Ghost lineage

Darren Naish

Among the arguments against birds being a type of dinosaur is the argument that the most birdlike of the dinosaurs, and therefore the taxa closest to birds, appear in the fossil record after the first known bird, Archaeopteryx. This is not a valid argument due to preservational bias in the fossil record. For example, an animal may have first evolved in an upland forest, where fossilization is highly unlikely, and it took millions of years before an individual wandered into, and died in, an area where it could be preserved.

A typical protest against the above refutation is that there would have to be a fossil gap of tens of millions of years for the refutation to hold. Although this period is unusually long, if it exists (and recent discoveries suggest it does not), it is not unique. Darren Naish wrote of one such example involving animals called Champosaurs. -- ed.

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 Cenozoic, 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 [a lineage for which no fossil record exists but is inferred by two related fossils separated by a large amount of time -- ed.]. 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 [taxa for which specimens are separated by large amounts of time with no intervening fossil record, thus making it seem as though the later specimens "rose from the dead" like Lazarus did in Christian mythology -- ed.], 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 in 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: paleontological knowledge is improving. Geology 22: 111-114.