Re: Spinosaurus - the authors respond

[Tim Williams <tijawi@gmail.com>]

Dale McInnes <wdm1949@hotmail.com> wrote:


> Holy Crap !! I didn't .. even for a moment=2C think about that. That would
> make post Cenomanian sediments potentially the most exciting late K. deposits 
> on
> the planet.


If (and this is a super-ginormous *if*) spinosaurs did give rise to
fully marine descendents, these critters could theoretically be found
anywhere in the world, not just Africa.  Alas, it may well be that
_Spinosaurus_ was an evolutionary dead-end, and its lineage went
extinct on the cusp of returning to the sea.  That seems to be how the
cookie is crumbling.


Across the spectrum of other non-avian dinosaurs, the prospects of
other semi-aquatic or aquatic lineages look fairly slim.  The burly
African iguanodont _Lurdusaurus_ has been touted as a possible hippo
analog, but the original description favored a more ground-sloth-like
(and implicitly terrestrial) ecology (Taquet and Russell. 1999):

  "In its squat posture the animal must have resembled ankylosaurs.
   However, its relatively small skull, circular chest, powerful and
   heavily clawed forelimbs, transversely flattened femoral shaft and
   generally massive skeletal elements probably even more strikingly
   recalled the form of giant ground sloths."


In passing, Bakker et al. (1992) proposed that _Spinosaurus_ was one
of several large African theropods that might have been "marine
predators" and thereby paralleled early cetaceans.  This assessment
might be prescient for _Spinosaurus_; but there is no evidence that
_Carcharodontosaurus_ or _Bahariasaurus_ (which is likely the same as
_Deltadromeus_) shared _Spinosaurus_'s adaptations for a
(semi-)aquatic lifestyle:

  "An ecological sidebar to the acrocanthosaur cycle is the complex
   of gigantic aquatic theropods in North Africa in the Early Cretaceous
   and Cenomanian. _Spinosaurus_, _Bahariasaurus_, and
   _Carcharodontosaurus_ were described from incomplete remains
  preserved in fluvio-deltaic and near-shore sediments (Stromer, 1915,
  1931, 1934). Body lengths and femoral lengths are in the range of
  _T. rex_. The long, crocodile-like snout of spinosaurs and the
  shark-like teeth of carcharodontosaurs indicate piscivorous habits,
  and hence these giant theropods may be a radiation of marine
  predators analogous to that of the early seals and toothed whales
  of the Tertiary. Both hindlegs and tail probably were used in
  swimming in theropods."


Finally, I've wondered if the peculiar ankylosaur _Liaoningosaurus_
might have been amphibious, on account of its extensive ventral armor
(abdominal plate).  In turtles (Testudinata), the evolution of a
ventral dermal armor (plastron) has been used as an indicator of
aquatic habits (e.g., Rieppel and Reisz, 1999; Li et al., 2008 - the
description of _Odontochelys_).  But on the basis of ventral armor
alone, even I'd say that the concept of _Liaoningosaurus_ as an
'aqua-ankylosaur' is a stretch.






Cheers

Tim