From jawless fish to toothless jaws: Hemicyclaspis to Chondrosteus

Updated December 16, 17, 2019
with Thelodus moving to the most basal position in this phylogenetic sequence of jawless fish. Chondrosteus is removed, replaced with Pachycormus due to a reinterpretation of bones and the resulting tree topology shift.

Adding jawless fish
to the large reptile (LRT, 1611+ taxa) sheds new light on the origin of jaws and the basic topology at the base of the LRT.

Figure x. Chondrosteus was revised and no longer fits here. Pachycormus is inserted in its place.

Figure x. Chondrosteus was revised and no longer fits here phylogenetically.  Pachycormus is inserted in its place.

Thelodus
(Fig. 1) was crushed to a thin film with a ventral exposure. Here the round lacrimal and angular jaw bones are highlighted. The lateral armor (green) is barely ossified.

Osteostraci,
like Hemicyclaspis (Fig. 1), have a ventral opening at the front of ventral surface of the skull, similar to their ancestors, like Birkenia, which retain lancelet-like cilia surrounding the oral opening. Perhaps Hemicyclaspis did, too.

Sturgeons,
like Acipenser (Fig. 1), have a longer rostrum and a posterior tube mouth. The maxilla and dentary are not yet present. Those bones grow teeth. Teeth are not present. Neither are the bones that grow them. So the lacrimal and surangular create the protrusible rim of that tube mouth and neither connects to the quadrate. Nesting sturgeons at the base of fish with teeth is the opposite of traditional cladogram topologies, in which sturgeons are considered ‘aberrant’ or ‘regressive’ (see below).

Figure 1. Old woodcut illustration labeling the upper mouth tube bone the lacrimal. Mn = mandible. h = quadrate. g = hyobranchial. Weave of bones above the lacrimal are palatal bones (pterygoid, ectopterygoid, palatine and vomer, plus a remnant gill bar. This taxon really exaggerates the rostrum, similar to the related spoonbill.

Figure 1. Old woodcut illustration labeling the upper mouth tube bone the lacrimal. Mn = mandible. h = quadrate. g = hyobranchial. Weave of bones above the lacrimal are palatal bones (pterygoid, ectopterygoid, palatine and vomer, plus a remnant gill bar. This taxon really exaggerates the rostrum, similar to the related spoonbill.

As you can see (Fig. 2), I am not the first worker 
to determine that the traditional ‘maxilla’ on sturgeons is instead the lacrimal.

Sturgeons, continued.
Gill covers (operculum) appear. While feeding on the bottom with the mouth buried in sediment, water cannot enter the mouth. So instead water enters the top of the operculum and exits out the back for respiration.

Note the close correspondence
between the torso ossifications, fin placement, tail shape and skull shape on the sturgeon and its osteostracan ancestor, Hemicyclaspis (Fig. 1).

Figure 1. Chondrosteus skull re-illustrated and compared to the original reconstruction and in situ drawing. Compare to Trachinocephalus in figure 2.

Figure 1. Chondrosteus skull re-illustrated and compared to the original reconstruction and in situ drawing. Compare to Trachinocephalus in figure 2.

Chondrosteus
(Fig. 1) is now deleted from this list, now nesting with lizardfish.

Are sturgeons jawless fish?
In the LRT sturgeons are transitional between jawless fish and traditional gnathostomes.

Jollie 1980 reported in his growth study on sturgeons,
“It is a conclusion that the endocranium has been drastically altered in form and in the reduction of its ossifications but that the dermal head skeleton is basically that of an actinopterygian fish which shows many regressive tendencies such as the variable multiplication of ossified units. The jaws in this group are unique both in terms of suspension and in lacking a premaxilla. The post-temporal of the pectoral girdle has a unique relationship with the endocranium which involves the exclusion of the lateral extrascapular. An interclavicle is present. In spite of such features, the developmental story and adult ossifications of the sturgeon support the idea of a common, and understandable, bone pattern in actinopterygians and osteichthians.”

Jollie did not place Acipenser and Hemicyclaspis
in a phylogenetic context. In the LRT (subset Fig. 4) Pseudoscaphorhynchus is a tested sturgeon.

Figure 6. Subset of the LRT focusing on fish.

Figure 4. Subset of the LRT focusing on fish.

Are sturgeons bony fish?
Not according to the LRT. Much of their skeleton is cartilaginous and they nest basal to cartilaginous taxa. So between cilia and jaws, the transitional trait is a tube. Marginal teeth seem to have appeared three times by convergence in this scenario and once gained, were quickly lost in placoderms + catfish. Add to those palatal tooth carpets found in catfish, mantas and whale sharks.

Apologies for earlier errors.
As I’ve often said, I’m teaching myself vertebrate paleontology one taxon at a time using the LRT as a terrific tool for figuring things out.


References
Jollie M 1980. Development of head and pectoral girdle skeleton in Acipenser. Copeia 1980(2):226–249.

Actinopterygii = ray fin fish
Osteichthyes =  bony fish

wiki/Gnathostomata

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