GHOSTS, GAPS AND CHAMPSOSAURS

[darren.naish@port.ac.uk]

Pete Buccholz wrote...

> Again on this list, there are shouts that the mere thirty-million 
> year gap between _Deinonychus_ and _Archaeopteryx_ is indicative of 
> archaeopterygid ancestory of dromaeosaurs, rather than something 
> like the reverse.  This is clearly absurd.  Aside from the Bathonian 
> dromaeosaur teeth from the UK, the time gap is really not that big.

Surprise suprise, the example Pete gave to illustrate such a gap was 
the basal ornithopod _Bugenasaura_: clearly a primitive animal that 
Pete thinks (correctly) split off from all other ornithopods even 
before _Agilisaurus_ did. But the presence of _Bugenasaura_ in the 
Maastrichtian shows that there is a ghost lineage - representative of 
this animal's direct ancestors - that must extend back to the 
Jurassic.

OK, but a way better example is provided by champsosaurs (or 
chirostoderans). Champsosaurs (amphibious, gharial-like diapsids) 
were first described from late Cretaceous rocks of Montana by Cope in 
1876. Later, champsosaur fossils were found in the early Cainozoic, 
thereby proving that they survived across the KT boundary. Their 
geological range was not of a particularly significant length. 
However, in 1989 Susan Evans described champsosaur fossils from the 
Bathonian (mid Jurassic) - pulling the fossil record of these 
reptiles way way back and showing that they had a massive mid 
Jurassic----->late Cretaceous ghost lineage. Even more remarkable, 
and generally overlooked until very recently (Storrs, Gower and Large 
1996), though it was described as a champsosaur by von Huene in 1935,
 is that an Upper Triassic fossil - _Pachystropheus_ - also seems to 
be a champsosaur: this extends the fossil record of these reptiles 
back from the Bathonian another 45 Ma! Another ghost lineage for 
champsosaurs. 

But the surprises don't stop there: Max Hecht has recently 
(1992) described a champsosaur from the *Oligocene*. In recognition 
of Jablonski's work on ghost lineages and Lazarus taxa, he named it 
_Lazarussuchus_. Amazingly, cladistic analyses including all of these 
champsosaurs (Evans and Hecht 1993) indicates that _Lazarussuchus_ is 
actually very primitive, more so than Cretaceous champsosaurs. The 
genus itself, therefore, has a ghost lineage extending back from the 
Oligocene to, at least, the Cretaceous. Wow. 

Why champsosaurs are so good at phasing it and out of the fossil 
record I have no idea. However, ghost lineages are not indicative of 
incompleteness of the fossil record. I am rushed for time right now 
and have to leave, but I have to mention this paper..

BENTON, M.J. and STORRS, G.W. 1994. Testing the quality of the fossil 
record: palentological knowledge is improving. _Geology_ 22: 111-114.

"What is it with you and motorbikes?"

DARREN NAISH
darren.naish@port.ac.uk