Repost: Suminia: Life in the Trees 260 Million Years Ago

When I hear the phrase "early human relative" I cannot help but think of an ape-like creature. Something like Sahelanthropus fits the bill nicely - it may not be a hominin but it is still a close relative from around the time that the first hominins evolved. That is why I was a bit puzzled to see MSNBC.com parroting a story written by the Discovery Channel which proclaimed "Early human relative predates even dinosaurs"! Was this another fossil that would change everything? While not quite as startling as a Precambrian rabbit, a 260-million-year-old-hominin (or even primate) would certainly be a shocker!

The truth of the matter, however, is that the fossil described in the MSNBC story is only a distant relative of humans. Called Suminia getmanovi, it was a synapsid (the diverse group of vertebrates to which mammals and their closest relatives belong) that lived during the Permian in what is now Russia. More specifically, it was an anomodont, or a relative of the tubby Lystrosaurus and the small, tusked Robertia. The attempt to make Suminia relevant to human ancestry, therefore, was a quick and dirty way of grabbing attention, but in this case I think it stirred more confusion than enlightenment.

My gripes about the msm-coverage of this synapsid aside, it really was an interesting creature. While this is the first many people have probably heard of Suminia it is not a "new" fossil. It was originally described in 1994, and the new Proceedings of the Royal Society B paper that has stirred all the media coverage is based upon a jackpot of new, better-preserved specimens studied by paleontologists Jorg Frobisch and Robert Reisz. To understand why Suminia is making headlines, though, we need a little more background information.

As any good paleontologist knows different environments influence fossil preservation in particular ways. An oxygen-depleted lakebed may allow for exquisitely detailed fossils to be formed while animals that live in the canopy of forests are much less likely to be preserved. In this latter example, dead animals that make it down to the forest floor may be consumed or destroyed without ever being buried as there is no regular deposition of sediment to preserve them. Even if the body goes untouched by larger scavengers, detritivores like insects, fungi, and bacteria will still break the animal's body down. If a dead animal falls out of a tree into a nearby lake or stream it might be covered up quickly enough, but in general arboreal animals are rare in the fossil record because they did not live in environments amenable to good fossil preservation.

Thus the fossils of Suminia came as something of a surprise to paleontologists. Frobisch and Reisz determined that Suminia was probably an arboreal animal, and it is (so far) the earliest arboreal vertebrate known. It was clambering through the trees over 30 million years before the chameleon-like drepanosaurids were occupying a similar niche, for example, and it is astounding that so many remains of a tree-living synapsid have been found from deposits of such great age.

The new Suminia material consists of a single block containing the articulated remains of over a dozen individual animals. While such jumbles can be difficult to study they are also extremely informative, especially since variation between individuals from one location can be studied. Indeed, the individuals on the slab appear to range from sub-adults to adults, and the preservation of the bones suggests rapid burial caused by some sort of minor catastrophe.

This sort of excellent preservation allowed Frobisch and Reisz to study the anatomy of Suminia in detail. (All that was known of Suminia when it was described was a skull.) What the authors found most curious were the hands of the individuals, and it seemed that Suminia had hands and fingers similar to creatures that are arboreal. Suminia had very long fingers, large hands when compared to the rest of its arm, claw-like bones at the ends of its fingers, and a first digit set off at an angle to function like an opposable, grasping thumb. This is just the sort of arrangement that would be expected if it was grasping tree limbs to move through the canopy.

The hands of Suminia widely differed from those of terrestrial anomodonts and more closely resembled those of other arboreal vertebrates from various groups and time periods. It appeared that lineages that adopted an arboreal lifestyle evolved similar traits over and over again, these similarities being due to convergent evolution. Comparisons between Suminia and living arboreal vertebrates supported this hypothesis, and Frobisch and Reisz make a very well-supported case that Suminia moved through the trees by clinging and grasping.

Why the ancestors of Suminia moved into the trees, however, is another question. The Permian deposits in which Suminia was found is relatively well-sampled. During the time the little synapsid was skittering through the trees the local environment supported a large number of herbivores (about 83% of all the vertebrates present) and a much smaller number of carnivores (about 13% of all the vertebrates present). While it is true that an arboreal lifestyle may have helped keep Suminia out of the jaws of predators, the authors hypothesize that it was competition with other herbivores that might have driven some synapsids into the trees. These plants would have been a resource unexploited by other vertebrates, and this may have allowed for the evolution of Suminia from more terrestrially-adapted ancestors.

Dinosaurs and hominins make headlines much more frequently, but Suminia justly deserves its 15 minutes of fame. The problem is that Permian synapsids are not often in the news, so any time one is mentioned a considerable amount of background information is necessary to help people understand how they fit into the "tree of life." Indeed, "anomodont" is far from being a household word, but I hope this essay has provided a little more context for those who are curious about this strange creature from the distant past.

Jörg Fröbisch and Robert R. Reisz (2009). The Late Permian herbivore Suminia and the early evolution of arboreality in terrestrial vertebrate ecosystems Proceedings of the Royal Society B, Online First DOI: 10.1098/rspb.2009.0911