A bit of terminology this time out – neomorphs. The name suggests that these have a new shape, but while that’s true somewhat by default, the term is generally used to describe new bones. These are new in the sense that they are novel structures that have arisen in an evolutionary lineage. Given that my target audience on the Musings is one with a degree of biological / palaeontological knowledge, I’ll brush right over what homology is in these parentheses (homology being the idea that the humerus in say, a human, is the same bone [in terms of evolutionary inheritance] as the humerus in a bat, and a dinosaur and a bird and so on) and move onto what this means in terms of homology.

Obviously not all animals have exactly the same skeletons. Variation aside, for a start things can be modified (hence bats can have much longer fingers than say humans). Bones can fuse together to form apparently new or highly modified structures (like clavicles fusing to form a furcula). Bones can be lost too (which is why modern horses have only one functional toe, but their ancestors three and their ancestors five). But bones can also be *gained* and these are the neomorphs.

In archosaurs at least, these turn up quite regularly. Pterosaurs have that unique wrist bone – the pteroid that supports (in some way) the propatagium, and the prepubes that sit, well in front of the pubes. Ornithischian dinosaurs have two – the predentary that makes up the front of the lower jaw and the palpebral that sits over the orbit. Ceratopsians, perhaps unsurprisinglty have a few more of their own with the epoccipitals and epijugal making part of their horns and frills.

One could also argue that the numerous repeatedly novel osteoderms that appear in stegosaurs, crocs, saltasaurs and others are all neomorphs too, and arguably the ossified tendons of the ornithischians. In a sense of course they are, but since we don’t give names to all those little bits, listing them all seems a bit frivolous. The terminology is as ever, a slave to us and not a master, and the term ‘neomorph’ is there as a convenience for us to use as appropriate. So while there is inevitably quite a bit of grey to its exact definition and usage, it’s still a handy term to have around to describe those novel elements that turn up.

3 Responses to “Neomorphs”

  1. 1 Mike Taylor 27/10/2010 at 9:53 am

    Is the pteroid not a carpal?

    • 2 David Hone 27/10/2010 at 10:08 am

      Well, possibly. But if so, which and what it is homologous to is far from clear.

      And the pteroid itself articulates with a second carpal, it does not articulate directly with the wrist but with another bone that articulates with the carpus (the lateral carpal). There is *probably* a full set of carpals^ in the wrist already so it looks like they added one somewhere, and this would most likely be the pteroid.

      ^ These fuse up in the adults so you only see a proximal and distal syncarpls and then the lateral carpal and pteroid plus the occasional sessamoid, but in younger versions there are about half a dozen carpal elements plus the pteroid, sessamoid and lateral carpal). That’s quite a few. (Based on the Kellner & Tomida Anhanguera monograph).

  1. 1 What can you do with a fragment? « Dave Hone's Archosaur Musings Trackback on 27/03/2011 at 12:14 pm
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