Placoderms (“plated skins”) are armoured fishes, and are the most primitive of the gnathostomes (jawed fishes). The dominant group in the Devonian oceans, placoderms occupied a variety of niches, from the gigantic 6m long top predator of the seas Dunkelosteus, to the small riverine bottom feeder Bothriolepis. All went extinct right at the end of the Devonian, around 359 million years ago.
Placoderms have a distinctive armour of interlocking bony plates covering their head and torso. Their long bodies and tails resembled those of a shark. In some species, their head and body armour had a pair of ball and socket joints linking them, which allowed the head to move independently.
Placoderm plates are the most abundant fish remains at the Waterloo Farm site (apart from isolated fish scales). Four species representing two orders of placoderm, the Antiarchi and the Arthrodira, are described from Waterloo Farm. The single antiarch species is Bothriolepis africana, whereas there are three arthrodires species: Africanaspis doryssa, Africanaspis edmountainii and Groenlandaspis riniensis.
Placoderm diversity at Waterloo Farm shows some similarity to contemporary sites from around the world, including Red Hill (Pennsylvania), and Strud in Belgium. This is remarkable as these latter sites represent equatorial environments in the Late Devonian, whilst Waterloo Farm was at high latitude. This supports the idea of cosmopolitanism towards the end of the Devonian, whereby animal genera and species achieved a worldwide distribution at that time.
Placoderms have a distinctive armour of interlocking bony plates covering their head and torso. Their long bodies and tails resembled those of a shark. In some species, their head and body armour had a pair of ball and socket joints linking them, which allowed the head to move independently.
Placoderm plates are the most abundant fish remains at the Waterloo Farm site (apart from isolated fish scales). Four species representing two orders of placoderm, the Antiarchi and the Arthrodira, are described from Waterloo Farm. The single antiarch species is Bothriolepis africana, whereas there are three arthrodires species: Africanaspis doryssa, Africanaspis edmountainii and Groenlandaspis riniensis.
Placoderm diversity at Waterloo Farm shows some similarity to contemporary sites from around the world, including Red Hill (Pennsylvania), and Strud in Belgium. This is remarkable as these latter sites represent equatorial environments in the Late Devonian, whilst Waterloo Farm was at high latitude. This supports the idea of cosmopolitanism towards the end of the Devonian, whereby animal genera and species achieved a worldwide distribution at that time.
Antiarchs
Bothriolepis africana
Locality: Waterloo Farm Age: latest Devonian Bothriolepis was a relatively large placoderm, with adults at Waterloo Farm reaching over a metre in length (1). Juveniles smaller than ten centimetres are also found. This animal was almost certainly a bottom dweller, with a carapace that was anything but hydrodynamic. It had two closely spaced eyes positioned on top of its head, and long spiny arms, with an elbow like joint, which some have suggested it used to cover itself with sediment (2). The range of sizes of the fossils suggest that Bothriolepis was a resident in the Waterloo estuary throughout its life cycle. |
Arthrodires
Three species of arthrodires have been described from Waterloo Farm. These are all phlyctaeniid arthrodires, united by having ‘ski-runner’-like lateral spines, outside of holes in the armour through which their front pair of fins protruded. Their heads hinged on their body armour with a pair of ball-and-socket joints. Fossils include embryonic and juvenile specimens through to adults, indicating that these animals spent their entire lives and reproduced within the estuarine lagoon.
Africanaspis doryssa
Locality: Waterloo Farm
Age: latest Devonian
This species is characterized by an extremely tall spike on its back formed by the median dorsal plate (3). This fish only grew to about 30cm long. Its spiky body armour would have provided defence against a variety of larger predators that inhabited, or visited the lagoon.
Africanaspis doryssa
Locality: Waterloo Farm
Age: latest Devonian
This species is characterized by an extremely tall spike on its back formed by the median dorsal plate (3). This fish only grew to about 30cm long. Its spiky body armour would have provided defence against a variety of larger predators that inhabited, or visited the lagoon.
Africanaspis edmountainii
Locality: Waterloo Farm
Age: latest Devonian
This species is similar to A. doryssa, and differs in the possession of a lower dorsal spike, details of the ornament and proportions of the body plates. Unlike the former species, the head of this animal is not known.
Locality: Waterloo Farm
Age: latest Devonian
This species is similar to A. doryssa, and differs in the possession of a lower dorsal spike, details of the ornament and proportions of the body plates. Unlike the former species, the head of this animal is not known.
Groenlandaspis riniensis
Locality: Waterloo Farm Age: latest Devonian Most of the placoderm remains at Waterloo Farm are assigned to Groenlandaspis riniensis, representing individuals with reconstructed body lengths ranging from 25 mm to well over a metre in length. Occasionally, intact carapaces of small individuals are found and some, very rarely, even have associated impressions of the unarmoured tail end of the body. These are the “groenlandaspidids” with the best preserved information on their soft tissue. Groenlandaspis riniensis was the first fossil animal to ever have an isiXhosa derived name (named in 1997). After discovery, it was put in the existing genus Groenlandaspis, but given the new species name riniensis, meaning ‘from Rini’ the traditional name for Grahamstown. References: 1) Long, J. A., Anderson, M. E., Gess, R., & Hiller, N. (1997). New placoderm fishes from the Late Devonian of South Africa. Journal of Vertebrate Paleontology, 17(2), 253-268. 2) Janvier, P. 1996. Early Vertebrates. Oxford University Press, Oxford, New York. 3) Gess, R. W., & Trinajstic, K. M. (2017). New morphological information on, and species of placoderm fish Africanaspis (Arthrodira, Placodermi) from the Late Devonian of South Africa. PloS one, 12(4), e0173169. |