Re: [dinosaur] Elasmosaur prolonged tooth formation (free pdf) + Sauropterygia axial development

["David Marjanovic" <david.marjanovic@gmx.at>]

> Laura C. Soul and Roger B. J. Benson (2017)
>
> Developmental mechanisms of macroevolutionary change in the tetrapod axis: A 
> case study of Sauropterygia.
> Evolution (advance online publication)
> doi: 10.1111/evo.13217
> https://urldefense.proofpoint.com/v2/url?u=http-3A__onlinelibrary.wiley.com_doi_10.1111_evo.13217_full&d=DwIFaQ&c=clK7kQUTWtAVEOVIgvi0NU5BOUHhpN0H8p7CSfnc_gI&r=Ry_mO4IFaUmGof_Yl9MyZgecRCKHn5g4z1CYJgFW9SI&m=T8oLRi7TH3iUXUo1Qkzu9u4xdk3XziWzntfCV_XR4Wk&s=E30eP-s6Nejgg36luolJ8EU1jc_ztucjYyLZVESZjSk&e=
>  

This paper assumes that the somites form first, and then the Hox genes assign 
regional identities to the somites, so that every change in the number of 
cervical vertebrae has to be a homeotic mutation (a transformation of cervical 
into dorsal vertebrae or vice versa). Is that actually how it works? I'm told 
it's the other way around: first the Hox genes delimit the regions of the body 
axis, then the somite clock starts ticking – so that a change in the number of 
cervical vertebrae can be due to a change in the size of the cervical regions 
(a homeotic mutation), to a change in the speed of the somite clock (if it 
speeds up, more somites fit into the region – they'll be smaller than usual at 
first, but they can grow later), or of course to a combination of both.

I stress, however, that if all this were completely taken out of the paper, the 
paper would hardly suffer at all: it presents a large new dataset that will 
become very useful, and its reconstruction of the evolution of the numbers of 
vertebrae in the different regions of the sauropterygian body is highly 
interesting regardless of the causes of these changes.