Eriptychiida

Introduction

The Eriptychiida, or eriptychiids, are a group of armored, fossil jawless vertebrates, which lived in the Middle Ordovician (about 450 million years ago) in North America. It is represented by two species, Eriptychius americanus and E. orvigi, known by only scales and armor fragments. A single specimen shows a partly articulated snout, whith traces of a calcified endoskeleton. Isolated branchial plates with a notch for the branchial opening suggest that their overall morphology was rather similar to that of the Astraspida. Eriptychius is found in association with the Astraspida.

Characteristics

Eriptychiids are characterized by:

• Large, flattened tubercles made up by orthodentine with very large tubules, possibly capped with true enamel.

The structure of the dentine of eriptychiids is in many respects closer to that of heterostracans that to that of astraspids. This is the only argument to place them as the closest relatives to heterostracans, among the Ordovician vertebrates. However, eriptychiids differ from all other pteraspidomorphs in having a massively calcified endoskeleton, pervaded by canals for blood vessels.

Discussion of Phylogenetic Relationships

The Eriptychiida are represented by a single, poorly known genus, Eriptychius, which superficially resembles Astraspis in having a tessellate headshield, but whose bone and dentine microstructure looks closer to that of heterostracans than to that of Astraspis. Eriptychiids occur in North America, usually in association with Astraspis.

References

Denison, R. H. (1967). Ordovician vertebrates from Western United States. Fieldiana: Geology, 16, 269-288.

Ørvig, T. (1958). Pycnaspis splendens, new genus, new species, a new ostracoderm from the Upper Ordovician of North America. Proceedings of the United States National Museum, 108, 1-23.

Ørvig, T. (1989). Histologic studies of ostracoderms, placoderms and fossil elasmobranchs. 6. Hard tissues of Ordovician vertebrates. Zoologica Scripta, 18, 427-446.