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

Abstract

Elongate body plans have evolved independently multiple times in vertebrates, and involve either an increase in the number or in the length of the vertebrae. Here, we describe a new mechanism of body elongation in saurichthyids, an extinct group of elongate early ray-finned fishes. The rare preservation of soft tissue in a specimen of Saurichthys curionii from the Middle Triassic (Ladinian) of Switzerland provides significant new information on the relationship between the musculature and the skeleton. This new fossil material shows that elongation in these fishes results from doubling the number of neural arch-like elements per myomeric segment. This unique way of generating an elongate body plan demonstrates the evolutionary lability of the vertebral column in non-teleostean fishes. The shape and arrangement of preserved myosepta suggest that S. curionii was not a highly flexible fish, in spite of the increase in the number of neural arch-like elements.

Building the backbone: the development and evolution of vertebral patterning

“…This might be aided by the relative independence of teleost arch and chordacentrum development already noted, which perhaps promotes oscillatory swimming movements (Lauder, 1980). Enhanced axial flexibility might also explain the evolution of diplospondyly in elasmobranchs and holocephalans, where the ratio of centra to arches is 2:1 rather than 1:1; and, conversely, axial stabilization might be promoted where the ratio is 1:2 (Maxwell et al, 2013).…”

Regionalization of the axial skeleton in the ‘ambush predator’ guild – are there developmental rules underlying body shape evolution in ray-finned fishes?

“…Phylogenetic relationships were based on the hypotheses of [8,69]. Branch lengths were set using constrained ages of terminal taxa based on youngest stratigraphic occurrence; internal nodes within Acipenseriformes were constrained using fossil occurrence data from [70].…”

“…This landmark coincides with the osteological transition in all species referred to Saurichthys , and all saurichthyids except Saurorhynchus . Based on the findings of [8], counts of neural arch-like elements calculated from saurichthyids were divided by two to obtain the number of embryonic segments in each region, thus standardizing the data to that obtained for Birgeria and Acipenseriformes. Fineness ratio values were log transformed prior to analysis.…”

“…Body elongation in fishes has been attributed to several factors, including increase in the number of vertebrae, increase in the length of vertebrae, increase in the length of the skull, and decrease in the depth of the body (reviewed by [4]). An additional mechanism, duplication of elements within a somite, has been proposed for some non-teleost actinopterygians [8]. A detailed investigation into axial elongation in actinopterygians found that the addition of vertebrae occurred in either the abdominal region, the caudal region, or both, and suggested these body regions were capable of independent evolution, being organised into separate developmental modules [3].…”

Two new species ofSaurichthys(Actinopterygii: Saurichthyidae) from the Middle Triassic of Monte San Giorgio, Switzerland, with implications for character evolution in the genus

“…), trophically differentiated based on body size (Rieppel, ), and S. macrocephalus (Deecke, ) and S. curionii (Bellotti, ) from the overlying Ladinian Meride Formation, differentiated based on relative jaw length and opercular shape, and from the older species pair by qualitative reductions in the squamation (Rieppel, ). The four species appear to have a similar paleoecology (Rieppel, ; Furrer, ), and with the Late Triassic–Early Jurassic saurichthyids, form a monophyletic group to the exclusion of all Early Triassic species (Rieppel, ; Maxwell, Furrer & Sánchez‐Villagra, ). The low diversity in the Monte San Giorgio basin is surprising given the magnitude of faunal and environmental changes at the locality over the depositional period (Röhl et al ., ; Stockar, ), and in fact species‐level diversity of the saurichthyids from the Besano Formation is higher than formally recognized (Rieppel, ; Wilson et al ., ).…”