Iniopterygia : a new order of Chondrichthyan fishes from the Pennsylvanian of North America

Abstract

An entirely new group of Pennsylvanian fishes, belonging to the class Chondrichthyes, is described and their comparative anatomical and phylogenetic relations are discussed.Seven species belonging to five genera are distinguished and placed within the subclass Holocephali as a separate order, Iniopterygia. The iniopterygians are structural, but not phyletic, intermediates between the chimaeroids (as here denned) and the elasmobranchs. Present analysis permits the notion that the holocephalians and the elasmobranchs are sister groups sharing a common ancestor that never possessed a bony dermal armor but an even spread of lepidomorial denticles over the entire skin and the stomodaeum. Iniopterygians and chimaeroids, in turn, appear to be sister groups having evolved from a common ancestor that combined an autostylic jaw suspension with a generalized shark-like dentition.Iniopterygians are presently known only from carbonaceous, sheety shales of the Pennsylvanian basin complex of central North America. IX Logan Quarry PF6661, cf , LQS level J, articulated skeleton, lacking tail fin (XR: LQ237) PF6660, cf , LQS level J, isolated pectoral clasper hooks XR: LQ228)Hajji Hollow PF6587, cf, partly articulated skeleton, incomplete (XR: HH5) Characterization. -Anterior finrays of pectoral fins in males much enlarged and provided with a single file of about 13 fishhook-shaped denticles, diminishing in size distad. Description. -The largest skeletons (i.e., PF6678 or PF6645) are between 30 and 35 cm. in overall length. U.N.S.M. = University of Nebraska State Museum. "XR=X ray plate.

Skull and brain of a 300-million-year-old chimaeroid fish revealed by synchrotron holotomography

“…The iniopterygian skulls from Oklahoma and Kansas are very similar to that of Sibyrhynchus denisoni (1), from the Indiana black shale, in the shape of the jaw and scutes, and in the outline of the braincase in dorsal view (Figs. 1 A and E and 2 C and D).…”

“…Carboniferous ͉ chondrichthyans ͉ vertebrate ͉ X-ray phase imaging I niopterygians have been described on the basis of partly articulated specimens preserved as impressions in the 310-Myr-old Carboniferous (Late Pennsylvanian) black shale of the northern U.S. (1). These fossils show 2 main chondrichthyan characteristics: a fragile layer of prismatic calcified cartilage lining the endoskeletal elements and pelvic claspers (special copulation organs).…”

“…1A). All iniopterygians described until now were f lattened specimens that do not allow 3D reconstructions, although studies based on stereographic radiographs have suggested the presence of some chimaeroidlike characteristics (1)(2)(3)(4), but the detailed anatomy of the group has remained largely unknown. However, among the numerous 300 Myr-old fish-bearing concretions from the Pennsylvanian sites of Oklahoma and Kansas (see Materials and Methods), most of which yield braincases of palaeonisciforms (ray-finned fishes) (5), some have turned out to contain chondrichthyan remains.…”

Chondrichthyan life history styles as revealed by the 320 million years old Mississippian of Montana

“…The pectoral fins of INIOPl and INIOP2 resemble those of the teleostean family Dactylopteridae (flying gurnards) in proportions as well as in having a small number of very long, free, trailing radials. Most remarkably, the pectoral fins of INIOP3 are mounted along the dorsolateral margins of the trunk as they are in the Pennsylvanian members of this order (Zangerl & Case 1973). There are two families of teleost fish with pectoral fins mounted high on the dorsolateral flank of the body and with the lower lobe of the tail longer than the upper lobe, the characiform Gasteropelecidae or freshwater hatchetfishes, and the cyprinodontiform Exocoetidae or flyingfishes.…”

Holocephalan embryos provide evidence for gill arch appendage reduction and opercular evolution in cartilaginous fishes

“…A hyoid ray-supported opercular flap has been reported in the symmoriid Falcatus (36) (hyoid rays reported after reexamination in ref. 8), and has also been described in certain iniopterygians (37,38), although there is evidence that these groups may be stem holocephalans (8,11,12,39,40). With such variation documented in only a small number of known taxa, a thorough reevaluation of chondrichthyan phylogeny and pharyngeal endoskeletal anatomy is clearly needed to accurately reconstruct the frequency, timing, and polarity of chondrichthyan branchial ray expansion and reduction.…”