Reevaluation of the caudal skeleton of certain actinopterygian fishes: III. Salmonidae. Homologization of caudal skeletal structures

Abstract

The ontogenetic development of caudal vertebrae and associated skeletal elements of salmonids provides information about sequence of ossification and origin of bones that can be considered as a model for other teleosts. The ossification of elements forming the caudal skeleton follows the same sequence, independent of size and age at first appearance. Dermal bones like principal caudal rays ossify earlier than chondral bones; among dermal bones, the middle principal caudal rays ossify before the ventral and dorsal ones. Among chondral bones, the ventral hypural 1 and parhypural ossify first, followed by hypural 2 and by the ventral spine of preural centrum 2. The ossification of the dorsal chondral elements starts later than that of ventral ones. Three elements participate in the formation of a caudal vertebra: paired basidorsal and basiventral arcocentra, chordacentrum, and autocentrum; appearance of cartilaginous arcocentra precedes that of the mineralized basiventral chordacentrum, and that of the perichordal ossification of the autocentrum. Each ural centrum is mainly formed by arcocentral and chordacentrum. The autocentrum is irregularly present or absent. Some ural centra are formed only by a chordacentrum. This pattern of vertebral formation characterizes basal teleosts and primitive extant teleosts such as elopomorphs, osteoglossomorphs, and salmonids. The diural caudal skeleton is redefined as having two independent ural chordacentra plus their arcocentra, or two ural chordacentra plus their autocentra and arococentra, or only two ural chordacentra. A polyural caudal skeleton is identified by more than two ural centra, variably formed as given for the diural condition. The two ural centra of primitive teleosts may result from early fusion of ural centra 1 and 2 and of ural centra 3 and 4, or 3, 4, and 5 (e.g., elopomorphs), respectively. The two centra may corespond to ural centrum 2 and 4 only (e.g., salmonids). Additionally, ural centra 1 and 3 may be lost during the evolution of teleosts. Additional ural centra form late in ontogeny in advanced salmonids, resulting in a secondary polyural caudal skeleton. The hypural, which is a haemal spine of a ural centrum, results by growth and ossification of a single basiventral ural arococentrum and its haemal spine. The proximal part of the hypural always includes part of the ventral ural arcocentrum. The uroneural is a modification of a ural neural arch, which is demonstrated by a cartilaginous precursor. The stegural of salmonids and esocids originates from only one paired cartilaginous dorsal arcocentrum that grows anteriorly by a perichondral basal ossification and an anterodorsal membranous ossification. The true epurals of teleosts are detached neural spines of preural and ural neural arches as shown by developmental series; they are homologous to the neural spines of anterior vertebrae. Free epurals without any indication of connection with the dorsal arococentra are considered herein as an advanced state of the epural. Caudal distal radials originate fr...

Relationships of Lower Euteleostean Fishes

“…62, 594), for which the type locality is Salmo and other salmonids. Ontogeny is now well known in salmonoids (Arratia and Schultze, 1992), and contrary to earlier suggestions there is no indication that the structure is compound in origin. Stearley and Smith (1993, character 96) considered the large, fan-shaped membranous outgrowth in salmonines to be derived relative to the smaller outgrowth in Thymallus and coregonids ( Fig.…”

“…In salmonoids, Stearley and Smith (1993, character 97), in agreement with Arratia and Schultze (1992), referred to the short neural spine in coregonids and Thymallus as a neural arch with a detached neural spine. Thymallus and coregonids may give that impression, because in some specimens NPU2 is tipped with cartilage (Arratia and Schultze, 1992, figs.…”

“…14), in the bathylagid Bathylagichthys (Kobyliansky, 1986, fig. 19), in the coregonids Prosopium (Arratia and Schultze, 1992) and Stenodus, and in all other osmerids (Allosmerus, Mallotus, Spirinchus, and Thaleichthys). No urodermal is recorded in Recent osteoglossomorphs, non-elopid elopomorphs, clupeomorphs, ostariophysans, alepocephaloids, salmonids, galaxioids, salangids, esocoids, stomiiforms, or acanthomorphs.…”

“…Other salmonoids showing the condition are illustrated in Shaposhnikova (1968a,b), Nybelin (1971, pi. 6) and Arratia and Schultze (1992). Among osmeroids, the condition does not occur (the neural and haemal spines are slender distally) in Allosmerus, Osmerus (Fujita, 1990, fig.…”

Scale morphology and specialized dorsal scales of a new teleosteomorph fish from the Aptian of West Gondwana

“…Rhombic, modified ganoid scales covering parts of the bases of the upper epaxial caudal rays were defined as urodermals by Patterson (1968) and Arratia and Schultze (1992). The base of the uppermost caudal principal fin ray in specimens of Teleosteomorpha n. sp.…”