Euthycarcinoids

Introduction

A recent addition to the aquatic fauna of the Rhynie chert is a group of bizarre organisms which are termed euthycarcinoids. The euthycarcinoid body plan comprises a preabdomen with a variable number of segments or somites. On the ventral side these form distinct plates or sternites whereas on the dorsal side the plates (tergites) being fused appear larger and fewer in number so that two or sometimes three sternites appear to correspond with one tergite. Characteristically the appendages of the preabdomen are multi-segmented, almost antenna-like, with one leg pair per sternite. An apodous postabdomen with a variable number of segments ends in a pointed or styliform tail or telson. The systematic placement of euthycarcinoids remains somewhat problematic, since they show similarities with crustaceans and uniramian arthropods, including insects.

One species of euthycarcinoid has been found in the chert, Heterocrania rhyniensis (Hirst & Maulik 1926). It was originally assigned to the genus Crania after the collector and discoverer Rev. William Cran, but was subsequently changed to Heterocrania as the name Crania was preoccupied by a genus of brachiopod.

When first discovered, the affinity of Heterocrania was unknown, the fossil only being known from fragmentary remains lacking sufficient diagnostic features. It was only with the recent discovery of more complete specimens in the Windyfield chert that it became clear the animal was a euthycarcinoid (Anderson & Trewin 2003) (see inset below).

Euthycarcinoid reconstruction

Above: Reconstruction of the Rhynie chert euthycarcinoid Heterocrania rhyniensis (scale bar = 2mm) (Anderson & Trewin 2003).

Fossil Record

Euthycarcinoids are relatively rare as fossils. They are an extinct group of arthropods with a fossil record spanning from the Late Silurian to the Middle Triassic. The earliest euthycarcinoid fossil was described by McNamara and Trewin (1993) from the Tumblagooda Sandstone of Western Australia. The youngest fossil has been described from the Middle Triassic of Australia (Edgecombe & Morgan 1999). Upper Carboniferous euthycarcinoids have been recorded from Mazon Creek fauna (Schram & Rolfe 1982) and also in Europe (Anderson et al. 1997, 1999; Secretan 1980; Wilson & Almond 2001). Heterocrania is the first euthycarcinoid known from Devonian rocks.

Morphology

Heterocrania is a small euthycarcinoid, to date being the smallest of its kind described. In life it probably attained a length of approximately 15mm.

Head

The cephalic segment or the head of Heterocrania is domed, and flattened on the ventral surface (see inset below). The latter bears a pair of sub-rectangular plates, probably representing mandibles. The inner edge of these plates, where they meet, possess striated ridges (see inset right).

Right: Fragment of the cephalic segment of Heterocrania rhyniensis showing one of the ventral plates (p) with a striated inner margin (s) (scale bar = 500µm).

Fragmented cephalic segment of Heterocrania

Transverse cross section of cephalic segment of Heterocrania

Above: Transverse cross section through the head capsule of Heterocrania rhyniensis showing the domed dorsal surface (h) and what are most probably mouthparts (m) (scale bar = 500µm) (Copyright owned by The Natural History Museum).

Preabdomen

The preabdomen of Heterocrania is multi-segmented, comprising a series of two distinct ventral plates or sternites corresponding to a larger, fused tergite of which there were probably five in all (see inset right). Each sternite bears a pair of leg appendages.

The tergites are convex and markedly wider than the flattened sternites, the lateral margins of the tergites forming shelf-like projections (Anderson & Trewin 2003). In transverse cross section, these lateral projections appear to be reinforced by cuticular struts separating the upper and lower surfaces of the tergite cuticle (see inset below). In some thin sections of the animal curious rod-like tubes are present internally, with a disposition of one pair per sternite (see inset right) and appear to correspond with the position of the bases of the leg appendages, for which they probably provided support.

In a few specimens an internal sub-cylindrical gut trace occurs, comprising amorphous organic detritus (see inset below).

Longitudinal cross section of Heterocrania

Above: An almost complete specimen of Heterocrania rhyniensis in longitudinal section seen in dorsal aspect (the dorsal surface has been lost during sample preparation). This shows the margins of the fused tergites (t) with the corresponding sternites at depth in the section (s). The curious rod-like tubes are also shown (tr), together with the anterior segments of the postabdomen (a) (scale bar = 1mm).

Transverse cross section of body of Heterocrania

Above: Transverse cross section through the preabdomen of Heterocrania rhyniensis showing the convex tergite (t) with lateral shelf-like projections supported internally by cuticular struts (st). One of the leg appendages is visible (l) together with probable gut contents (g) (scale bar = 500µm).

Postabdomen and Tail

The postabdomen of Heterocrania comprises five apodous homologous segments. This arrangement suggests the euthycarcinoid belongs to the family Kottixerxidae. The tail or telson is of unknown length but appears to be similar in morphology to most other euthycarcinoids in being pointed and styliform.

Leg Appendages

Heterocrania possesses ten pairs of leg appendages, one pair attached to each of the preabdominal sternites. Each leg appendage comprises sixteen individual box-shaped segments or podomeres of consecutively decreasing size and each leg ends in a sharp distal spine. The podomeres bear a short, ventrally positioned, articulated spine or spur. The leg pairs decrease in their relative length towards the posterior or the preabdomen.

Right: A leg appendage of Heterocrania rhyniensis, showing the individual box-shaped podomeres (p) (scale bar = 250µm).

Leg appendage of Heterocrania

Palaeoecology

In the Rhynie chert Heterocrania is typically associated with the the crustacean Lepidocaris, the probable charophyte Palaeonitella, coprolites, algal and cyanobacterial filaments and often occurs in cherts displaying a clotted or 'mulm-like' texture. Heterocrania is therefore interpreted as an aquatic organism.

The morphology of the leg appendages suggests the animal was not suited to a predominantly swimming or nektonic lifestyle, as evidenced by the lack of paddle-like flattened podomeres and setae. Although Heterocrania may have been capable of limited swimming, it was probably more suited to burrowing or crawling on the substrate of freshwater ponds, feeding on organic detritus (Anderson & Trewin 2003).