Australia: The Land Where Time Began
Placoderms - "plate skin" - Class Placodermi
Placoderms are armour-plated gnathostomes.
The Placodermi includes 6 clades (Denison, 1978; Goujet & Young, 1995). In phylogenetic sequence from basal to most derived (Goujet, 2001; Smith & Johanson, 2003) they are the Acanthoraci, Rhenanida, Antiarchi, Petalichthyida, Ptychodontida and Arthrodira. Of these the Arthrodires are the largest group.
They appeared in the fossil record about 420 Ma in the Early Silurian and by 400 Ma in the Devonian all major placoderm orders were present. The most recent evidence suggests that placoderms share a common ancestor with sharks. In both sharks and placoderms the males have external clasping organs for internal fertilisation. But only 1 group of placoderms, the ptyctodontids, have these organs.
In the Devonian they were very successful, becoming the dominant fish type in Australia and throughout the rest of the world. By the start of the Carboniferous there were only 1 or 2 genera, and they went extinct soon after, leaving no known living descendants. Some have suggested that ghost fish (chimaeras) may be related.
The origin of the placoderms is problematic. Some of the oldest known placoderms show some similarity to the heterostracan agnathans, the head and trunk region of both types is covered with a bony shield, a short scaly tail extending from the armoured part of the body. The shield was of the same construction in placoderms and heterostracans - there were 3 distinct layers, and holes for bone-producing cells within the bone. The unique aspect of the placoderm armour is that the head and trunk shields act separately, being hinged to one another and moving relative to each other.
The relationship of placoderms to other fish types - sharks, acanthodians and bony fish - is uncertain. There are many theories. A big problem when sorting out the relationships is the lack, so far, of good quality fossils prior to the Devonian. The placoderms and other fish types are already highly specialised when they first appear in the fossil record in the Early Devonian. The lack of unspecialised forms makes it difficult to construct a family tree for placoderms, and for vertebrates in general, as they among the earliest know vertebrates. It is not known if the armour developed at beginning of the Early Devonian, on previously diversified forms that lacked the bony in their shields, or if earlier stages had already developed armour but have not yet been found.
In the oldest known placoderms the braincase is already heavily ossified and attached to the upper jaw and the cheek bones. Most placoderms had bony plates around the edge of their mouths that acted as teeth, 1 or 2 large ones in the upper jaw and a single pair in the lower jaw, but a few had small tooth-like structures. The vertebrae have bony arches above and below the centra, the neural and haemal arches, but the centra are still composed of cartilage. The front elements of this vertebral complex is usually fused, the structure being called the sinarcual, which is articulated in 2 places with the occipital condyles at the back of the skull. This arrangement was to aid in the up and down movement of the head during feeding. This arrangement increases efficiency of forward movement by restricting movement to the vertical, but at the expense of lateral movement of the head.
Placoderms had pelvic and pectoral fins that were paired, the tail fin being heterocercal, the dorsal lobe was larger than the ventral lobe. This design tends to push the fish down as it swims. From this tail structure and the fact that most placoderms showed some degree of dorso-ventral flattening, it seems they were probably bottom feeders, in oceans, lakes and streams. A process that continued through the Devonian was the gradual loss by some placoderms of the heavy armour, and even the de-ossification of the spinal column, apparently to allow a move to higher levels of the water column, where this bone loss would make them lighter for swimming and probably gave them greater speed.
There are 5 or 6 main groups of placoderms. The uncertainty relates to the difference in opinion of some palaeontologists, disagreeing on the groups the various taxa should be divided into.
A placoderm was found in Antarctica in 1997 in which pigment cells had been preserved. Its ventral surface was iridescent and its dorsal surface was red (2).
see Oldest Mother
|Author: M. H. Monroe Email: firstname.lastname@example.org Sources & Further reading|