WHEN THE FISH CRAWLED OUT OF THE SEA (was Re: An end to miracles

[Pieter.Depuydt@rug.ac.be]

Betty wrote:
> Since I have an interest in living fish (I have 4 aquariums) it had
> just occured to me that all the land-crawling fish-types nowadays are
> fresh water fishes-mud skippers, lung fish, bichers, rope fish, etc-
> with only occasional land-crawling by such saltwater fishies as grunions
> (which flop about more than any actual crawling).  
>   Furthermore, the amphibians (supposedly very similar to the first land
> dwelling vertbrates) of today seem rather intolerant of salt exposures
> on their sensitive skins (and I don't believe they will lay eggs in
> alkaline pools of water in deserty areas either).

The classical view, which was mostly shaped by Romer, that tetrapods 
originated in a fresh water habitat originated from the fact that all 
living amphibians (Lissamphibians) are strictly tied to freshwater, 
and that in those times ancient 'fishes' were best known from the Old 
Red Sandstone and the Coal Measures, which were considered as 
sediments laid down in freshwater circumstances.

First, extant lissamphibians however may not provide a good model for 
the earliest tetrapods, as current research by Laurin and Reisz 
suggests that their origins may have to be sought much 'higher' in 
the tetrapod tree (tetrapods are here used to designate all 'limbed' 
vertebrates), i.e. within the 'lepospondyls' whether or not this is a 
monophyletic assemblage. This would mean that lissamphibians are more 
closely related to amniotes than to the old "amphibians" as 
temnospondyls, loxommatids, embolomeres etc..

Second, recent studies of the fossil assemblages in which the 
earliest tetrapods occur have casted doubts on the strictly 
"freshwater" nature of their habitat.
The Andreyevka fauna, where the primitive tetrapod Tulerpeton occurs 
together with antiarchs, groenlandaspids, osteolepiforms, dipnoi 
(lungfishes) has been interpreted, based on the sedimentology, as 
been laid down in an estuarine and even occasionally marine 
environment: i.e. probably a large tidal flat, with large deltas and 
frequent marine incursions.
(Lebedev and Clack 1993. Paleontology 36(3), p721-734)
In their description of Hynerpeton from Pennsylvania, Draeschler et 
al. describe the paleoenvironment as "a broad coastal plain with 
fluvial systems menging into a muddy shoreline of an extensive delta 
plain" (Daeschler, Shubin et al 1994. Science 265, p639-641). 
Ichthyostega and Acantostega are accepted as being preserved in 
fluvial sediments, but with tidal flats, lagoons and deltas nearby
(Janvier 1997. Early Vertebrates, Oxford University Press).
So it seems that at least some of the early vertebrates had their 
habitat in at least partially marine (estuarine, tidal) 
circumstances.

For the matter of completeness, the "crawling out of the water" of 
the earliest vertebrates in search for food or to escape a drying 
pond (the Romer view) is not longer accepted. As has already been 
pointed out on the list (i.a. by Dr. Holtz), recent study has 
revealed that the earliest vertebrates probably still were mainly 
aquatic animals, and that the primitive limbs and digits may have 
evolved in the first place to facilitate movement in very shallow, 
possibly weed- or algae choked waters (although it cannot be excluded 
that Ichtyostega and Acantostega were secondarily aquatic...)

Pieter Depuydt
(off-topic as always)