Devonian – Fish Faunas of the Late Devonian

Australia: The Land Where Time Began

Devonian – Fish Faunas of the Late Devonian

The Devonian has often been referred to as the age of Fishes. Detailed information has been found in many locations dating to the Late Devonian about fish faunas in a variety of different habitats. One of the most intensively studied is the Gogo Formation in Western Australia, the site of a reef that dates to the Late Devonian. Though this reef on land has been known of since the 1940s, in recent years excavations at this site have uncovered many fishes with minute detail of their internal soft tissue that has been preserved. The reproductive structures of placoderms and their embryos have been described (Long et al. 2009; Ahlberg et al., 2009), and among the amazing finds is evidence of internal fertilisation in these fishes. Long (2006) has described the fossil fish of the Gogo formation in Swimming in Stone [Also The Rise of Fishes, 1995, 2011]. In the Late Devonian this locality was a marine tropical reef. A tetrapod-related animal has been recovered from this locality, Gogonasus, the closest known form related to a tetrapod in this deposit, though to date none of the complement of genera that are usually also present in the same deposits as those that yield tetrapods have been found, and according to Clack¹ this suggests that the Gogo deposit may have been a deposit that was rather different from those previously known to contain tetrapods.

The Miguasha or Escuminac Bay locality in eastern Canada is one of the richest localities for Devonian fossil fish, providing a picture of the kinds of fish that lived in the same habitats as the elpisostegalian fish and their environments. As recognition of the importance which has been attributed to this fossil fauna it is now a World Heritage Centre. The sequence in this locality has been dated to the early Frasnian and recent work has provided a detailed picture of this locality and its fauna (Schultze & Cloutier, 1996; Cloutier et al., 1996). Previously this locality had been interpreted as representing a freshwater environment (Chidiac, 1996), it has been shown by geochemical and faunal studies to have been a coastal brackish to marine environment. Schultze and Cloutier edited a compendium volume in 1996 that described the flora and fauna.

In Miguasha the chondrichthyans were the only vertebrate group that was not represented, and the anaspids and the osteostracans represented the jawless (agnathan) fishes that are the primitive fishes related to lampreys of the present. The anaspids, which are elongated, narrow-bodied fish with a downturned tail and no paired fins are believed to have been filter feeders. The osteostracans, such as Alaspis, had flat, semicircular head shields that were composed of bony plates up to 300 mm across, and paired eyes that were close together on the top of the shield, and a single nostril. The head shields of some osteostracans had fields of what has been believed may have been electric organs. They had a pair of pectoral appendages that were flaplike attached to the back of the head shield; with the rest of the body being covered by scales, and they are believed to possibly have been detritus feeders on the sea floor.

The heavily armoured placoderms, antiarchs and arthrodires, were common at Miguasha. It has been suggested recently that the placoderms as a whole may not have been a natural group (Brazeau, 2009). The head shields of both forms were heavily armoured and there was a joint between the head amour proper and the shoulder region. Bothriolepis, one of the best known of the antiarchs was a moderately sized form about 200-250 mm in total length. This genus had a worldwide distribution, being found in comparable sites around the world, and it is the most common fish at Miguasha. Compared to the shoulder shield the head shield of antiarchs was short, and there were a pair of appendages near the junction that articulated at a complex shoulder joint. These appendages were armoured externally and jointed in the middle, which gave them the appearance of a crab’s claws. Though they had jaws they had no true teeth and have been suggested to have been scavengers. They have been suggested to probably have had lungs, based on some that were preserved at Miguasha, though Clack¹ says this is debatable (Denison, 1941).

Unlike antiarchs, arthrodires were major predatory forms. Some reached quite large size, having heads that were almost a metre long and with bodies that were possibly a further 2 m long. They had enormous jaws that had built-in toothlike pincers or scissors. Their pectoral appendages and pelvic girdles were unarmoured, though their pelvic fins are not well known. Much of the body doesn’t appear to have had scales. The arthrodire present at Miguasha was Pleurdosteus, which measured up to 250 mm in length.

Placoderms are gnathostomes, (jawed fishes), though their relationships with other gnathostomes is not certain. Placoderms and another group of early gnathostomes, the acanthodians (the spiny sharks), are all extinct. There were several genera of placoderms at Miguasha, some of which were larger, being up to 159 mm long, long-bodied, small-finned forms, and some with shorter bodies and longer fins, up to 80 mm long. The most notable feature of acanthodians was the fins. A bony spine supported the midline and paired fins along the leading edge. There were more than 2 pairs of paired fins ventrally on some acanthodians.

The tetrapods are the only one of the true osteichthyans groups not to be represented at Miguasha. Cheirolepis, a primitive form, that grew up to 500 mm long, represented the ray-finned fishes, and the lobe-fins, lungfishes, coelacanths, porolepiforms, and osteolepiforms are all represented there. The lungfishes were short-bodied forms that had elongated second dorsal fins. Fleurantia, that was rather rare, and occurred only in the lower half of the sequence (Cloutier et al., 1996) was 420 mm in maximum length, had a long snout and a denticulated palate, while Scaumenacia, the 4^th most abundant genus at Miguasha, occurred throughout the sequence, grew to a maximum length of 645 mm, had a short snout and tooth plates on the palate, which indicated the 2 genera had different habits and diets. Miguashaia was a primitive coelacanth that grew to 450 mm, did not have the characteristically modified tail, dorsal and anal fins that are present in all later coelacanths. Holoptychius is the porolepiforms that is best known from this locality, grew up to 470 mm, and Eusthenopteron, which is the most common tristichopterid, occurred throughout the sequence, is one of the most famous and best-known fossil fish in the world. It is known from individuals that cover a wide range of sizes, growing to about 500 mm in length. With regard to tetrapod origins and relationships Elpistostege, the closest to a tetrapod in the Escuminac Bay fauna, is one of the most significant fishes in the fauna, though only 3 incomplete specimens have been recovered. The skull of Elpistostege, at about 210 mm, is slightly smaller than that of Panderichthys. It has been found only near the top of the sequence (see Schultze & Cloutier, 1996).

This overview of the Miguasha fauna gives an idea of the range of vertebrates known from the Middle to Late Devonian, against which the record of the earliest tetrapods can be viewed.

Sources & Further reading