BSCI392
11-07-07
The evolution of jawed vertebrates

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Among living vertebrates, we have only vertebrates with jaws and lampreys. During the Early Paleozoic, however, a wide range of jawless vertebrates occupied numerous ecological niches. Traditionaly, these were termed Ostracoderms. Although you will find this term in older literature, it is sadly paraphyletic. But first, consider an attribute of almost all vertebrates:

Vertebrate hard tissues:

Fossil veretbrates are mostly known from hard tissues - bone, dentine, and enamel. Bone is composed of:

In contrast, dentine (aka "acellular bone") lacks the living component, and enamel consists entirely of interlocking crystals of the mineral component.

Bone is secreted by living cells that inhabit some preexisting connective tissue - either:

As a result, two corresponding types of bone are known:

These distinctions are significant because dermal and endochondral bone have distinct, separate evolutionary origins.

Our earliest suggestion of vertebrate bone is Anatolepis from the Late Cambrian - isolated plates of scale-like dermal bone.

By the Silurian we see a great diversity of jawless fossil vertebrates, lacking paired fins and endochondral bone, including:

Pteraspidomorphi (Cambrian (assuming Anatolepis) - Devonian). Includes Sacabambaspis (right), the first well-preserved vertebrate fossil, and a significant diversity of others.

Morphology:

Without a living analog, speculations about the ecology of these creatures is very limited. We not the following:

Feeding:

Locomotion:

Leads to the general picture of pteraspidomorphs as creatures that rested on the bottom to suspension feed, but would swim haphazardly when disturbed, only to settle to the bottom again. Their probably poor control over swimming orientation and direction makes it unlikely that they could perform ram suspension feeding.

Anaspida (Silurian)

Morphology:

Overall, anaspids seem adapted for active swimming. exactly how they ate is mysterious, but they lack the obvious adptations to suspension feeding of pteraspidomorphs. Phillipe Janvier has argued persuasively that anaspids might be the sister taxon to lampreys (Hyperoartia). that would make lampreys secondarily cartilagenous.

Thelodonti (Ordovician - Devonian)

Morphology:

One interesting group of thelodonts is the Silurian - Devonian Furcacaudiformes. These show the presence of a stomach, along with some very surprising morphologies resembling those of highly maneuverable reef fish, but without paired fins, how did they maneuver. Strains the abilities of the comparative method.

In all of the above taxa, there is no consensus about their interrelationships or closeness to jawed vertebrates. But then we get two groups that share a major synapomorphy with gnathostomes: Endochondral bone

Galeaspida: Restricted to southern China and Indochina, then a separate continent. (Silurian - Devonian)

Morphology: