F Rosa Rubicondior: Something Fishy About Jaws!

Friday 21 October 2016

Something Fishy About Jaws!

Life reconstruction of Qilinyu, a 423-million-year-old fish from the Kuanti Formation (late Ludlow, Silurian) of Qujing, Yunnan, in Silurian waters.
Photograph: Dinghua Yang
Early fossil fish shows where our jaws came from - Uppsala University, Sweden

A couple more of those little unknowns that drives science forward may have been solved this week by a team from Uppsala University, Sweden and the Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, China.

The first is where did our jaws come from in our evolutionary past and the second is where exactly do the 'armour-plated', fish-like placoderms fit in with the evolution of bony fish and via them, the tetrapods, including us mammals. The Silurian placoderms lived over 400 million years ago and it was not clear whether they were the direct ancestors of the bony fish or a sister clade with the ancestral bony fish along with the cartilaginous fish.

The question of the origin of jaws is complicated because not all jaws are the same, even though they serve the same basic functions. The shark, skate and ray jaw, for example arises in the developing embryo from a cartilaginous arch resembling a gill arch and this simply grows into a jaw. The jaws of bony fish, begin as a cartilaginous arch but then new bones form outside this arch. These go on, in our case, to form the maxilla, premaxilla and dentary bones which make up the entire jaw and carry our teeth. These bones are found in the entire clade which includes mammals, crocodiles and cod, in other words, everything that descended from the bony fish including all the terrestrial and marine tetrapods.

What the Uppsala/Beijing team have found is additional evidence that the placoderms, which are known to have a similar set of bones but which was thought to form a 'gnathal plate', not a true jaw in the sense of the mammalian jaw, may in fact be an early version of the same structure after all. In fact, the accepted wisdom that this was not the same structure began to change in 2013 with the discovery of Entelognathus, a 423 million year old fossil fish from Yunnan, China, which combined placoderm skeletal characteristics with maxilla, premaxilla and dentary bones.

The discovery of Entelognathus revealed the presence of maxilla, premaxilla, and dentary, supposedly diagnostic osteichthyan bones, in a Silurian placoderm. However, the relationship between these marginal jaw bones and the gnathal plates of conventional placoderms, thought to represent the inner dental arcade, remains uncertain. Here we report a second Silurian maxillate placoderm, which bridges the gnathal and maxillate conditions. We propose that the maxilla, premaxilla, and dentary are homologous to the gnathal plates of placoderms and that all belong to the same dental arcade. The gnathal-maxillate transformation occurred concurrently in upper and lower jaws, predating the addition of infradentary bones to the lower jaw.

Min Zhu,Per E. Ahlberg,Zhaohui Pan,Youan Zhu,Tuo Qiao,Wenjin Zhao,Liantao Jia,Jing Lu
A Silurian maxillate placoderm illuminates jaw evolution
21 Oct 2016: Vol. 354, Issue 6310, pp. 334-336 DOI: 10.1126/science.aah3764

© 2016 American Association for the Advancement of Science. Reprinted with kind permission under licence #3973620723757.

This new evidence is in the form of a new species identified as Qilinyu from the same formation and era as Entelognathus and again combining a placoderm skeleton with maxilla, premaxilla and dentary bones, but with a different appearance and, presumably, a different lifestyle to that of Entelognathus. This suggests this structure was present in a shared ancestor to these two very different species.

As the authors say in their press release:

The simplest interpretation of the observed pattern is that our own jaw bones are the old gnathal plates of placoderms, lightly remodelled. It seems like substantial parts of our anatomy can be traced back, not only to the earliest bony fishes, but beyond them to the strange ungainly armoured placoderms of the Silurian period.

And this, of course, adds weight to the argument that the placoderms are not only directly ancestral to the bony fish but also to us.

Creationists who are under the impression that the scientific Theory of Evolution is a theory in crisis which is about to be overthrown in favour of magic (sic), might like to read this paper to see if there is any evidence to support this notion, rather than the idea that the Theory of Evolution is a coherent and complete explanation for the observed evidence.

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