Australia: The Land Where Time Began

A biography of the Australian continent 

Porolepiformes  

These were relatively large predatory fish from the Devonian (Jarvik, 1972). They had a worldwide distribution, being most common in Middle to Late Devonian freshwater deposits of Early Devonian age. The scales of this form give them their name; the cosmine-covered scales had rows of pores. Other features characterise these fish, a broad head, and very small eyes, in the cheek they had a prespiracular bone, and the large fangs had a special style of infolding of the enamel and dentine - called a dendrodont tooth structure. They had a large whorl of stabbing teeth at the front of the jaw. It is believed that most porolepiforms were ambush predators, based on the body and tail.

The oldest known porolepiform, Porolepis, was found in Spitsbergen and other parts of Western Europe. It grew to about 1.5 m, considered to be fairly large for its time, had a thick cosmine cover on all its bones and scales, and the eyes were very small. It has been assigned to the family Porolepidae, together with some other forms known only from fragmentary remains, that had thick rhombic scales. Most Middle and Late Devonian porolepiforms have been assigned to the family Holoptychiidae, because they have rounded scales and lack cosmine. In the Early Devonian seas and in-shore environments, Porolepis was among the largest predators of the Early Devonian seas and near shore environments. Their appearance seems sluggish, though they were probably more efficient than the primitive placoderms and acanthodians with their heavy armour that they probably preyed on. Nasogaluakus, from the late Early Devonian of the Canadian Arctic is the earliest known holoptychioid.

 The holoptychioids, the chief group of porolepiforms, arose in the Middle Devonian, growing to 2.5-3.0 m long, which was enormous for their time, and were ambush predators. Most had invaded the river systems where they were safe from the giant predatory dinichthyid placoderms, though some have been found in marginal marine deposits, where they became the top aquatic predators in their habitat. According to Long the holoptychioids are considered to be more evolutionarily advanced than the porolepids, having lost the thick cosmine covering from their bones and scales, and the scales had become rounded, and the skull had a specialised set of bones around the external nostrils, which included a “nariodal” bone, and in the front of the lower jaw there are enlarged tooth whorls which differ from those of onychodontids in having a series of large teeth in parallel rows. The predatory diet of porolepiforms is reflected in their whole dentition, with large teeth, or fangs, along the lower and upper jaws that are flanked by several series of gripping teeth. There were many small bones bearing tooth-like denticles present on the robust gill arch bones.

Glyptolepis, one of the holoptychioids that was more widely spread in the Middle Devonian, known from several species found in East Greenland and Scotland, have been the subject of a detailed study by Erik Jarvik of Stockholm. Glyptolepis, which thrived in the Middle and Late Devonian of the Old Red Continent, may have grown to almost 1 m in length. Recent discoveries of material in Scotland of so-called Glyptolepis and Holoptychius species has been redescribed by Per Ahlberg of the University of Uppsala as new forms, such as Duffichthys, from the Scat Craig Beds of Elgin (Ahlberg, 1994). Duffichthys is known only from lower jaws, unusual in having very large attachment area for the symphyseal tooth whorl (Ahlberg, 1994).

Holoptychius, one of the largest of the porolepiform group, was also one of the more widespread members of the group, lived near the end of the Devonian. Based on the scales of the largest species, Holoptychius nobilissimus, it is indicated that the reconstructed fish may have reached up to 3 m long, which would make it a formidable predator in the ancient river and lake systems of North America, Greenland, Europe, parts of Asia, and possibly Australia. In the Dura Den site in Scotland whole body fossils of Holoptychius are well known, as schools of these fish were rapidly covered by wind-blown sands soon after their death. The Holoptychius from Dura Den are, on average, a relatively small species less than 1 m long. The genus is indicated by the worldwide distribution of their scales that have been found to have been widespread and capable of invading new river systems by crossing saltwater. They are generally rarely found outside of river and lake deposits, though marine porolepiforms are known from Latvia, e.g. Laccognathus, from the Lode deposit.

Sources & Further reading

1. Long, John A, 1995, The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press.
2. Long, John A, 2011, The Rise of Fishes - 500 Million years of Evolution, 2^nd edition, University of New South Wales Press.

Links

New Onychodontiform (Oeteichthyes; Sarcopterygii) from the lower Devonian of Victoria, Australia