Ida the Fossil Primate

You probably know that there is a new primate fossil, nicknamed "Ida," and that there is quite a buzz about it.

Ida comes from fossil deposits in Germany, and was originally excavated in two different parts by private collectors, and only recently rejoined and recognized for the amazing fossil it is. This is considered to be a new genus, and is named Darwinius masillae

Ida is a 47 million year old adapid primate of outstanding, unprecedented state of preservation that seems to have some very interesting and possibly unexpected features that could shed light on the evolutionary relationships among the extinct primates. But before we get to that, we need to cover some background on primates, extinct and otherwise.

The first thing you need to know is that order of living primates can be divided into two groups: the suborders Strepsirrhini and the Haplorrhini.

Strepsirrhini includes the lemurs of Madagascar, and the lorises, pottos and galagos of Africa and Asia. In other words, Strepsirrhini are the Prosimians, more or less.

The Haplorrhini include the old world monkeys and the new world monkeys, and the apes, as well as this one strange group called the Tarsiers (which used to be in the Prosimians, which has caused some confusion.) The Haplorrhini are the "Anthropoid primates," more or less.

Based on morphology and DNA and so on, it is believed that these two groups diverged from one another perhaps as far back as 80 million years ago (Murphy et al 2009). Subsequent to that time, the different smaller groups of primates (old world vs. new world monkeys, for instance) diversified.

Now, here is a basic problem that plagues primate evolutionary research. If you look at all the features that make a primate a primate (as opposed, say, to a tree shrew or some other mammal) using only living species, you get a workable set of features. If you take standard lemurs and, say, Old World Monkeys and you make a set of distinctions between those two groups, you get a reasonable set of criteria to distinguish among them. But, when you either add in Tarsiers (or some other primate groups) or start looking at fossils that are tens of millions of years old, it starts to get tricky. It becomes difficult to distinguish between convergence and common ancestry for certain traits. In other words, it is hard to tell if two traits are the same in two groups because the common ancestor of those groups had the trait and the specimens you are looking at both inherited this, or if the two lineages independently evolved the same trait.

"How likely is that to happen?" you may be asking yourself. Answer: Under some conditions, very very unlikely. Under other conditions, very likely. Let me explain.

Imagine we wanted to do a phylogeny of sedans. Once a line of car develops a square-back or pickup design, we eliminate it from our analysis. Sedans only. At the same time, some other research team is analyzing "powered vehicles" .... things that go with engines. This would include cars, trucks, boats, trains, and space ships. Even though both lineages may have been around for about the same amount of time, the sedan lineage would be much more prone to convergence because all sedans are almost exactly the same length, width, and height, have almost exactly the same number of seats, the same method for driving (like, auto vs. one stick) and so on, compared to the vehicles in the second study. The comparisons across vehicles that can carry nine tons of gravel, vehicles that can go under water, and vehicles that can fly are going to result in only the most trivial and easily exposed convergences.

Same with primates. At many important levels, all primates are the same. Compared to carnivores, all primates have almost the same pattern of teeth ... there are very few variants on tooth pattern among all the primates that exist today, but many many variants for the carnivores. Even body size is fairly restricted for primates. Yes, there are a few whopping big ones, but compared to the elephants or the hyracoidea, not so much. With only a few exceptions, primates live in moist to wet heavily vegetated environments. Compare this to antelope, who have water-dependent and water-independent species. And so on.

Perhaps because of this limited range of variation, or perhaps causing this limiting range, is the simple fact that morpholgically all primates are primitive. So there is not some group of primates where the radius and ulna are fused, and a different group where they are not. All primates have unfused radius and ulna. There are not primates with vs. without some kind of grasping hand (I simplify slightly here). All primates have the same number of fingers and toes. In comparison, for instance, carnivores have varying numbers of toes and different patterns of bone fusions among them.

So that is the background. If you are going to look at ancient fossils, you've got a very conservative set of lineages so a) you'll always recognize a primate, quite easily, when you see one and b) convergence will haunt you for your entire life if you are a paleo-primatologist.

For various reasons, especially the item noted above about the moist heavily vegetated habitats, but also the sparseness on the landscape and lightly built skeletons, primates make lousy fossils. There are very few places in the world where we have primate fossils, and they are much restricted in geographical space and time range. In short, the primate fossil record sucks. The primate fossil record is mostly teeth, and the teeth look all the same. All the work I've done in the primate fossil record has been in the Miocene, and that is not so bad. There are better conditions: You get some real fossils, even postcranial (body) bones, sometimes. But the pre-Miocene record is really scary.

Paleontologically, primates are anatomically ambiguous ghosts.

Within the fossil record, there are probably four or five sets of fossils that represent groups different from living primates, but most "crown" fossils can be placed into two major groups: The adapids and the omomyids. If either of these two groups could be linked to later, living primates, this would have to be done using a number of physical characteristics of the bone ... there is not much chance of finding ancient DNA in these very old fossils.

This part of the fossil primate record includes the Adapoidea, including the widespread Adapids and some other fossil groups, but no living forms, and the Tarsioidea, which includes the fossil Omomyids, some other fossil forms, and the living Tarsius. The Adapids and Omomyids date mainly to the Eocene about 55 to 34 million years ago. Note that this is well after the Haplorrhini-Strepsirrhini split. Over the years, various groups of primatologists have attempted to link either of these groups to the major living groups of primates.

One subgroup of the Adapoidea is the Cercamoniinae, identified in the 1970s by one of the present paper's authors, Philip Gingerich. This group exhibits a few traits that seem to link it with the living Haplorrhini (monkeys and apes). For example modern monkeys often have a premolar that is shaped a certain way to "hone" the canine. This is a bit unusual, and is a marker for this kind of modern primate. Something that looks like such a tooth appears, more or less, among the Cercamoniinae. This sort of connection (and other factors) has led some (Gingerich included) to link the Adapoidea in general to the modern anthropoid primates (monkeys and apes). Others disagree.

The fossil being reported now, Ida, is grouped by the authors into the Cercamoniinae. If that phylogenetic conclusion ends up being verified by further study, this excellent, well preserved fossil will be an important touchstone in interpreting early anthropoid (non-lemur) primate evolution and behavioral ecology.

The phylogenetic argument that is being made in this paper is admittedly preliminary (more work is promised on this) but so far it is very tricky and is likely to remain tricky. The structure of the prior arguments that links either of the two main fossil groups to either of the two main living groups has always been tenuous. The reason for this, as I've alluded to above, is that those two fossil groups fall about half way in time between the present and the original split of these groups, and there are not enough fossils between 80 and 55 million years ago to understand the details of that early split. Then, there are not enough fossils from about 34 million years ago to recent times to understand this later period during which the modern forms arose. While more work will be done on the phylogenetic relationships, considering that the contemporary fossils .... the old fossils roughly of the same age as "Ida" ... are mostly teeth, and tens of millions of years separates Ida from the modern forms, I do not expect much more in the way of a resolution until more fossils like Ida, but of different species, are found.

Getting away from phylogeny, let's have a look at other aspects of this fossil. This is a remarkably well preserved specimen. The animal probably died from volcanic gas (like C02) and fell into the water, and was slowly buried in fine sediment. Once encapsulated, the body began to rot, and the slime layer that started out as the animals' flesh, skin, and fur approximated the outer surface of the body and was preserved in the fine sediment as well. The sediments in which this fossil is preserved are compressed, so the entire skeleton is uniformly crushed, slightly, but everywhere, affecting every bone. In order to visualize and measure the specimen, fancy 3D imaging and image processing techniques were applied.

The animal was a juvenile female and weighted between 385 and 580 grams, and would have grown up to be about 660 grams by one estimate, or a whopping 1600 or 1700 grams by another estimate. Since the larger estimate is based on molar size, which in turn can be secondarily influenced by adaptations to diet, I'd go with the lower estimate. Indeed, using the brute force method of holding the fossil up to full scale pictures of living primates to find one that matches, the estimated adult body size is about 845 to 892 grams.

The contents of the digestive track were preserved (that is extraordinary) and include leaves and fruit. There are no insect remains in the gut.

You are going to hear more about this fossil, as more analysis is done. I have two meta-remarks to make about this finding. First, there was a lot of hype about how this fossil is a "missing link" and so on and so forth. That was overdone. But equally overdone is the reaction to the hype. Almost every blog post or other secondary report I've seen on this has talked almost as much ... or more ... about the hype than about the fossil. Second, please note that this find was reported in PLoS ONE, an open access on line journal. This means that you can see the report yourself, look at all the cool pictures, and try if you must to slog through the highly technical text. This is big for Open Access publishing.

Franzen, J., Gingerich, P., Habersetzer, J., Hurum, J., von Koenigswald, W., & Smith, B. (2009). Complete Primate Skeleton from the Middle Eocene of Messel in Germany: Morphology and Paleobiology PLoS ONE, 4 (5) DOI: 10.1371/journal.pone.0005723

Murphy, W., Pringle, T., Crider, T., Springer, M., & Miller, W. (2007). Using genomic data to unravel the root of the placental mammal phylogeny Genome Research, 17 (4), 413-421 DOI: 10.1101/gr.5918807

There is a LOT of blogospheric and media coverage on this find. I was going to provide a list of links and then realized that there is only one good way to do this ... I'm sending you here, to A Blog Around the Clock. Bora has every single link.

All of the images used above come from the original on line article.