This is Mazon Monday post #9. Acanthodes beecheri is the subject of this Species Spotlight. Some of the material in this post was provided by David Duck. Dave has a deep appreciation and interest in the Mazon Creek fauna and flora. He has even constructed 3-D models of quite a few of the Mazon animals, including the Tullymonster gregarium, Euproops danae, Mamayocaris jaskoskii, and Acanthodes beecheri among others. The models are very detailed! Thanks for your contribution, Dave!
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Species Spotlight: Acanthodes beecheri
Acanthodes is an extinct genus of spiny shark. Fossils have been found in Europe, North America, and Australia. Their geologic range is from the Middle Devonian until the Lower Permian. Their actual position on the tree of life is controversial. The following quote comes from the wikipedia page for Acanthodes.
The scientific classification of acanthodians is still a subject of great dispute, due to the fact that they share qualities of both bony fish (osteichthyes) and cartilaginous fish (chondrichthyes). A recent study has suggested that Acanthodes may have been, or closely related to an early common ancestor to all cartilaginous and bony fish, including humans.[6][7] Acanthodian internal anatomy is primarily understood by Acanthodes bronni because it remains the only example preserved in substantial detail, central to which is an ostensible osteichthyan braincase. For this reason, Acanthodes has become an indispensable component in early gnathostome phylogenies.[8] Acanthodes is quantifiably closer to chondrichthyans than to osteichthyans. However, phylogenetic analysis places Acanthodes on the osteichthyan stem, as part of a well-resolved tree that also recovers acanthodians as stem chondrichthyans and stem gnathostomes. As such, perceived chondrichthyan features of the Acanthodes cranium represent shared primitive conditions for crown group gnathostomes. There has been increasingly detailed findings of early gnathostome evolution highlights ongoing and profound anatomical reorganization of vertebrate crania after the origin of jaws but before the divergence of living clades.[8]
Two species of Acanthodes have been identified in the Mazon Creek fauna. One species identified in this paper is Acanthodes beecheri. The pictures below are of 3-D models laid next to two specimens found in Mazon Creek. The specimens are from the collection at the Milwaukee Public Museum and Yale. The 3-D model was created by David Duck.
A Creature Corner article on Acanthodes appeared in the April 1989 edition of the ESCONI bulletin.
Mazon Creek concretions have preserved what is considered to the most diversified Late Pennsylvanian fish fauna known. Represented are lampreys, lungfish, coelacanths, shark-like fishes, various bony fishes, and acanthodians. Two species of acanthodians have been described from this location. More recent studies have noted that the vagaries of preservation preclude descriptions down to the species level, therefore, only the genus Acanthodes is recognized.
The Acanthodii are known from the Silurian and are the oldest known group of vertebrates with jaws. Superficially, acanthodians resemble sharks; heterocercal tail (vertebral column turning up at the end with tail fin suspended from it), large orbits on a stout body with shark-like proportions. Fins on this Class are an unusual type; all fins were paired and consisted of a membrane strengthened by a long spine at the front of each fin. In some species, the spine greatly exceeded the fin length. Despite the old designation "spiny sharks", they were completely armored with small diamond shaped scales, not the isolated dermal denticles of sharks. Scales on our 3-inch long genus, Acanthodes, averaged out at a square millimeter in size. Early acanthodians had teeth fixed to jaw bones; Acanthodes lacked any tooth-like structures. Braincase, head features and large orbits are comparable to those of ray-finned bony fish. The poor preservation of acanthodians hinders study of their internal features. Most were small and easily crushed, yet portions of an internal skeleton are visible.
Acanthodes has been found in the Essex and in the Braidwood biota. Most typical acanthodians were freshwater dwellers, so it is assumed that Acanthodes ventured out into the Mazon delta during the periods of high water. Acanthodians became extinct in the Lower Permian.
Where and how to place these strange fish is the evolutionary scheme puzzles scientists. Paired fins with bony spikes resemble primitive sharks (Class Chondrichthyes), yet the well-developed skeleton and scale covering resemble the Class Osteichthyes (the higher bony fishes). It is no wonder that Romer (1966) and Carroll (1987) have placed the genus Acanthodes and other acanthodians in "Class incertae sedis Acanthodii". There the Class "sits" awaiting further study.
