The Pteraspidomorphi, or pteraspidomorphs, is a group of fossil jawless vertebrates which lived from the Early Ordovician to the Late Devonian (i.e. from 470 to 370 million years ago). They possess and extensive dermal armor covering the head, and a scaly tail. They have no other fin than the caudal fin. Their head armor, which always comprises large, median dorsal and ventral plates (or shields), is generally oblong, and this gives them the aspect of a tadpole. Pteraspidormorphs have long been regarded as closely related -or even ancestral- to jawed vertebrates, but the few characteristics they share with the latter are now considered as primitive for all vertebrates.
Most pteraspidomorphs were marine, but lived very near to the shore, in lagoons and deltas. Some groups are thought to have been fresh water. They were certainly bottom-dewellers, as shown by traces of abrasion of the ventral surface of their headshield.
Pteraspidomorphs are characterized by:
- Large, median ventral and dorsal dermal plates, or "discs", which form most of the head armor.
- Oak leaf-shaped tubercles on the dermal plates (lost independently several times)
The Pteraspidomorphi include four clades, the Arandaspida, Astraspida, Eriptychiida and Heterostraci. Their dermal plates and scales are made up of acellular bone, or aspidine, but this is thought to be a primitive condition for the dermal bone of all bony vertebrates. Pteraspidomorphs have no calcified endoskeleton, except for eriptychiids and possibly arandaspids, which show traces of calcified cartilage. Pteraspidomorphs generally have a fusiform shape, with a head armor almost as long as the body, and have no other fin than the caudal fin, which is generally pad- or fan-shaped. The mouth is armed with elongate dermal plates which could expand as the mouth opened, and served as a scoop. The internal surface of the dermal plates -at any rate in heterostracans- shows impressions of the gills, brain, two vertical semicircular canal and distinctly paired nasal capsules. The nasal capsules of heterostracans seem to have opened ventrally into a separate median duct comparable to the nasopharyngeal duct of hagfishes. They are put among the Vertebrata because some heterostracans show impressions of arcualia (dorsal components of the vertebrae), and because they have a well-developed sensory-line system.
Evolution and Systematics
Discussion of Phylogenetic Relationships
The main characteristics supporting the nodes of this phylogeny are:
- Node 1: Gill openings along the dorsolateral margin of the head armor.
- Node 2: Dentine with large tubules in tubercles.
Pteraspidomorphs have been first regarded as related to bony fishes, then to sharks, then ancestral to hagfishes, and finally as the closest jawless relatives of the gnathostomes. This last theory was based on the fact that they seem to have a paired olfactory organ and a sensory-line pattern which is quite similar to that of the gnathostomes. These characteristics are, however, likely to be general for either the vertebrates or, at any rate, for the ensemble of all "ostracoderms"+ the gnathostomes. Other "ostracoderms", such as the Galeaspida are now known to have a paired olfactory organ. Current phylogenetic analysis using a large number of characteristics (see Vertebrata page) now place pteraspidomorphs as the sister-group of all other "ostracoderms" and the gnathostomes.
The interrelationships of the Pteraspidomorphi are still poorly corroborated. The Astraspida share with the Heterostraci the dorsolateral position of the gill openings (single in heterostracans). The poorly known Eriptychiida seem to share with the Heterostraci the same type of dentine in the tubercles.