by Greg Mayer
In a paper in press in Nature, Min Zhu and colleagues describe a new species of placoderm from the Silurian period of China. Placoderms are an extinct group of (usually) heavily armored jawed fishes that lived in the Silurian and Devonian. The new species is based on a beautifully preserved 3-D specimen, and is interesting, but it is being widely misreported in the press.
To understand why this new species is interesting requires some background information. First, we need to know that while most modern vertebrates (backboned animals such as ourselves) have jaws, and are called gnathostomes (“jaw mouths”), not having jaws is the primitive condition (jawless vertebrates, represented today only by hagfish and lampreys, are called agnathans). The origin of jaws is thus a key episode in the vertebrate story.
Second, we need to know that there are four great groups pf gnathostomes, the placoderms, the acanthodians (another extinct group, often called ‘spiny fish’), chondrichthyans (cartilaginous fish: sharks, rays, and their relatives), and osteichthyans (bony fish: tuna, gars, goldfish, etc.; the tetrapods are descended from osteichthyans, and for our purposes can be included with them).
And finally, we need to know that the vertebrate skull is a composite of bones from three different sources: the chondrocranium, bones preformed in cartilage that surround the brain, the splanchocranium, bones preformed in cartilage that support the gill arches, and the dermatocranium, bones that ossify directly and cover most of the outside of the skull. Gnathostome jaws are formed by the anteriormost bones of the splanchocranium (the palatoquadrate in the upper jaw, and Meckel’s cartilage in the lower jaw), which are often covered over or replaced by dermal bones in development. (Chondrichthyans, lacking bone, have only the first two components in their skulls.)
Okay, so what’s interesting? Placoderms have jaws, including the palatoquadrate and Meckel’s cartilage, which are accompanied by dermal bones that have usually been thought not to correspond very precisely to the dermal bones of osteichthyans. In the new fossil, Zhu and colleagues identify some dermal bones as being the same as in osteichthyans, most prominently the maxilla in the upper jaw and the dentary in the lower jaw (see first figure above). This is what’s interesting, because if true, it would mean that the osteichthyan condition is more widespread than previously known, and thus perhaps change some of our ideas on the relationships of the various gnathostome groups.
Another thing Zhu and colleagues do is a phylogenetic analysis of 75 taxa with 253 characters, but unfortunately for them the results are quite muddled, with no clear evidence that the ‘maxilla’ or other dermal jaw bones of the new placoderm are homologous to those of osteichthyans. These large data set analyses rarely produce convincing results, because it is the interpretation and analysis of the individual characters that most strongly influence the results, and these individual analyses are usually de-emphasized (or as in this case, hidden in the online supplement).
So where has the press gone wrong? First, some commentary, especially by scientists, has been directed toward the differing trees gotten by Zhu and colleagues, versus the one obtained by Davis et al. (2012) last year using essentially the same data set. This is inside baseball– the relationships among the four great gnathostome groups is quite interesting, but this paper does not resolve the question.
Popular media have been implying that jaws were not previously known in placoderms or fish in general, or that we would not expect jaws in fish this primitive or early. None of this is right. Placoderms are jawed fish, they are not the oldest known jawed fish, and the bones in this specimen do not apparently show a new or previously unknown condition (rather, the claim is that the condition in this fish resembles an already known condition). There is definitely something of interest here, but it’s not quite all that the media are portraying it as.
Davis, S. P., Finarelli, J. A. & Coates, M. I. 2012. Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes. Nature 486: 247–250.
Zhu, M., X. Yu, P.E. Alberg, B. Choo, J. Lu, Q. Qu, W. Zhao, L. Jia, H. Blom and Y. Zhu. 2013. A Silurian placoderm with osteichthyan-like marginal jaw bones. Nature in press.