By the inspiring Ray Troll, the king of fish artists.
Remember fish? Today we revisit the true aquatic vertebrates to look at the big surprises of the last decade in gnathostome evolution. First, a review:


Squalus skull and arches from British Chalk Fossils

The gnathostome mouth:



The state of knowledge at the turn of the century

Gnathostome diversity:

In the year 2000, three major groups of unknown relationships were recognized:

A fourth group, Acanthodii was more of a puzzle. Maybe monophyletic, but presumed to be closer to Osteichthyes in any case.



Coccosteus from Wikipedia
Placodermi: Silurian to Devonian armored gnathostomes experienced a rapid worldwide diversification and sudden decline.

Placoderms were very diverse and occupied a wide range of ecological roles. Their specializations included:

Janvier, Phillipe. 1993. Early Vertebrates - find the two placoderms
Fossil record: Fragmentary records of placoderms appear in the Middle Silurian. This is followed by a rapid diversification. During the Devonian, placoderms were the dominant vertebrate group. Both marine and fresh water forms are recorded with a worldwide distribution except for puzzling absence in South American sediments. Placoderm diversity was greatly reduced by an extinction event in the Late Devonian. They were completely extinguished by the mass extinction event at the end of the Devonian. Thus, entire radiation took up only about 50 million years, but while it lasted, it was spectacular.


Placoderm surprise: Over the decades, numerous phylogenetic analyses have yielded radically different interpretations of placoderm phylogeny. These analyses may have differed in their:

But they all assumed that Placodermi was monophyletic, so there was no need to include a wide variety of other basal gnathostomes in the analyses. In such an analysis, is it even possible to test whether placoderms were monophyletic or not.

Martin Brazeau, 2009 has analyzed basal gnathostome phylogeny and found that placoderms are paraphyletic, with some groups, including the arthrodires being more closely related to living gnathostomes and others, including the antiarchs, being more basal.


Eugnathostome Diversity


Cladists until recently used the following monophyletic group names in referring to these creatures:


Synapomorphies of Eugnathostomata:



Ctenacanthus from Julius T. Csotonyi web site
Chondrichthyes - Cartilaginous fish (Silurian - Rec.)

Basic definitions:

Fossil Material: Chondrichthyans are arguably the oldest known gnathostomes, but their fossil record is poor because their bodies lack preservable hard parts that stay articulated when they die. Two notable exceptions:


Phylogeny:


Cladoselache fyleri model at American Museum of Natural History
Basal chondrichthyans: The earliest unambiguous chondrichthyan remains are from the Early Silurian, but consist of isolates scales. We get a good idea of what a primitive chondrichthyan looked like from Cladoselache: (Devonian) Earliest known chondrichthyan from numerous well preserved skeletons. Roughly half meter adult length. Superficially shark like, but hardly a proper modern shark.
Anatomically suited as fast pursuit predator with tall tail, narrow trunk, and finlets to reduce drag of tail base. Stomach contents can include fish swallowed tail first, euconodonts, and invertebrates. One odd feature is the reduction of the pelvic fins and absence of anal fins.



Stethacanthus from Wikimedia Commons
  • Some fossil chondrichthyans like Symmorida (Late Devonian - Pennsylvanian) had a tendency to develop sexually dimorphic display structures on their heads and anterior dorsal fins. Possible uses include:



    Spotted ratfish, Hydrolagus colliei from Wikipedia
    Holocephali - Chimaeras and kin:

    Hybodus sp. model at American Museum of Natural History
    Elasmobranchii: (Devonian - Recent) Proper sharks. Synapomorphy is a specialization of the branchial arches (don't ask). Earliest known elasmobranch is Antarctilamna from the Middle Devonian.
    Note: each dorsal fin has a fin spine. Although these are prominent in sharks, they are probably plesiomorphic for chondrichthyes as a whole. Cladoselache has them, as do many non-chondrichthyan gnathostomes.


  • Like placoderms, elasmobranchs have occupied many ecological niches and embrace great diversity. Some interesting cases include:

    From YouTube
    Reproduction: Although all living elasmobranchs practice internal fertilization and produce relatively small numbers of large developed young, they display an amazing variety of reproductive strategies.


    Take-home messages: