Acanthodes BW.jpg

''Acanthodes'' (from el, ἄκανθώδης , 'provided with spines') is an extinct genus of spiny shark. Fossils have been found in Europe, North America, Asia, and Australia. ''Acanthodes'' was most common in the Carboniferous and Early Permian. A few putative species ("''Acanthoides dublinensis", Acanthodes guizhouensis'') have been reported from Devonian strata, but their referral to the genus may not be valid.

Description

Compared with other spiny sharks, ''Acanthodes'' was relatively large, at long. The genus had no teeth, instead gills. Because of this, it is presumed to have been a filter feeder, filtering plankton from the water. The ''Acanthodes'' has been found to have only a couple of skull bones. It was covered in scales that were cubical in shape.

It also had fewer spines than many of its relatives. Each of the paired pectoral and pelvic fins had a spine, as did the single anal and dorsal fins, giving it a total of just six, less than half that of many other species. Due to its slender branchiostegals, singular dorsal fin, lack of fin spines, and its pelvic fin that is similar to pectoral fins, Acanthodes are considered to be part of the Acanthodidae subdivision of the Acanthodii. A fossil discovered near Hamilton, Kansas in the Upper Carboniferous Hamilton Formation, and published in 2014 as ''Acanthodes bridgei '' was so well-preserved that traces of its eye tissue were sufficient to establish that ''Acanthodes'' had both rod and cone photoreceptor cells, and thus profited from color vision.

Discovery

Several specimens of ''Acanthodes'' were recovered from an abandoned quarry (Hamilton Quarry) which contained individuals ranging in total length from 54 mm to 410 mm. ''A. bridgei'' was one of the species found there. ''A. bridgei'' has large orbits, a shorter pre-pectoral region, and shallower insertions of the fin spines.

Taxonomy

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.
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. ''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.

As mentioned earlier, ''A. bronni'' is an acanthodian, a group of stem gnathostomes more derived than placoderms, but fairly close to the origins of chondrichthyans and osteichthyans. ''A. bronni'' lived about 290 million years ago during the Carboniferous period.
Researchers took 138 characteristics of various skulls of ''A. bronni'' and compared these with skulls of both chondrichthyans and osteichthyans, and determined that acanthodians are closer to cartilaginous fishes.

References

* Parker, Steve. Dinosaurus: the complete guide to dinosaurs. Firefly Books Inc, 2003. Pg. 60

External links

entry at the Fossil Museumentry at Saint Joseph's University

{{Taxonbar, from=Q2335100

Acanthodii genera
Carboniferous acanthodians
Permian acanthodians
Paleozoic fish of North America
Paleozoic fish of Europe
Prehistoric fish of Australia
Taxa named by Louis Agassiz