Re‐evaluation of the ontogeny and reproductive biology of the Triassic fish Saurichthys (Actinopterygii, Saurichthyidae)

Abstract

Viviparity has evolved independently at least 12 times in ray-finned fishes. However, the fossil record of actinopterygian viviparity is poor, with only two documented occurrences. Both of these are from the non-teleost actinopterygian Saurichthys, and include S. curionii and S. macrocephalus from the Middle Triassic Meride Limestone (Monte San Giorgio, Switzerland). Here, we present new data on the reproductive biology of these species, giving unprecedented insights into their life-history. Based on positional and preservational criteria, six specimens were identified as unambiguously gravid. Embryos were positioned dorsal to the gastrointestinal tract, parallel to the axial skeleton and to each other, in the posterior two-thirds of the abdominal region. A minimum of 16 embryos are preserved in the most fecund females and, based on the largest preserved embryos and smallest preserved neonates, birth must have occurred at 7-12% of maternal fork length. Embryonic crania and teeth are relatively well-ossified, however ossification of the parietal region is delayed. In the postcranium, the median scale rows and lepidotrichia are ossified, but not the lateral scale rows. Ossified squamation and gradual allometric growth suggests that neonates did not undergo metamorphosis and were relatively precocial. When considered in a phylogenetic context, neither live birth nor internal fertilization appears to represent the primitive state for saurichthyid fishes.

Internal cranial anatomy of Early Triassic species of †Saurichthys (Actinopterygii: †Saurichthyiformes): implications for the phylogenetic placement of †saurichthyiforms

“…† Saurichthys is known from thousands of specimens belonging to over 40 nominal species, associated with marine, freshwater and brackish settings and occurring on all continents except Antarctica and South America [ 14 , 25 ]. Abundant and well-preserved fossils permit investigation of soft-tissue features, with studies revealing reproductive mode and details of ontogeny [ 33 – 35 ], mode of axial elongation [ 26 , 36 ], swimming mode and efficiency [ 37 ], as well as gastrointestinal anatomy [ 38 ]. Although the wealth of potential paleobiological information about † Saurichthys is unrivalled among early fossil actinopterygians, some basic anatomical aspects of this genus are known in limited detail relative to other taxa.…”

A Portrait of a Young Fish: Redescription ofPteronisculus gunnari(Nielsen, 1942) from a juvenile specimen from the Early Triassic of East Greenland, with implications for ontogenetic development in early actinopterygians

“…Although complete growth series are known for some Palaeozoic and early Mesozoic fishes (e.g. R Cloutier, 2010a;Maxwell et al, 2018), juvenile actinopterygians are comparatively rare. Growth series of styracopterid actinopterygians from the Mississippian of Scotland reveals allometric growth of the fins and demonstrates an increase in the number of scales through ontogeny (Lauren Cole Sallan & Coates, 2013).…”

“…Similarly, juvenile actinopterygians from Bear Gulch (Namurian) and Mazon Creek (Pennsylvanian) show scale covering spreading from the lateral line and the tail, and progressive ossification of the skull and pectoral girdle (Karen Anne Schultze & Bardack, 1987). Isolated juvenile palaeopterygian specimens are relatively more common, and have been recognized by the reduced scale covering and/or skull dermal plate development (M. Coates, 1993;Hutchinson, 1973;Karen A Lowney, 1980), the degree of sensory canal enclosure (Maxwell et al, 2018) or limited endochondral ossification of the skull and pectoral girdle (Nielsen, 1942(Nielsen, , 1949.…”

The marine conservation deposits of Monte San Giorgio (Switzerland, Italy): the prototype of Triassic black shale Lagerstätten

“…A Mixosaurus , PIMUZ T 4830 (e.g., Brinkmann, 1996 ; Miedema et al, 2023 ). B Saurichthys , PIMUZ T 3917 (e.g., Maxwell et al, 2018 ) …”