Sunday, 5 August 2018

False Deer-Llamas of Bolivia

(the nasal bones are that 'bump' on the forehead
just forward of the eyes)
Australia is an island continent, separated from the rest of the world's landmass for millions of years, allowing it to develop its own unique wildlife, from kangaroos and wombats to bandicoots and emus. The only other island continent we have today is Antarctica, which has no native land mammals at all (or emus, obviously).

In geological terms, however, South America was also an island continent until relatively recently, only joining North America three million years ago, towards the end of the Pliocene epoch. Even today, the strip of land connecting the two is only 35 miles (60 km) or so wide at the narrowest point, both narrower and longer than that connecting Eurasia to Africa. This means that, like Australia, South America had a long history of so-called "splendid isolation", and it evolved a number of unique animals in the process.

While some of the strange animals from this time did in fact, survive, which is why we have armadillos, sloths, and so on, the majority did not, dying out when they had to face competition from big cats, deer, bears, and other more familiar animals from the north. Among many such creatures were a group called the litopterns.

These were relatively large, herbivorous, creatures, most of which probably looked a little like llamas, and had a lifestyle similar to that of modern deer. However, while llamas are related to camels, and are among the groups that entered the continent from the north (where they subsequently died out), it's long been obvious that the litopterns were something else entirely. One key difference is that they typically had three toes on each foot (rather than two), and that they rested most of their weight on the central one, as three-toed horses once did, and tapirs still do today.

It turns out that that's not a coincidence; some of the last litopterns may well have survived past the end of the Ice Ages, and we have been able to recover proteins from their bones that demonstrated that horses and their kin are likely their closest living relatives today. Unsurprisingly, they must have separated from this line long before actual horses and so on existed, so they aren't the same group, but they are at least in that corner of the mammalian family tree.

[Note: The previous paragraph previously said 'DNA' instead of proteins. This was a mistake.]

The best known family within the litopterns, are the macraucheniids. Most of these animals are not very well known, leaving us enough remains to know that they existed, but not enough to tell us an awful lot about them. There are, however, two exceptions, that have left us relatively complete fossils. One is Macrauchenia itself, that last known survivor, and almost universally picked as the example when somebody needs an illustration of what litopterns in general looked like.

The other is Theosodon. First identified in the late 19th century, this is much older than Macrauchenia, and represents an earlier branch of the family tree. In fact, the macraucheniids are traditionally placed into two subfamilies; one that contains the later species, which are clearly related to one another, and another, including Theosodon, that are more older and more primitive, and probably represent a number of disparate evolutionary lines, all of which ultimately failed.

Most of these "primitive" forms lived no later than the end of the Early Miocene, about 16 million years ago, while the first of the "modern" forms (which happens to be something called Huayqueriana) appears around the beginning of the Late Miocene, just 7 million years ago. It's not quite true that we have nothing from the rather large gap in between these dates, but what there is fragmentary, and difficult to clearly identify.

Until now, that is, because a description of two new species of Middle Miocene macraucheniids from southern Bolivia has just been published. Significantly, these are more complete fossils than others of their kind from this era, and can perhaps tell us more about what was going on at the time.

The larger of the two newly described species has a number of similarities with known Theosodon specimens, although it also has enough differences, especially in the arrangement of the teeth, to indicate that it is a new species. Unfortunately, because the name Theosodon has been applied to so many fragmentary specimens, it's not completely clear what, if anything, really defines the genus as it's currently understood. As a result, the researchers who named the new species aren't absolutely certain that it really is a Theosodon, rather than something closely related, and have only placed it in the genus temporarily, until somebody can sort the mess out. For the time being, though, it's called "Theosodon" arozquetai.

The other species is distinctive enough that it can clearly be given an entirely new name: Llullataruca shockeyi, with the first half coming from the Quechua words for "false deer".  While it's exact position in the wider machraucheniid family tree is unclear, it does appear to be more "primitive" than Theosodon, more closely resembling much older species. It may therefore represent some late survivor of an earlier group that had long since gone extinct elsewhere on the continent.

So, "primitive" in what respect? There are two key trends in the evolution of macraucheniids over time. Perhaps he more obvious is that, generally speaking, they became larger as the geological epochs passed. Macrauchenia was the largest known species, and also the last to exist, while early species were generally much smaller. It's hard to know exactly how large an extinct animal was from its bones alone, especially when, as in this case, there are no close living relatives to compare it with. And, while these new fossils may be more complete than previously known ones of their age, there are still plenty of bits missing.

In the end, the authors of the paper describing the new fossils plump for an approximate weight of somewhere around 100 kg (220 lbs), somewhat larger than most fallow or white-tail deer stags, but noticeably smaller than a red deer or elk. This fits pretty well with what we'd expect, while Llullataruca is considered primitive in part because it's so much smaller; the estimate here averages around 45 kg (100 lbs), roughly the same as a typical fallow deer doe.

The second change in macraucheniids over time is an alteration in the shape of their nasal bones, which form the roof of the snout (or the bridge of the nose, in humans). These look fairly normal in Llullataruca, as they do in other early macraucheniids. In later species, however, they become increasingly short, which, since they're directly behind the nostrils, means that the nose slowly pushes its way back along the snout, away from the tip, and towards the forehead.

Once again, this pattern seems to be partially developed in the new 'T'. arozquetai, with the nasals being short enough that the nose must have been quite a bit further back than you'd expect for an animal with a snout, but nowhere near as far back as in the known later species. In those animals, it's proposed that's what going on is that they had a short trunk, perhaps similar to that of a tapir. Since the flesh of the trunk won't preserve in fossils, what appear to be the nostrils in the skull would actually be at the back of the trunk, while the ones that were visible in life would have been at the tip (however far from the end of the bony snout that might have projected).

It's perhaps unlikely that 'T'. arozquetai itself had a trunk, but it might at least have been part way along the path to developing one, with an exceptionally long fleshy nasal cavity that we can no longer see. Either way, this reinforces the idea that it was a very close relative to all those later, and generally more familiar, macraucheniids that you see in popular science books, and that we do have a glimpse here into how small, deer or llama-like animals evolved into the long-necked, short-trunked giants that would later struggle through the Ice Ages.

[Photo by "Ghedoghedo" from Wikimedia Commons.]


  1. I've been told that it's unlikely that Macrauchenia did have trunk because, IIUC, it has much less vascular foramina and whatnot to supply it than what tapirs and proboscideans do. Do you have anything to add on that?

    1. I have certainly heard that argument, although it's unclear what else is going on that might cause the nostrils to be so far back on the head. It's not as if something with that shape would be likely to breathe while partially submerged as seals do. But, yes, without the soft tissue, we don't know, and it might be a subject I'll return to some day.

  2. I had thought it was proteins, not DNA, from Toxodon and Macrauchenia, that was used in the linked Welker, Collins, et al paper?

    1. Quite right. I shall amend accordingly. Thanks!