FILTER FEEDING REPTILES

[darren.naish@port.ac.uk]

On filter feeding in Mesozoic reptiles.. so far as we know, no 
Mesozoic marine reptile had the required specializations to be an 
obligate plantivore [or planktonivore.. take your pick] as are 
megachasmid, cetorhinid and rhincodontid sharks, and mysticetes. The 
reptiles all lack both a filtering apparatus and a hugely spacious 
mouth (e.g., evolved by way of a proportionally enormous head or 
tremendously vaulted jaws and palate). That reptiles cannot be filter 
feeders, as was argued by Collins and Janis in _Ancient Marine 
Reptiles_, isn't quite right because everyone agrees that 
ctenochasmatid pterosaurs were filter feeders, plus of course 
flamingos are filter feeders too:) Also, what on earth were mesosaurs 
doing? 

Obligate filter feeding and the presence of a baleen-like structure 
was hypothesised by Carroll and Dong for _Hupehsuchus_, an animal 
that Mike Lee and Ryosuke Motani have recently regarded as a basal 
ichthyosaur. Carroll and Dong's speculation probably isn't 
right, but it was interesting.

As goes facultative plantivory, it looks plausible that a number of 
plesiosaurs practised this mode of feeding in much the same way as 
some living seals, such as _Hydrurga_. _Hydrurga_, the leopard seal, 
is famed as a macropredator that tears penguins and other seals to 
pieces, but in some studies krill are found to be a major part of 
its diet. _Lobodon_, the so-called crabeater seal, is meanwhile famed 
for eating krill. What's interesting here is that, according to 
Michael Bonner (seal ecology expert), neither of these seals 
really 'filter' their planktonic prey in the way that people have 
long assumed: instead, they apparently individually grab prey items, 
hold them up against the palate with the use of the tongue, and then 
grab another. If this is correct, it means you do not need 
extravagant sieving teeth*, or a baleen analogue, to effectively feed 
on plankton. *Both of these seals do have multicuspate posterior 
teeth that interlock, but this apparatus is not as effective as 
baleen.

Fine intermeshing teeth that might aid in the trapping of plankton 
while water is expelled are seen in cryptoclidid plesiosaurs - most 
notably in the British mid Jurassic _Kimmerosaurus_, the Antarctic 
Cretaceous form _Morturneria_ (formerly _Turneria_), and the South 
American ?Maastrichtian _Aristonectes_ (the one Dan was trying 
to remember the name of). _Aristonectes_ has something absurd like 
200 teeth and its jaws are bowed outwards, forming a rorqual-like 
inverted V. If it wasn't filter feeding, I don't know what it was 
doing (incidentally, known only from drawings done by Cabrera in the 
1940s [where is the material??], _Aristonectes_ has been variously 
interpreted as an aberrant elasmosaur or pliosaurid - it is the new 
stuff from New Zealand that Craig Jones, Ewan Fordyce and Arthur 
Cruickshank are working on that apparently shows it is a 
cryptoclidid). Therefore, at least some plesiosaurs may have been 
filter feeders, but relatively crude ones compared to things like 
sharks and baleen whales.

HOWEVER, it may be that there weren't so many Mesozoic filter feeders 
because, quite simply, there wasn't so much available plankton. For 
starters, cold-water currents like the circum-Antarctic current  
(responsible for most of the world's extant net plankton tonnage) did 
not exist, and plantivorous invertebrates like cephalopods were 
phenomenally abundant and may have 'taken' the ecological role of 
plantivore from macrovertebrates. This assumes that ammonites were 
filter feeders, a controversial theory that I know at least some 
ammonite workers staunchly defend. Also, if this is correct, it sets 
a remarkably precedent for Mesozoic ecosystems: little creatures 
controlling energy flow - a marked contrast to what appears to have 
been happening in terrestrial ecosystems (big creatures controlling 
energy flow). Also, does the Cainozoic (with its cetaceans) represent 
a switch to big plantivores 'stealing' the plankton prey from the 
little plantivores? 

It might also be that plantivorous marine reptiles were rare or 
nonexistant because big fishes were already doing the job. In the mid 
Jurassic, the gigantic pachychormid 'holostean' fish _Leedsichthys_ 
was a certain planktivore. As it grew to something between 15 and 30 
m (biggest fish of all time), and is reportedly common at some Oxford 
Clay localities, it might have had a major ecological effect on 
plankton populations. A new species of _Leedsichthys_, _L. 
notocetus_, was described last year from Chile. Dave 'Leedsichthys' 
Martill therefore refers to _Leedsichthys_ as a 'global planktivore' 
(though, of course, we do not _know_ that its distribution was 
actually global). As yet another aside, _Leedsichthys_ must have had 
a massive effect on other aspects of Jurassic marine life. It would 
have provided prey for macropredators like pliosaurs and some 
metriorhynchids, its floating carcasses would have produced tons of 
carrion for sharks and all manner of other creatures, its dung would 
have fertilized the sea floor, its probably millions of eggs and fry 
would have resulted in feeding frenzies amongst other Oxford Clay 
fishes, and its carcasses, when on the sea floor as a 'deadfall', 
would have produced rich sea floor 'islands' colonized by diverse 
invertebrates. Unfortunately we can only speculate about the 
complexity of these extinct food chains, while gazing in wonder at 
the living ones. 

In the Cretaceous, filter feeding sharks evolved: rhincodonts (whale 
sharks) first appear in the late Cretaceous (a new one was described 
posthumously by Nessov last year I think) and there is *supposed* to 
be a giant plantivorous shark in the Pierre Shale (I have even heard 
it called the 'Cretaceous megamouth'). This shark appeared in a Doug 
Henderson painting (together with an elasmosaur), but does anyone 
know any more about it?

DARREN NAISH 
PALAEOBIOLOGY RESEARCH GROUP
School of Earth, Environmental & Physical Sciences
UNIVERSITY OF PORTSMOUTH
Burnaby Building
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Portsmouth UK                          tel: 01703 446718
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http://www.naish-zoology.com]