The restored lower jaw of Arcanotherium (formerly Numidotherium savagei). (From Delmer, 2009)
Unlike the folks at this past weekend’s ScienceOnline 2010 meeting, fossils don’t come with nametags. The identification of preserved bits of ancient life relies upon careful comparison with what is already known, a task made all the more difficult for vertebrate paleontologists by the fragmentary nature of many of their subjects. Scraps of bone given one name could turn out to be parts of another partial skeleton given another name, or other bits of bone attributed one name could turn out to be something entirely new. Reexaminations and revisions are ongoing, and one effort to rearrange a petrified puzzle led to the origin of a new elephant from an old one.
Since the beginning of the 20th century the place to took for the early relatives of elephants has been northern Africa. There, in the Fayum desert of Egypt and in Dor El Talha, Libya, numerous proboscideans (the group to which elephants belong) have been discovered. At first it seemed that the fossils could be organized in a step-by-step fashion, documenting a linear march of elephant evolution, but continued discoveries have presented scientists with a diversity of strange early elephants quite different from the paltry collection of living species. Among this early radiation was Numidotherium, a medium-sized genus known from a nearly-complete skeleton.
Even though Numidotherium koholense was originally described on the basis of fossils discovered in Algeria, the deposits of Dor El Talha yielded very similar bones (themselves found alongside the bones of a close relative, Barytherium). They were distinct enough to lead paleontologist Nicholas Court describe them as a new species, Numidotherium savagei, in 1995, but according to a new study by Cyrille Delmer these specimens may be so different that they deserve a new genus name. As described in the journal Acta Palaeontologica Polonica the Dor El Talha bones do not belong to Numidotherium at all but a new genus of fossil elephant, Arcanotherium.
Given the history of the bones used to describe the mammal, it is little wonder that Arcanotherium remained hidden for so long. Many of the original specimens from the locality in Libya were not given official catalog numbers, a good number of them had originally been attributed to other early elephants, and altogether the recovered parts did not even come close to representing the complete skeleton of the beast. Among the numerous fossils Delmer attributes to Arcanotherium are enough bits to comprise most of the lower jaw, bits of the shoulder, parts of the lower limbs, a few vertebrae, and a small collection of teeth. Not bad, but not quite enough to make it stand out unless you really know what you’re looking for.
When Delmer studied the fossils, though, it became clear that the anatomical details did not fit with those of Numidotherium as well as had been previously supposed. The nearly fully-restored lower jaw was a good example of this. It had a long Y-shape, akin to that of a divining rod, and in the front there were spots for two sets of incisors. This most certainly marked it as an early proboscidean as members of this group of mammals had the number of their incisors reduced during their evolutionary history, with many later forms (including modern elephants) retaining only the modified second incisors commonly called “tusks” in the upper jaw. (Though, as Delmer notes later in the same paper, it may be that the tusks in the lower jaw are derived from the first incisor while those in the upper jaw are modified second incisors. Different proboscideans throughout history have had different complements of upper and lower tusks.) Altogether this arrangement makes the lower jaw of Arcanotherium appear more similar to those of later “shovel tusked” proboscideans than the weird, early types that it lived alongside.
Furthermore, the original description of the bones by Court did not seem to rest on solid anatomical comparisons. Some of the traits Court used to distinguish genera and species could not actually be compared due to a lack of fossils and other characteristics taken to signify the uniqueness of the taxa in question were present in close relatives, too. After sorting through all this Delmer ran a cladistic analysis of the proposed Arcanotherium fossils. If it was a distinct genus as hypothesized then it would occupy a distinct place in the early proboscidean family tree.
A phylogenetic tree of early proboscideans, with Arcanotherium between Barytherium and Moeritherium. (From Delmer, 2009)
As Delmer proposed, Arcanotherium was recovered as a distinct genus, but it was not a close relative of the true Numidotherium species from elsewhere in northern Africa. Instead it nested between the large, still poorly-understood genus Barytherium and the smaller, semi-aquatic genus Moeritherium. This arrangement suggests that forms previously taken to be ancestral to elephants, such as Moeritherium, were probably too specialized to be good transitional forms. Instead the transition between the radiation of early proboscideans and the later, more mastodon-like forms was affected by creatures similar to Arcanotherium. As Delmer notes, there is still plenty of work to do, but enough has already been found to start a serious debate about this early part of proboscidean history.
Delmer, C. (2009). Reassessment of the generic attribution of Numidotherium savagei and the homologies of lower incisors in proboscideans Acta Palaeontologica Polonica DOI: 10.4202/app.2007.0036