The early Gnathostomes (jawed vertebrates) have traditionally been split by taxonomists into four groups, the Placoderms, the Chondrichthyans, the Acanthodians and the Osteichthyans.
The Placoderms appeared sometime during the Silurian, though the fossil record for Silurian Placoderms is not good, and became extremely successful during the Devonian, but dwindled rapidly during the later part of the period, becoming extinct at the end. The head and foreparts of Placoderms were encased in armored boney plates, the rear parts either naked or covered in boney scales. Placoderms lacked teeth (or dentine), their mouthparts being comprised of Most Placoderms are thought to have been bottom-dwellers, though some were apparently adapted to live in the water column. At least one species gave birth to live young.
Reconstruction of Dunkleosteus terrelli, a six meter Devonian Placoderm, by paleoartist Davide Bonadonna.
The Chondrichthyans appear in the fossil record during the Devonian, though they may be older as a group, and are still extant; modern sharks, rays and chimeras (ratfish) are Chondrichthyans. Chondrichthyans lack boney parks; their skeletons are comprised entirely of cartilage and their scales (made of bone in other fish) are made of dentine; the body of a shark is in effect covered in thousands of teeth. Modern (and probably ancient) Chondrichthyans occupy a large range of environmental niches. Most Chondrichthyans lay eggs, though some species produce live young.
Artist's impression of Hybodus, a highly successful two meter shark that survived from the Late Permian to the Late Cretaceous (about 160 million years). Andrea Morandini.
The Acanthodians appeared in the Ordovician, were a widespread group of fish in the Silurian and Devonian (when they colonized freshwater habitats as well as the oceans) and survived to the end of the Palaeozoic, going dying out in the End Permian Extinction. They had cartilaginous skeletons like Chondrichthyans, a dentine spine at the fore of their fins (a feature also seen in primitive Chondruchthyans) and boney scales, like Osteichthyans. Although the majority of the skeleton of Acanthodians was cartilaginous, it contained boney elements in the pectoral (shoulder) and pelvic (hip) girdles; these were recruited from the dermal elements of the skin. The pectoral and pelvic joints of Placoderms also contained bones recruited from the dermal layers. Osteichthyans have dermal elements in their pectoral, but not their pelvic girdles. Chondrichthyans lack boney elements in their dermal layers (or anywhere else); no dermal elements are recruited into their skeletons.
Reconstruction of the Early Devonian Acanthodian Parexus, by palaeoartist Nobu Tamura.
All other jawed fish are Osteichthyans; Boney Fish. These have skeletons and scales made of bone, and teeth made of dentine. Technically Tetrapods (terrestrial vertebrates) are specialized Osteichthyans. Ostoeiththyans can be divided into two group, the Actinopterygians, or ray-finned fish, and the Sarcopterygians, or lobe-finned fish. One group of Actinopterygians, the Teleosts (ray-finned fish with expandable mouthparts) dominates modern marine and freshwater environments, though a few other groups, such as sturgeon, birchirs and gars, still survive. The Sarcopterygians comprise the Ceolocanths, Lungfish and Tetrapods.
Reconstruction of the Devonian Sarcopterygian Fish Osteolepis, by palaeobiologist Martin Brazeau.
Recent studies of the origin of Gnathostome groups have tended to conclude that the Placoderms split away from the other groups earliest in their history, then the Chondrichthyans. The Acanthodians, despite their mixture of features, are now seen as closely related to the Osteichthyans, or even primitive members of the group.
In a paper published in the journal PLoS One on 3 April 2012, a team of scientists led by Min Zhu of the Key Laboratory of Evolutionary Systematics of Vertebrates at the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences, discuss the discovery of specimens of two early boney fish (both from previously described species) which show recruitment of dermal bones into the pelvic girdle.
The first of these is a new specimen of Guiyu oneiros, a Late Silurian Osteichthyan showing features of both Actinopterygians and Sarcopterygians, which may predate the division between the two groups; it is sometimes described as the earliest Sarcopterygian, though this is probably an example of 'shoehorning'; the tendency of taxonomists to push specimens to existing groups (even if they don't fit well) rather than admit the need for new groups. G. oneiros has previously been described from a single specimen with a skull and partial post-cranial skeleton, but poor preservation of the pelvic girdle.
The new specimen lacks a head, but shows much better preservation of the post-cranial skeleton, including the pelvic girdle. This contains elements clearly derived from the boney dermal plates (scales) of the fish. Re-examination of the original specimen of G. oneiros shows these elements to be present, but not well enough preserved to have been identified (though this does support the diagnosis of the two specimens belonging to the same species).
The new specimen of Guiyu oneiros. (Top) Photograph. (Middle) Interpretive drawing. (Bottom) Reconstruction, using both the original and the new specimens. Red arrow points to the front end of the fish. Abbreviations: ba.sc, basal scales of pelvic fin; cla, clavicle; cle, cleithrum; icl, interclavicle; ipelv, interpelvic plate; pelv.sp, pelvic fin spine; scap, scapulocoracoid; sdf.sp, second dorsal fin spine; tr.anf, lepidotrichia of anal fin; v.dpg, ventral lamina of dermal pelvic girdle; vrs, ventral ridge scale. Zhu et al. (2012).
The second 'specimen' is a series of disarticulated pelvic girdles attributed to the Late Silurian Osteichthyan Psarolepis romeri. P. romeri is less well understood than G. oneiros, being known only from disarticulated bones. Like G. oneiros it has been described as a Sarcopterygian, though not apparently for any good reason. The limited nature of the P. romeri material would prevent any analysis based upon this alone, but the similarity between it and the G. oneiros pelvic girdle enables it to be used as supporting evidence for the presence of dermally derived elements in the pelvic girdles of early Osteichthyan Fish.
Disarticulated elements from the pelvic girdle of Psarolepis romeri. Abbreviations: art.pf, articulation facet for pelvic fin; endo.pg, endoskeletal pelvic girdle; l.dpg, lateral lamina of dermal pelvic girdle; pbr, postbranchial lamina; pf.sp, pelvic fin spine; po, foramina for pterygial nerves and vessels; sp, pectoral fin spine; v.dpg, ventral lamina of dermal pelvic girdle. Zhu et al. (2012).
Based upon this evidence Zhu et al. suggest that recruitment of dermal bones into the pelvic girdle is the primitive state in all Gnathostome Fish, and that it has been lost separately in Chondrichthyans and Osteichthyans; in the later case either being lost prior to the division of the group into Actinopterygians and Sarcopterygians, or lost separately in each group.
Family tree for Gnathostome Fish, based upon the state of the pelvic girdle. Zhu et al. (2012).
Reconstruction of Guiyu oneiros in life. Zhu et al. (2012).
See also New species of Deepwater Tilefish from the Philippines, New Tetrapodomorph Fish from the Devonian of Nevada, New species of Armored Catfish from Ecuador, New species of Parrotfish from the East Atlantic and Boney Fish on Sciency Thoughts YouTube.
Follow Sciency Thoughts on Facebook.