Interopercle absent; premaxilla and maxilla rigidly attached to the ectoptery-goid and dermopalatine; spiracle usually present; myodome absent in the most primitive taxa.
The classification of this group is very insecure. It is a group of great structural diversity, and evidence is lacking for monophyly not only for this subclass but also for most of the groups herein recognized. Given the many phyloge-netic uncertainties that exist on the relationships of many taxa, I have not made many changes to the classification of this taxon from that used in Nelson (1994), except for the exclusion of the Polypteriformes. As noted by Cloutier and Arratia (2004) and other authors, the selection of outgroups and the varied inclusion of extant and fossil taxa play a significant role in phylogenetic analyses. There is great need for extensive work, involving both fossil and extant material, such as that done by Grande and Bemis (1991, 1996, 1998).
The arrangement of fossil taxa given by Cloutier and Arratia (2004) is a good hypothesis to follow. The sister group to the remaining actinopterygians is thought to be Dialipina (based also on a 1997 work of L. Taverne and a 2001 work by H.-P. Schultz and S. L. Cumbaa), followed in a successive comblike branching pattern, with each group sister to all remainging taxa, by perhaps i) Cheirolepididae, ii) Mimia and Moythomasia, iii) Osorioichthys and Kentuckia, and continuing. No phylogenetic classification is attempted here, and for convenience only for this classification, chondrosteans are shown as the sister group of neopterygians until more convincing evidence to the contrary is available. An early chondrostean family not otherwise classified here is Haplolepidae (with two Pennsylvanian genera, Haplolepis and Pyritocephalus).
Extant taxa in two families, six genera, and 27 species.
fOrder CHEIROLEPIDIFORMES. Includes only the one family, the Devonian Cheirolepididae with the one genus, Cheirolepis. One species, C. canadensis, may hold the record for having the largest number of pelvic fin rays, up to 124, as noted in a 1996 paper by G. Arratia and R. Cloutier. Although classified here within the chondrosteans, this taxon, after Dialipina, is probably the sister group for all remaining actinopterygians (e.g., Cloutier and Arratia, 2004).
fOrder PALAEONISCIFORMES. In many primitive palaeoniscids, the cheekbones form a solid unit (the maxilla, preopercles, and suborbitals are firmly united), the hyomandibular is oblique, the eyes are large and far forward, and the tail is strongly heterocercal. More advanced forms had a hyomandibular in the vertical plane and a breakup of the cheekbones. This permitted more flexibility in the oral-branchial chamber. The dorsal lobe of the tail became reduced to an abbreviated heterocercal tail. Numerous other evolutionary trends can be noted in proceeding from the chondrostean level of organization to the holostean level.
Coccolepis. incertae sedis. The morphology of Coccolepis bucklandi was described by Hilton et al. (2004).
Suborder Palaeoniscoidei. Families or genera placed in this heterogenous group of primitive chondrosteans include Aeduellidae, Acrolepidae (with, for example, Acrolepis and possibly Boreosomus and Pteronisculus), Amblypteridae (Amblypterus and Paramblypterus) (Dietze, 2000), Birgeriidae (e.g., Birgeria), Canobius, Commentryidae, Elonichthyidae, Palaeoniscidae, Pygopteridae, Rhabdolepidae (includes the Devonian Osorioichthys), Rhadinichthyidae and the related Aesopichthyidae (Poplin and Lund, 2000; Cloutier and Arratia, 2004), and Stegotrachelidae (with, e.g., the Devonian Stegotrachelus and Tegeolepis).
Suborder Redfieldioidei. Body fusiform; mouth terminal or subterminal; dorsal and anal fins positioned far back, opposite one another, and with fin rays more numerous than radials; branchiostegal rays reduced to one or two platelike bones; single external naris surrounded by a distinctive "premaxilla," rostral, nasal, and adnasal bones. Triassic and Lower Jurassic, freshwater.
About 15 genera, herein treated as belonging to one family, Redfieldiidae (e.g., Brookvalia,, Dictyopyge, Helichthys, Redfieldius, and Schizurichthys).
Suborder Platysomoidei. Body deep and compressed (zeidlike).
Three families, Bobastraniidae, Chirodontidae, and Platysomidae. Marine and freshwater. Mississippian to Lower Triassic.
Suborder Dorypteroidei. Body deep and mostly scaleless; pelvic fin in front of pectorals (jugular); caudal peduncle very narrow.
One Upper Permian genus, Dorypterus.
fOrder TARRASIIFORMES. Dorsal and anal fins continuous with the diphycercal caudal fin; pelvic fins absent; scales variously reduced or absent; body elongate; pectoral fins with a rounded fleshy lobe; frontal bones distinct (e.g., Taverne, 1996). Mississippian.
fOrder GUILDAYICHTHYIFORMES. Highly compressed, discoidal bodies, tall rhombic "ganoid" scales with peg-and-socket joints. Marine fishes of Mississippian age.
Lund (2000) found in a cladistic analysis a stable sister group relationship between Polypterus and the Guildayichthyiformes as a crown group within the Paleozoic Actinopterygii, and he rediagnosed the Cladistia as a superorder to reflect this relationship. However, I follow the placement of Cloutier and Arratia (2004) in showing a close relationship with the Tarrasiidae and Guildayichthyidae.
Two genera, Guildayichthys and Discoserra, from Montana (Lund, 2000).
fOrder PHANERORHYNCHIFORMES. Body superficially like that of a sturgeon.
One Pennsylvanian genus, Phanerorhynchus.
Order ACIPENSERIFORMES (18)—sturgeons. Caudal fin heterocercal; myodome and preopercle reduced or absent; gulars absent; skeleton largely cartilaginous; fin rays more numerous than their basals; intestine with spiral
valve. Grande and Bemis (1991) give derived characters for this order and for the taxa of the suborder Acipenseroidei. Their classification, in their detailed osteological study, is followed here.
Extant representatives in two families with six genera and 27 species (Grande and Bemis, 1996; Bemis et al., 1997).
tFamily PEIPIAOSTEIDAE. Incertae sedis. Two genera, Peipiaosteus and Stichopterus, and probably Spherosteus and Yanosteus, extending back to the Upper Jurassic (Grande and Bemis, 1996; Bemis et al., 1997).
fSuborder Chondrosteoidei. One family, Chondrosteidae (mouth subterminal), with Chondrosteus and Strongyylosteus, and perhaps Gyrosteus from the Jurassic of Europe. The chondrosteids are considered to be the primitive sister group of the acipenseroids (Grande and Bemis, 1991, 1996).
Suborder Acipenseroidei. Opercle lost, gill cover made up primarily by the sub-opercle; one to three elements that may be homologous to the branchiostegal rays of other actinopterygians; endocranium with an extensive rostrum.
Family ACIPENSERIDAE (60)—sturgeons. Anadromous and freshwater; Northern Hemisphere.
Five rows of bony scutes or plates on body; four barbels in front of mouth; mouth inferior and protrusible; gill rakers fewer than 50; teeth absent in adults; pectoral fin with anterior spinous ray made up of fused rays; swim bladder large. The freshwater Kaluga, Huso dauricus, and the anadromous Beluga, H. huso, are among the largest if not the largest fish in freshwater. H. huso definitely reaches 4.2 m, and longer lengths have been reported for both species.
Four genera with 25 species (Bemis et al., 1997; Birstein and Bemis, 1997). Many of the species are difficult to identify. The historical biogeography of sturgeons is explored in Choudhury and Dick (1998). One fossil genus, the Upper Cretaceous Protoscaphirhynchus, from Montana.
Subfamily Acipenserinae. Three genera in two tribes (Grande and Bemis, 1996; Bemis et al., 1997), although this arrangement may be incorrect (Birstein et al., 2002).
tribe acipenserini. Spiracle present; snout and caudal peduncle subconical.
Acipenser. Range of family. Gill membranes joined to isthmus, mouth transverse. Seventeen species (five of which occur in North America).
tribe scaphirhynchini. Spiracle absent; snout depressed.
Pseudoscaphirhynchus. Aral Sea basin. Caudal peduncle short, slightly depressed, and not completely armored. Three species.
Scaphirhynchus. Mississippi basin. Caudal peduncle long, depressed, and completely armored. Three species.
Huso. Adriatic Sea to Caspian Basin; Amur River. Gill membranes joined to one another, mouth crescentic. Two species.
Family POLYODONTIDAE (61)—paddlefishes. Freshwater, rarely brackish; China and United States.
Snout paddlelike; body lacking the large scutes of acipenserids but with small "scales" in some regions, such as the caudal peduncle and caudal fin, and large Psephurus with trunk "scales"; minute barbels on snout; gill rakers long and in the hundreds in the plankton-feeding Polyodon (shorter and fewer in number in Psephurus); teeth minute; spiracle present; gill cover greatly produced posteriorly. Maximum length perhaps up to 3 m, attained in Psephurus gladius.
Fossil taxa are Protopsephurus (Lower Cretaceous, China, the oldest and most primitive paddlefish known and sister to all other members, Grande et al., 2002), Paleopsephurus (Lower and Upper Cretaceous, freshwater, Montana and Wyoming, and considered to be the primitive sister group to the remaining polyodontid taxa), Crossopholis (Lower Eocene, freshwater, Wyoming, and the sister group to Polyodon), and Polyodon tuberculata (lower Paleocene, freshwater, Montana). Grande and Bemis (1991, 1996) and Grande et al. ( 2002) described this family and its included taxa. Two living species.
Polyodon spathula. United States (Mississippi drainage). The Paddlefish, plankton-feeding, with a nonprotrusible mouth.
Psephurus gladius. China (Yangtze River and lower reaches of some other rivers and adjacent sea). The Chinese Paddlefish, piscivorous, with a protrusible mouth.
fOrder PTYCHOLEPIFORMES. Triassic and Jurassic. North America.
fOrder PHOLIDOPLEURIFORMES. Triassic. One family, Pholidopleuridae (e.g., Australosomus and Pholidopleurus).
fOrder PERLEIDIFORMES. Triassic and Lower Jurassic. Example families placed in this artificial group are Cephaloxenidae, Colobodontidae, Platysiagidae, Peltopleuridae, Cleithrolepidae, and Perleididae (e.g., Aetheodontus, Dipteronotus, and Meridensia, e.g., Tintori, 1990; Bürgin, 1992). Tintori and Sassi (1992) provided evidence for a sequenced ranking of Australosomus, Peltopleuriformes (with Peltopleurus, Habroichthys, and Thoracopterus, placed in the family Thoracopteridae and thought to be capable of gliding), Cleithrolepis, Perleidus, Luganoia, and the Neopterygii. Further studies on members placed here include that of Bürgin (1996), Lombardo and Tintori (2004), and Mutter (2004).
tOrder LUGANOIIFORMES. Triassic.
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