Vertebral column and associated elements in dipnoans and comparison with other fishes: Development and homology

Abstract

A vertebral column consisting of a persistent notochord and ossified arcocentra is the primitive condition for Gnathostomata; it still persists in primitive actinopterygians and sarcopterygians. Advanced actinopterygians and sarcopterygians develop numerous types of centra that include, among others, the presence of holocentrum, chordacentrum, and autocentrum. The chordacentrum, a mineralization or calcification of the fibrous sheath of the notochord, is only found in actinopterygians, whereas an autocentrum is a synapomorphy of teleosts above Leptolepis coryphaenoides. The chordacentrum, formed by migration of cartilaginous cells from the arches into the fibrous sheath of the notochord and usually covered by a thin calcification, is a unique feature of chondrichthyans. The actinopterygian chordacentrum and the chondrichthyan chordacentrum are not homologous. The postcaudal cartilaginous centrum is only known in postcaudal vertebrae of living dipnoans. The holocentrum is present in certain fossil dipnoans and actinopterygians, where it has been independently acquired. It is formed by proliferation of cartilage cells around the elastica externa of the notochord. These cells later ossify, forming a compact centrum. A vertebral column formed by a persistent notochord without vertebral centra is the primitive pattern for all vertebrates. The formation of centra, which is not homologous among vertebrate groups, is acquired independently in some lineages of placoderms, most advanced actinopterygians, and some dipnoans and rhipidistians. Several series of structures are associated with the vertebral column such as the supraneurals, interhaemals, radials, and ribs. In living dipnoans median neural spine, "supraneural," and dorsal radial result from growth and distal differentiation of one median cartilage into two or three median bones during ontogeny. The median neural spine articulates with the neural arch and fuses with it in the caudal vertebrae early in ontogeny. Two bones differentiate in the anterior abdominal vertebrae, i.e., the proximal neural spine and the distal "supraneural." Three bones differentiate in front of the dorsal fin, i.e., the proximal neural spine, the middle "supraneural", and the distal radial; the same pattern is observed in front of the anal fin (the proximal haemal spine, the middle interhaemal, and the distal radial). Considering that the three dorsal (and also the three ventral) bones originate from growth of only one cartilage, they cannot be serial homologs of the neural spines, or "supraneural." They are linear homologs of the median neural cartilage in living dipnoans. The development of these elements differs within osteichthyans from sarcopterygians to actinopterygians, in which the neural spine originates as a continuation of the basidorsal arcualia and in which the supraneural and radial originate from independent cartilages that appear at different times during early ontogeny. The ribs of living dipnoans are unique in that they are not articulated with parapophyses, like in primi...

Exceptional fossil preservation demonstrates a new mode of axial skeleton elongation in early ray-finned fishes

“…3), there is no evidence of a large increase in the number of somites between Saurichthyidae and Birgeria (contra Schmid 14 , who counted each neural arch-like structure as a separate segment in Saurichthys). Although this is the first instance of two neural arch-like segments reported in vertebrates, diplospondyly (two mineralized vertebral centra per segment) is relatively widespread among fishes 16,25 and is thought to increase flexibility of the vertebral column 26 . Axial flexibility in saurichthyids will be discussed in more detail below; however, a perceived trend towards features increasing the stability of the axial skeleton in these fishes 12,27 suggests functional differences from diplospondyly.…”

“…1). The basidorsals give rise to the neural arches and spines, and the basiventrals give rise to the haemal arches and spines 16 . Groups thought to be closely related to saurichthyids have either chondrified 17,18 or ossified 19,20 small, nodular intercalaries.…”

The macrosemiiform fish companion of the Late Jurassic theropod Juravenator from Schamhaupten, Bavaria, Germany

“…To avoid possible confusion of names with those of the traditional terminology, those of the traditional terminology are presented in parentheses the first time that the name is cited and also in the illustrations. Names of vertebral elements follow Schultze & Arratia (1989); Arratia (1997); Arratia et al (2001). Names and abbreviations used in the identification of different fin rays are those in Arratia (2008).…”

A juvenile Early Carboniferous (Viséan) coelacanth from Rösenbeck (Rhenish Mountains, Germany) with derived postcranial characters

“…Following the definition provided by Forey (1998, p. 213), the anteriormost caudal vertebra has a haemal spine that supports a radial (or fused interhaemal plus ventral radial sensu Arratia et al (2001), see below) but does not carry a fin ray. There are at least 32 abdominal vertebrae (identified by neural arches in the trunk region), but the vertebrae of the anterior trunk region are not preserved.…”

“…Forey 1998). However, Arratia et al (2001) interpreted the single elements distal to the neural spines as consisting of fused supraneural plus dorsal radial and the elements distal to the haemal spines as fused interhaemal plus ventral radial. The distally located fin rays, which are segmented distally, correspond in number to the endoskeletal elements and do not bifurcate.…”