr, Kenya; 14, Rusinga, Kenya; 15, Loperot, Kenya; 16, Ryskop, South Africa; 17, Wadi Moghara, Egypt; 18, Ad Dabtiyah, Saudi Arabia; 19, Malembe, Angola. Map and caption from original paper.

The time period in question, the Oligocene, is circled on this time chart from the USGS.

African “higher” primates, the Old World Monkeys and the Apes, are collectively known as the Catarrhini. The Catarrhini split from the New World monkeys at least 40 million years ago, though this date is subject to revision. That would be somewhere in the middle of the Eocene. Later on during the late Eocene and subsequent Oligocene, around 20-something mya to 30-something mya, Catarrhini gave rise to populations that would have been the ancestors of the major living groups as well as some lineages that have gone totally extinct. The split between living Old World Monkeys and the apes would have been somewhere in the middle of this time range.

Saadanius hijazensis in situ. The fossil, found in 2009, preserves most of the face, the front upper portion of the skull, the temporal bone, and the palate, with some of the left and right upper teeth. The specimen was found with the palate and teeth facing upward, imbedded in an iron-rich clastic conglomerate in the middle part of the Shumaysi Formation. Credit: Iyad S. Zalmout, University of Michigan Museum of Paleontology

Frontal view of Saadanius hijazensis (holotype SGS-UM 2009-002). source

Within a relatively short time span, several ape lineages arose, causing much subsequent confusion among palaentologists. The short version of the story is that some four major ape lineages emerged between the middle Oligocene and the end of the Oligocene (around 23 mya), most of which we tentatively refer to today as “Apes of ancient aspect,” with all of those being extinct, and one lucky lineage that is not extinct, the “Apes of modern aspect.” (The latter get to be “modern” because they are more derived than the others.) You would be safe thinking of the latter as the “hominoids” and the others as “Miocene apes” since they mostly lived in the Miocene and the “hominoids” kept it simple by leaving very few fossils behind. Today, when we try to piece these apes together into a coherent pattern, to understand what form of ape gave rise to what other form of ape, we become confused and argue. If only we had a better idea of what the ancestral species of all these apes looked like, in order to test hypotheses about ape evolution in the early days.

Videoscan of the face of Saadanius hijazensis, a new genus and species of primate that lived in the Arabian Peninsula during the late Oligocene epoch, 29-28 million years before present. nsfnsf

Meanwhile, over on the Old World Monkey lineage, there was much less diversification and a relatively straight forward body plan easily represented by macaques or baboons persisted to modern times (though it is a bit more complex than that).

Thick solid vertical lines indicate known temporal ranges of taxa; thick dotted vertical lines show intervals of occurrence or possible extension for temporal ranges of taxa. Diagram and caption from original paper.

Now, here’s the rub: We have some fossils (Propliopithecoidea)from around 30-35 mya that show us what the ancestor to this complex series of developments look like, and we have what are essentially modern Old World Monkeys and Apes, and a fairly large collection of Miocene apes (of ancient aspect) post dating 23 million years ago. For the monkeys specifically, there are bits and pieces but mainly fossils that look pretty much like modern moneys and date to the last 5 mya or so.

In order to understand the evolution of a set of species, it is necessary to know about the nodes … the common ancestors of various sets of species. Ideally, we would have a good understanding of the population that gave rise to the Old World Monkeys and the Apes, so we could sort out differences among subsequent lineages with a knowledge of what specific traits are expected to be present in given animal because its ancestors had it. In other words, is a certain trait seen in one species and not another, sister species, because it was added by the first species, or lost in the second species?

There is a major difficulty in figuring this out with primates: The primates, with respect to untangling fossils, have three overwhelmingly important characteristics. First off, Many of the lineages are extinct and left incomplete fossil records. DNA can’t help us with them an the scrappy fossil are not exactly coming to the plate.

Second, the physical form of Catarrhini (many of the living and extinct apes and monkeys of the old world) is highly selected to adapt to arboreal lifeways. All species have selective forces working on them, but some selective forces are stronger and more overwhelming than others. The key characteristic of birds is flight. There are few features of flying birds that are not shaped directly or (barely) indirectly by the requirements of flight. The key characteristic of a moose is that it eats aquatic vegetation seasonally and lives in the snow the rest of the year. So it has long legs. Big whoop. The shape of, say, the top of the head of the moose is not adapted to this characteristic, but for birds, every part is shaped by the flight adaptation. Large mammalian arboreal species are not as constrained as vertebrates that fly, but they are fairly constrained. (I do simplify … some primates are not as arboreal as others, and the constraint I mention here is more severe in the New World where arboreality is much more intense.) The result of this constraint is that some features are either very conserved (once they emerge) or are converged on over time from multiple directions. This confuses us.

Third, we are talking about a long period of time with a spotty fossil record. Any kind of confusing convergence or random loss of a feature or other complexities that might occur over time is more likely in a very time-deep fossil record. Think of it this way: Monkeys and apes are evolving (from monkeys and apes) over and over across vast periods of time. In enough time to see the evolution of monkeys or apes from non-monkey or ape ancestors occur several times, the same lineages are traveling the adaptive landscapes altered by both random and adaptive forces that themselves are changing over time.

So, imagine a family tree with prosimians and New World Monkeys represented as lower branching events, and then two focal living groups, the Old World Monkeys and the Apes as two additional tips that we presume to join subsequent to the split with Old World Monkeys, but with no real fossil record at the point of that split. We can’t be sure of much of what is going on there. For instance, if a fossil that looks ape like (but this is only one bone or some teeth) is found from a deposit around 25 million years ago, does a particular feature of that fossil indicate that it post-dates the monkey-ape common ancestor (because it is novel, not seen in the last common ancestor) or does it indicate nothing other than membership in the Catarrhini (becuase the feature is seen in the last common ancestor)? Without a detailed set of information about the last common ancestor, we can’t say, and can thus not be sure of this fossil or what it tells us about the timing and nature of the ape-monkey split or other important questions.

So, Saadanius hijazensis is a missing link right? It represents the last common ancestor of Old World Monkeys and the Apes. Problem solved!

Well, no, unfortunately not, but Saadanius hijazensis is close to the last common ancestor and is thus very helpful. Saadanius hijazensis is considered to be close to the base of the ape-human clade. Not “the missing link” but very very helpful in understanding what the ape and monkey lineages we know of for later periods evolved from.

Saadanius hijazensis has a tubular ectotympanic (the bone that contains the canal runnig to the ear from the outside). This is a features that separates the Old World Monkesy from the earlier-split-off New World Monkeys, shared by the Old World Monkeys and living and Miocene Apes. So Saadanius hijazensis is a Catarrhini. Miocene apes have frontal sinuses, palates that are less uniform in size front to back than monkeys, really large male canine crowns, a few other esoteric tooth-related features, and are typically large. Saadanius hijazensis lacks these features, making it not an ape. Compared to the above mentioned really early primate fossils (from the Eocene), Saadanius hijazensis is similar but different in the ways one would expect if it was an Old World Monkey. And, the fossil seems to date from prior to, but just prior to, the monkey-ape split.

These facts together put Saadanius hijazensis near the common ancestor of Old World Monkeys and Apes, in both time and morphology. The fossil is found in the Arabian Peninsula, which at the time was part of Northeast Africa (the Red Sea did not exist yet) which places the fossil in space within a region (a very large region) thought to be the location of the evolution of these monkeys and apes. And, although this could quite accidently turn into a tautology, Saadanius hijazensis helps to pin down the timing of the split to about 23 to 25 million years ago, simply because it fits nicely with a morphology representing the pre-split form and dates to that period. This does not rule out an earlier split, of course, because Saadanius hijazensis could certainly represent an earlier evolutionary event. With respect to the timing of the monkey-ape split Saadanius hijazensis also provides hope that more fossils of this time period can be found, and although a single species may be hard to place in fossil space-time, a set of species can reveal a pattern that may make for interesting study.

Given this position, Saadanius hijazensis can help resolve conflicts regarding early Miocene apes, since Saadanius hijazensis approximates the ancestor from which they evolved, and help to understand (given further analysis) the behavioral biology of the Old World primates of that period. For instance, did the ancestor of the living apes have a flat face (like a gibbon) or a long snout (like many monkeys including baboons). According to my old friend and schoolmate, Laura MacLatchy, this fossil should cause us to lean towards the baboon-face model.

Here’s a Nature video about the fossil:

Zalmout, I., Sanders, W., MacLatchy, L., Gunnell, G., Al-Mufarreh, Y., Ali, M., Nasser, A., Al-Masari, A., Al-Sobhi, S., Nadhra, A., Matari, A., Wilson, J., & Gingerich, P. (2010). New Oligocene primate from Saudi Arabia and the divergence of apes and Old World monkeys Nature, 466 (7304), 360-364 DOI: 10.1038/nature09094

A University of Michigan press report on the find is here.

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3 thoughts on “New Primate Fossil Informs Us of the Ape-Monkey Split During the Oligocene

  1. Yup, need more fossils. Danged ancient bones – why aren’t there more of them just everywhere? Except for saying that it’s different enough from Old World Monkeys and the Apes to deserve its own branch, I doubt anything more can be said based on the fragments of this one specimen. For all we know it could be yet another evolutionary dead end with nothing to do with modern apes. Still, it’s just awesome to discover your own ancient monkey bones which are the first known of its type.

  2. That’s a cool USGS graphic – the spiral of the time periods. I’ve never seen that before. Thanks for posting it.

  3. I always find evolution an intriguing subject. Thank you to all those that contribute to a better understanding of human evolution.

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