Re: Could Plateosaurus engelhardti adjust its growth rate?

[K and T Dykes <ktdykes@arcor.de>]

<<"The evidence suggests that some plateosaurs had reached their maximum
size by the age of 12, while others were still growing at 27. The smallest
specimen was 4.8m long when fully grown, whereas others reached a giant 10m
long. This variation in growth rate and adult size has not been found in
other dinosaurs studied to date..."

With consequences for the warm-blooded/cold-blooded debate:  "The paper adds
weight to emerging evidence challenging the idea that the first dinosaurs
ran on two legs and were warm-blooded.">>

I wonder if it does, (bearing in mind that I'm far from convinced the first
dinos were 'warm-blooded').  Due to a whole slew of characters, it seems
safe to assume that cynodonts of a /Thrinaxodon/ grade were endothermic to
some degree, and probably a strong degree.  This was a high-performance,
fox-sized predator of the Lower Triassic known from southern Africa and
Antarctica.  If this assumption is correct, than endothermy is a reasonable
conclusion for all further derived cynodonts, (including myself), and the
vast majority are more derived.  (There's some evidence for endothermy
emerging in more basal therapsids, but never mind that for the while.

Certainly some endothermic cynodonts did not go in for determinate growth.
As they got older they got bigger.  I think I'll restrict myself to
mentioning two, but there are further examples.

1.  Fairly recently, leading vets for ailing taxa offered some much needed
treatment to predominantly Middle Triassic South American therapsid
carnivores.  This study of a broader sample of specimens concluded that
/Chiniquodon/, /Belesodon/ and /Probelesodon/ represent a growth series of
one genus (and two species) rather than three genera and six species.  That
isn't consistent with determinate growth.

Abdala F & Giannini NP (2000), Gomphodont cynodonts of the Chanares
Formation: the analysis of an ontogenetic sequence. Journal of Vertebrate
Paleontology, 20 (3), p.501-506.

2. Coming very close to the roots of Mammalia, (or Mammaliaformes if using a
crown-group approach), is /Sinoconodon/ from China and Europe; Upper
Triassic-Lower Cretaceous.  Many researchers accept /Sinoconodon/ as a
mammal, as the similarities vastly outweigh the distinctions.  However, it
was still doing a couple of freaky things.  One of this was serial
replacement of some teeth (albeit presumably much restricted).  There are
also several other aspects of the dentition which could fairly be termed:
"not quite mammalian enough for my tastes".

In 1994, Luo observed: "The alternating replacement of the incisors and
canines and partial replacement of the postcanines continued as the skull
increased in size. Based on this information, I interpret that the skull of
Sinoconodon experienced continuous slow growth, as in modern diapsid
reptiles and in nonmammalian cynodonts", (p.116)*.  I've not heard anybody
disagree with his conclusion.  This isn't a strategy of determinate growth,
but using that as a basis for ruling out endothermy would be simply
ridiculous.

I've got a few relevant figures, should anybody fancy testing their tape
measures, so I might as well include them.  The range of skull lengths for
/Sinoconodon/ from Yunnan runs from 2.2 to 6.2cm, and this gave a range of
bodymass estimates of 13 to 517 grammes.  This is big-mouse- to
big-rat-sized.  Compared to the other couple of further derived mammals (or
mammaliformes) in the fauna, the upper end of that spectrum is sauropodon
enormous!  The latest figures I've seen for /Morganucodon/ from Yunnan date
from 2001: skull lengths 2.7 - 3.8cm; bodymass estimates 17 - 89g.  I could
also mention /Hadrocodium/ but there's only a single (very possibly
subadult) specimen: 1.2cm; 2g.  Ah, ain't it cute?

* Luo Z-X (1994), Sister-group relationships of mammals and transformations
of diagnostic mammalian characters, p.98-128 (Chapter 6 of) In the Shadow of
the Dinosaurs - Early Mesozoic Tetrapods, (eds Fraser NC and Sues H-D),
Cambridge University Press.

I make so bold as to contend, that the fossil record shows a strategy of
indeterminate growth is compatible with endothermy.  Therefore, assuming
that strategy can only correlate with 'clood-bloodedness' (should anybody
wish to do so) would be unsafe.
Cheers
Trevor

Mesozoic Mammals, basal Mammaliaformes
http://home.arcor.de/ktdykes/morganu.htm
The Mesozoic - more than just the dinosaur