More About Placoderms
Diverse Armored Fishes
Placoderms are arguably the most diverse groups of the early jawed fishes. They first appeared in the Early Silurian and diversified dramatically during the Devonian, but became extinct at the end of that period. About 200 genera of placoderms evolved during this interval, with the vast majority restricted to the Devonian. Some developed into the first vertebrate giants.
Placoderms (=plate skin) are armored by their extensive dermal skeleton. These dermal bones (or plates) form head and thoracic shields that are either articulated by distinctive joints or fused into a single unit. Pectoral fins are typically well developed. Bony shearing or crushing structures on the jaws substitute for true teeth, which are absent. The jaw joint is simple. Most placoderms are known only from plates of the head or thoracic shields, but some exceptionally preserved specimens reveal heterocercal (shark-like) tails, a pair of pelvic fins, and single dorsal fins; there is no clear evidence for anal fins.
Other than the earliest placoderms which come from the Early Silurian of China, the fossil record of placoderms is limited to the Devonian. All the major groups were present at the beginning of the Devonian so its apparent that a degree of diversification occurred sometime in the Middle and Late Silurian. Placoderms lost 6 out of 14 Late Frasnian families during the Frasnian/Famennian Mass Extinction, an event which is estimated to have eliminated 20% of all marine animal families. Most of the remaining families persisted until the very end of the Devonian. This second extinction event also claimed some early sharks, ray-fins and lobe-fin fishes, but apparently had a modest effect on other fauna.
There are five or six major groups of Devonian placoderms. One group, the Arthrodires, account for nearly 2/3 of the genera. Arthrodires are distinguished by having two pairs of gnathal (tooth-like) plates extending from the upper jaw. The more advanced arthrodires also have an unusual neck joint that considerably increases gape size. Most placoderms were benthic but several of the more advanced arthrodires have reduced armor and were probably adept swimmers. Arthrodires are typically less than 1 m in length, but some (e.g., Dunkleostus) became the giants of their day. Two species of groenlandaspid arthrodires (Groenlandaspis pennsylvanica and Turrisaspis elektor) were found at Red Hill. Coccosteus, Eastmanoesteus, Holonema and Rolfosteus are well known arthrodires from elsewhere.
Authorities differ on whether phyllolepids belong within the arthrodires or in a separate group. These dorsoventrally flattened fishes lack the second pair of gnathal plates and jointed neck that characterize most arthrodires, but they share some other features that may warrant inclusion. A species of Phyllolepis was also found at Red Hill.
Antiarchs are the second most diverse group of placoderms. They are also the strangest. The jointed head and thoracic shields formed a heavily armored box with a small anterior opening for two protruding eyes. Each pectoral fin was completely encased in bone and in the more advanced genera it was jointed in a fashion more reminiscent of arthropods than of vertebrates. Asterolepis, Bothriolepis and Remigolepis are antiarchs.
An interesting side story concerns one antiarch, Bothriolepis. Serial sectioning of several well-preserved specimens by R.H. Dennison (1941) revealed a pair of sac-like structures that opened from the pharynx and extended well into the trunk. Dennisons interpretation that these novel structures were lungs was not widely accepted. The case was taken up by Wells and Door (1985), who proposed that Bothriolepis could have been semi-terrestrial. They argued that the boxy armor supported the body out of water, the jointed pectorals helped the fish to move on land, and the sac-like structures allowed the fish to breath air.
Peltalichthyida, Rhenanida, and Ptyctodontida are the three other main groups of placoderms. Peltalichyids (e.g., Lunaspis) are dorsoventrally compressed placoderms with shortened thoracic armor, expanded pectoral spines and dorsally oriented eyes. Rhenanids (e.g., Germendina) are even more dorsoventrally compressed placoderms with reduced thoracic armor and horizontally expanded pectoral fins that are reminiscent of skates and rays. Ptyctodontids (e.g., Cambellodus, Ctenurella and Rhamphodopis) have enlongate bodies, whip-like tails and reduced head and thoracic armor.
The males of one ptyctodontid, Rhamphodopis, have clasper-like intromittent organs similar to those found on chondrichthyans (sharks and their kin). This finding has been use by some authorities to propose that placoderms and chondrichthyans are related, but this view has not been widely accepted.
- Douglas Henderson's reconstruction of Dunkleosteus:
- Palaeos.com's overview of placoderms:
- Univ. of Bristol's web pages on the Gogo Formation:
- U.C. Museum of Paleontology's Introduction to the Placodermi:
- Carroll, R. L. 1988. Vertebrate Paleontology and Evolution. New York: W. H. Freeman & Co.
- Janvier, P. 1996. Early Vertebrates. Oxford: Claredon Press.
- Long, J.A. 1995. The Rise of Fishes: 500 Million Years of Evolution. Baltimore and London: John Hopkins Univ. Press.
- Maisey, J.G. 1996. Discovering Fossil Fishes. New York: Henry Holt & Co.
- Scientific Papers:
- Denison, R.H. 1941. "The soft anatomy of Bothriolepis." Journal of Paleontology 15: 553-561.
- Miles, R. S. and T. S. Westol, 1968. "The placoderm fish Coccosteus cuspidatus Miller ex Agassiz from the Middle Devonian Old Red Sandstone of Scotland. Part 1. Descriptive morphology." Trans Roy. Soc. Edinburgh 67: 373-476.
- Stensiö, E.A. 1969. "Elasmobranchiomorphi. Placodermata. Arthrodires." In: J. Piveteau (ed.) Traité de Paléontologie 4(2):71-692. Paris: Masson S.A.
- Wells, N.A. and J.A.Door, Jr. 1985. "Form and Function of the Fish Bothriolepis (Devonian; Placodermi, Antiarchi): The First Terrestrial Vertebrate?" Michigan Academician 17(2): 157-173.
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