Head and shoulder girdle with dermal bony plates (with bone cells); endochon-dral bone known in some taxa; head shield usually articulated (movable or not) with the trunk shield, with a double cervical joint; gill chamber extending anteriorly under neurocranium and may be covered laterally by dermal bone; probably five gill arches, no good evidence for spiracles; notochord unconstricted with vertebrae consisting only of neural and haemal arches and spines; tail diphycer-cal or heterocercal; anal fin probably absent. Although many features carry over from the osteostracans and other ostracoderms such as the notochord and head being mostly encased in bone, there are many features that are unique to placo-derms. A few Silurian records are known with greatest abundance in Lower to Upper Devonian; there is no clear evidence of placoderms surviving a major extinction event into the Lower Mississippian (see also Carr, 1995; Maisey, 1996).
Most primitive and at least many advanced groups of placoderms were marine. At least some arthrodiriforms, most antiarchiforms, and all phyllole-pidiforms are inferred to have been freshwater (e.g., but see Schultze and Cloutier, 1996). Except for the arthrodires, most were bottom-living fish with depressed bodies; only two families had species with compressed bodies. Although placoderms have been found almost worldwide, very few Devonian ones are known from South America (Maisey, 1996). A rapid replacement of placoderms by the chondrichthyans occurred at the end of the Devonian. Maximum length 6 m, but most are much shorter.
There is now strong evidence that placoderms are monophyletic, and five features are given in Goujet and Young (2004) supporting this conclusion. The hypothesis that placoderms are the sister group to all higher gnathos-tomes (Chondrichthyes, Acanthodii, and the Euteleostomi), as favored by Goujet and Young (2004) and suggested by B. Schaeffer in 1975, is accepted here (that is, placoderms are phylogenetically the sister group of all otherjawed vertebrates). Two other hypotheses as discussed by Janvier (1996) and Goujet and Young (2004) are i) placoderms and chondrichthyans are sister taxa, and ii) placoderms and osteichthyans (euteleostomes herein) are sister taxa.
The classification of this group is based primarily on Goujet and Young (2004), except that details for the antiarchs are from the papers noted for that group. The Stensioelliformes from the Lower Devonian (marine) of Germany, and the Pseudopetalichthyiformes, with one family, Paraplesiobatidae, from the Lower Devonian (marine) in Europe are not placed in the present classification.
fOrder ACANTHOTHORACIFORMES. Several genera (e.g., Brindabellaspis, Murrindalaspis, Palaeacanthaspis, Radotina, and Romundina) from Lower Devonian (marine) in Europe, Asia, and Arctic Canada.
Goujet and Young (2004) hypothesized that this taxon, with some of the oldest placoderm fossils, is the basal placoderm group. This taxon thus represents in classification the first known jawed vertebrate. They propose that one pectoral fin element (as opposed to three as in the traditional tribasal theory), an anterior insertion for the internal rectus extra ocular muscle, and two abducens innervated eye muscles may be primitive for placoderms, and hence for all jawed vertebrates.
fOrder RHENANIFORMES. One family, Asterosteidae (including
Gemuendina and Jagorina), with a raylike body, and several genera from the Lower to Upper Devonian (marine) in the United States, Bolivia, and Germany.
fOrder ANTIARCHIFORMES (antiarchs). Pectoral fin a slender appendage covered by small dermal plates; bottom feeders with mouth subterminal, and eyes dorsal and closely placed; pineal organ between eyes; sockets of the head-body joint on the head shield (opposite to the relationship in arthrodires). Maximum length about 1.2 m.
About seven families are recognized; at least Lower Devonian (but see Shimenolepis below) to end of Devonian (perhaps primarily freshwater) on, as a group, all major land masses. Classification based primarily on the phyloge-ny of Zhu (1996) and Zhu and Janvier (1996) with other details from Burrow and Turner (1999), Johanson (1997a,b), and Young and Zhang (1996). The names Goujet and Young (2004) gave to the two major clades are Yunnanolepida and Euantiarchi. The names used for certain taxa above family level and their rank are provisional.
Suborder Yunnanolepoidei. Zhu (1996) gave the orthography as Yunnanolepidoidei.
Chuchinolepidae—Chuchinolepis (synonym Quijinolepis). Yunnanolepidae—e.g., Phymolepis, Yunnanolepis.
With a number of unassigned genera: e.g., Heteroyunnanolepis, Shimenolepis (early Silurian and oldest probable placoderm and thought to be an antiarch, but this is a very poorly known fossil from China as noted by Zhu, 1996:296), and Zhanjilepis.
Suborder Bothriolepoidei. Zhu (1996) gave the orthography as Bothriolepidoidei.
Infraorder Sinolepida. Sinolepidae—e.g., Grenfellaspis and Sinolepis. Infraorder Euantiarcha
Microbrachiidae—e.g., Microbrachius. This and the remaining families are the euantiarcha (those with an articulated pectoral fin). Bothriolepidae—e.g., Bothriolepis. Gerdalepidae—e.g., Gerdalepis.
Asterolepidae (= Pterichthyidae) (in figure)—e.g., Asterolepis, Remigolepis. Related genera: Stegolepis.
With a number of unassigned genera: e.g., Dianolepis, Minicrania (sister to the other members of this suborder), and Pterichthyodes.
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