The caudal skeleton of basal teleosts, its conventions, and some of its major evolutionary novelties in a temporal dimension

The present study represents an evaluation of the current knowledge of the caudal endoskeleton of basal fossil and extant teleosts and gives new information on the origin, development and homology of the elements of the caudal skeleton. One of the major problems is the lack of metamerization in the posterior region of the body that makes identification of elements and homology statements difficult. The definitions of preural region, ural region, and preural centrum 1 are analyzed. Other landmarks that facilitate the identification and homologization of certain caudal elements are also reviewed. New studies on the early development of the caudal skeleton of basal extant teleosts demonstrate that the ural region develops from an early polyural skeleton into a diural skeleton or into a compound terminal centrum in different ways in different teleosts. The two ural centra present in adult teleosts develop ontogenetically and phylogenetically from a polyural stage independently in different teleostean lineages origin. Thus, the two ural centra of the diural skeleton are not homologous across teleosts. Consequently, we propose to study the origin and composition of the ural region of different teleosts using the polyural terminology. This assumes a one-to-one relationship between ural centra and their respective hypaxial (e. g., ural 1/hypural 1; ural 2/hypural 2; ural 3/hypural 3) and epaxial elements. Polyural terminology facilitates interpretation of the composition of the two ural centra and their relationships to epaxial and hypaxial elements of the caudal fin. The compound terminal centrum (synonym: urostyle) present in most ostarioclupeomorphs (or otocephalans) and many euteleosts is currently assumed to be the result of a fusion involving preural centrum 1 and the first ural centrum. According to our studies based on day-to-day ontogenetic series, the compound terminal centrum is the result of an early fusion of preural centrum 1 with different ural centra in different teleosts. From the highest number of 13 hypurals found in the Early Jurassic †Pholidophorus bechei, a decreased number of 8 or 7 hypurals is observed in Late Jurassic elopiforms and 6 or fewer hypurals in extant teleosts. In most cases the reduction in number of hypurals has been interpreted as a fusion of elements, but this has not been shown ontogenetically. A complete series of true uroneurals occurs first in “true” teleosts (†Leptolepis coryphaenoides plus more advanced teleosts) at the base of the teleostean radiation. The homology of uroneurals is still not understood for most fossil and extant teleosts, with a reduction in number ranging from 7 to 3 to none in different extant teleostean lineages. In fossil basal “true” teleosts, the anterior-most uroneural seems to be a modification of ural neural arch 2 or 3, whereas the anterior-most uroneural is a modification of ural neural arch 4 in elopiforms, some osteoglossomorphs and salmonids. The origin and development of the pleurostyle (currently interpreted as a modified uroneural) in ostarioclupeomorphs remain unclear. The pleurostyle differs between groups, being chondral in some, but a membrane bone in others. “Uroneurals of a peculiar sort” develop as modified epaxial elements of preural as opposed to ural centra in fossil †pachycormiforms, some †aspidorhynchiforms and †’pholidophoriforms’. The homology of epurals is not fully understood for most basal teleosts. Epurals of basal teleosts are neural spines separated from neural arches. Basal teleosteomorphs and a few basal teleosts (and salmonids) possess simultaneously epurals derived from neural spines of both preural and ural centra. However, aspidorhynchiforms lack epurals. In †Leptolepis coryphaenoides plus more advanced teleosts the anterior-most epural corresponds to the neural spine of ural centrum 1, the second epural to ural centrum 2, and so on. In fossil and extant elopiforms, the three epurals correspond to ural centra 1-3 (polyural terminology), whereas in basal osteoglossomorphs the only epural present seems to belong to ural centrum 2 (polyural terminology). According to the present evidence, the origin of the one or two epurals present in ostarioclupeomorphs, as well as their homology, remains unknown.