Getting to know "Ida"

It has been three days now since an international team of paleontologists promised to deliver the change we need change everything, but when I woke up this morning I was pleased to find that things had still not gone "Bizarro World" around here. There is still a lot going on with Darwinius (better known as "Ida"), though, and while I am sure we will still be talking about her for some time to come I wanted to take a moment to step back and answer a few questions that keep cropping up about this spectacular fossil.

As I attempted to follow the frenzy over this fossil primate I noticed that many people did not understand how scientists knew Ida was a female and that she was not yet fully mature. How can you tell such a thing from a fossil? It is not like they come with a birth certificate. True enough, but there are tell-tale characteristics the paleontologists who studied Ida used to come to these conclusions.

First, how do we know that Ida was a female? It all comes down to a missing penis bone, or baculum. Many, if not most, mammals have a penis bone, and in fact our species is one of the "oddballs" in that males of our species do not. Take a look at the restoration of the transitional pinniped, Puijila, that was announced a few weeks ago. See that long bone sticking out from in front of its pelvis? That's a baculum, and the presence of such a bone indicates that this specimen of Puijila was a male.

While our species might not have a baculum, other primates do, including fossil ones. Darwinius lived alongside another kind of lemur-like adapid primate called Europolemur, and fossils from the same Messel shales show that male Europolemur had large baculums. Given their close evolutionary relationship between Europolemur and Darwinius it can be reasonably assumed that male Darwinius had baculums, too, but Ida's skeleton does not have a penis bone. Is it possible that this specimen of Darwinius could have been, pardon the expression, dis-membered sometime after death and before fossilization? It is possible, but given the exceptional preservation of the fossil, including gut contents and a body outline created by bacteria, it is doubtful. The lack of a baculum attached to this fossil makes it highly probable that Ida was indeed a female.

Second, how do we know Ida was not yet a mature adult? To answer that question, I'll need you to think back to your childhood days when you anxiously waited for the tooth fairy to come and leave a quarter under your pillow for your baby teeth. If you could look inside your jaws during this time you would probably be put off by what you saw; developing teeth in "crypts" beginning to push out your milk teeth as they grew. Indeed, for a time we all have more than one set of teeth in our mouths, even if we can only use one. Other primates go through the same thing (tooth eruption, not visits from the tooth fairy), including adapids.

The eruption of permanent teeth can be correlated to life stage events in primates. The eruption of permanent molars, especially, often indicate that the individual is no longer being nursed by its mother and is starting to acquire its own food. In Ida the first and second permanent molars had already erupted, with a third well one its way to formation, showing that while she was not an infant she was still pretty young when she died. Indeed, looking at her mouth as a whole she still has her "milk teeth" in the front of her jaws and you can see the crowns of her permanent teeth beginning to form underneath. It would be hard for her to be the "ancestor of us all", then, if she probably had not even reached sexual maturity when she died.

Even though I have been critical of the way this entire "primate roll-out" has been handled, I have tried to stress how amazing a fossil Darwinius is. The sex and age of a fossil might seem like unimportant matters, but how often do we get such a clear window into the biology of an extinct species? Right now the public is still being deluged with the message that Ida is the "missing link", but I hope that what Ida's skeleton can actually tell us about how she lived and died receive greater attention as we continue to discuss her bones.

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