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Great white shark from The Guardian

Engima: - the origin of jaws

Gnathostomata - the jawed vertebrates combine a great range of evolutionary novelties. The odd thing is that there is no fossil of a partially jawed vertebrate.



Radinsky, Leonard, 1987 Evolution of Vertebrate Design




Squalus skull and arches from British Chalk Fossils

The Big News: The first two arches are specialized

This list invokes some challenging problems in gnathostome evolution. These are take up below, after an anatomy review.

Jaw anatomy: Up until now, when we have spoken of a "skull" we have meant only a plating of dermal bone around the head, or, in the case of Galeaspids and Osteostraci, that plus in endochondrally ossified neurocranium or braincase. With the appearance of the gnathostome jaws and branchial skeleton, the skull becomes a complex composite structure. The following illustrations show its components using the fossil bony fish Eusthenopteron as an example.



Orthacanthus sp. cranium
In sharks, which lack dermal skull roofs, the relationship of the braincase, palatoquadrate, hyoid arch, and Meckel's cartilage are easier to see. Ventral view

Origin of jaws: Many gnathostome synapomorphies are continuations of longstanding vertebrate trends toward increased skeletal ossification, brain enlargement, and improvement of swimming. Big exceptions are changes to the pharynx, especially jaws. We have:

Oddly, fossil intermediaries between jawed and jawless vertebrates just don't seem to exist in the fossil record. What's up?

Four general hypotheses for this radical transformation:

In any event, changes in inductive relationships between neural crest ectoderm and other tissues were probably involved in the rise of both: A side note: The velar skeleton of the ammocoetes larvae of lampreys has both internal and external components that might be homologous to the internal branchial arches of gnathostomes and the external arches of lampreys.

Gnathostome diversity:

Traditionally, three major groups of unknown relationships were recognized:


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
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.



From the SUNY Orange Online Biology Library
What placoderms lacked:

The myomeres of living gnathostomes differ from those of lampreys (right) and hagfish in that their myomeres are divided into upper and lower halves by a horizontal septum of connective tissue. (See transverse cross-section) Lampreys' are not. Trinajstic et al. on placoderm fossils with soft tissue preservation show that the placoderms, like lampreys, lacked the horizontal septum. This feature, therefore, is a synapomorphy of the two remaining gnathostome groups: Chondrichthyes and Osteichthyes.


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.



Climatius, close to the ancestry of Eugnathostomata, from Karl Corti on Libero
Eugnathostomata: The gnathostome crown-group. The last common ancestor of Chondrichthyes and osteichthyes and all of its descendants.

Synapomorphies of Eugnathostomata include:



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, conodont animals, 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 plesiomorphich 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 message:

    Chondrichthyans have been embarked on their evolutionary trajectory for as long as bony "fish" (land vertebrates included) and have evolved a range of body shapes, life strategies, and ecologies that rivals them and that has changed substantially over time. Any talk of sharks being "living fossils" is just ignorant.