Potential genetic bases of morphological evolution in the triassic fish Saurichthys

Abstract

Originating from an ancestor covered entirely by uniform rhomboid scales with numerous, highly segmented fin rays, the Triassic basal actinopterygian fish Saurichthys radiated into species diagnosed by different degrees of loss in scales, rays, and dermal bones. Such changes are analogous to those reported in mutants of different extant species, such as sticklebacks, zebrafish, and medaka. With this background, we infer a loss of function or a regulatory change of a signaling pathway as a key mechanism behind the morphological diversity of Saurichthys. Either the fibroblast growth factor pathway was affected, assuming that gene duplication had occurred, as for example in the closely related acipenserids or in teleosts, or the ectodysplasin pathway was involved, assuming that its pleiotropic effects led to viable morphological diversification. In contrast to a gradualistic model of evolution, a change in a major developmental gene serves as a viable explanation for the essential differences among the species of Saurichthys in the paleoecological context in which they originated.

Saurichthys and other fossil fishes from the late Smithian (Early Triassic) of Bear Lake County (Idaho, USA), with a discussion of saurichthyid palaeogeography and evolution

“…Based on the observed trends towards a reduced dermal skeleton, we can infer that saurichthyids are derived from a more general lower actinopterygian ancestor with, for instance, full body squamation, segmented lepidotrichia in all fins (Schmid & Sánchez-Villagra 2010), fringing fulcra on all fins, and a more common cranial bone pattern. In this context, the proposition that reduced body armour (Coates 1999, Gardiner et al 2005) and absence of fringing fulcra (Coates 1999) are synapomorphies of Saurichthys and recent sturgeons (mainly based on the use of the better known but more derived Middle Triassic species of Saurichthys for character establishment) is misleading.…”

“…Schmid & Sánchez-Villagra 2010). While attempts have been made to arrange the morphological chan- Table 1): a -maximal distance between upper margin of praeoperculum and the ventral margin of the angular bone; b -distance between jaw joint and posterior border of orbital opening; c -length of orbit; d -distance between anterior margin of the orbit and the posterior narial opening; e -maximal width of the skull at the level of the dermosphenotic bones.…”

“…The structure of the fins of Saurichthys is also not typical compared to most other lower actinopterygians. While the pectoral fins are mostly small and fan-shaped (but see Wu et al 2011) and their lepidotrichia always unsegmented, the fin rays of the pelvic, dorsal, anal, and caudal fins exhibit a general decrease in the degree of segmentation during the evolution of saurichthyids (Rieppel 1985(Rieppel , 1992Schmid & Sánchez-Villagra 2010). The lepidotrichia of both the pelvic and the unpaired fins consist of numerous segments in most early forms (e.g.…”

“…Besides the shortening of the postorbital portion of the head and the reduction of the number of dermal cranial bones, the dominant morphological changes were the loss in scales and fin ray segmentation (e.g. Mutter et al 2008;Rieppel 1985Rieppel , 1992Schmid & Sánchez-Villagra 2010). The last-mentioned modifications are directly linked to the swimming and chasing behaviour (see e.g.…”

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

“…Although both an unconstricted notochord and an increased number of axial elements-and therefore joints-in the skeleton are associated with greater axial flexibility 5,44,45 , the myoseptal morphology of Saurichthys is inconsistent with this inference. Geological and ecological data suggest that S. curionii was associated with a carbonate platform environment 24,32,46 , and ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3570 some morphological cues such as body outline and reduced squamation 12,47 are inferred to be related to good acceleration performance constrained by piscivory [48][49][50][51] . Dietary inferences are supported by stomach contents 24 .…”