Heterostraci

by Philippe Janvier  

Table of Contents

Introduction
Characteristics
Discussion of Phylogenetic Relationships
 

Introduction

The Heterostraci, or heterostracans, is a large clade of the Pteraspidomorphi, i.e. a group of fossil, armored jawless vertebrates, which lived from the Early Silurian to the Late Devonian (about 430 to 370 million years ago). They generally possess a fusiform head armor and a fan-shaped tail. Some forms, however, can have a rather depressed armor, with broadly expanded branchial plates. Some text-books still include the Astraspida, Eriptychiida and Arandaspida in the Heterostraci, but heterostracans clearly differ from these groups in having a single, common branchial opening on each side.

The protopteraspidid Doryaspis nathorsti, from the Lower Devonian of Spitsbergen.

Heterostracans are represented by nearly 300 species. They were marine but lived in sandy lagoons or deltas. Some species, however, are regarded as fresh water. They are known exclusively from North America, Europe and Siberia. They probably fed by scraping the bottom with their fan-shaped oral plates that armed their lower lip. They were poor swimmers and probably bottom-dewellers. It has been suggested that their posteriorly placed common gill opening could serve as a jet propulsion device and that made them moved like Cousteau's diving saucer.

Although most heterostracans are relatively small (5 to 30 cm in total length), some of them, the Psammosteidae, could grow to a very large size (up to 1.5 m in length) and developed steer-like branchial plates.

Characteristics

Heterostracans are characterized by:

Heterostracans are characterized by a single, common external opening on either side of the head armor (red arrow).

The heterostracan head armor comprises large ventral and dorsal shields and a variable number of separate plates, laterally (cornual, branchial plates) and around the mouth (oral, postoral, orogonal plates). In some taxa (Pteraspidiformes, Traquairaspidiformes), the dorsal shield can be compound of several distinct plates (orbital, pineal, rostral), but there always remains a large median dorsal "disk".

Since heterostracans have no calcified endoskeleton, their internal anatomy is only known from the impressions of the internal organs on the internal surface of the dermal armor. One may trace the impressions of the brain, gills, eyeballs, paired olfactory organ, and two distinct vertical semicircular canals of the labyrinth. Although paired, the olfactory organs seem to have opened ventrally into a large, median inhalent duct, as in extant hagfishes (see Hyperotreti).

Discussion of Phylogenetic Relationships

Most heterostracans fall into two major clades, the Cyathaspidiformes (Silurian-Early Devonian) and the Pteraspidiformes (Late Silurian-Late Devonian). There is, however, a number of minor taxa of debated affinities (Traquairaspidiformes, Lepidaspis, Tesseraspis, Cardipeltida, Tolypelepida). Cyathaspidiforms are characterized by the parallel and finely crenulated dentine ridges which form the ornamentation of the dermal plates. Some cyathaspidiforms, the Amphiaspidida, display a remarkable adaptation to benthic habits, with entirely fused plates of the armor and a peculiar opening near the orbit, which may as served as a spiracle. Pteraspidiforms are characterized by concentric dentine ridges with serrated margins. They usually have a median, spine-shaped dermal plate, and their dorsal shield consists of five separate plates (a dorsal "disc" and two orbital, one pineal, and one rostral plate). They also have separate branchial and cornual plates. Pteraspidiforms and cyathaspidiforms are probably sister-groups, and the other, minor groups are regarded as "basal" heterostracan taxa.

About this page

Philippe Janvier
E-mail: janvier@cimrs1.mnhn.fr.
Muséum National d'Histoire Naturelle
Laboratoire de Paléontologie
URA 12 - CNRS
8, rue Buffon - 75005 Paris
France

Page copyright © 1997 Philippe Janvier

Last saved 27 February 2001

Title Illustration

Heterostracans are the most diverse group of pteraspidomorphs and lived during the Silurian and Devonian periods. Among the most primitive heterostracans are tolypelepids (top right). Most heterostracans are pteraspidiforms, such as the pteraspidids (bottom right), protopteraspidids (bottom left) and the huge psammosteids (top left), which are the youngest known members of the group. (After Janvier 1996; Soehn & Wilson 1990.)
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