Abstract
Current knowledge about the evolutionary morphology of the vertebrate gastrointestinal tract (GIT) is hindered by the low preservation potential of soft tissues in fossils. Exceptionally preserved cololites of individual †Saurichthys from the Middle Triassic of Switzerland provide unique insights into the evolutionary morphology of the GIT. The GIT of †Saurichthys differed from that of other early actinopterygians, and was convergent to that of some living sharks and rays, in exhibiting up to 30 turns of the spiral valve. Dissections and literature review demonstrate the phylogenetic diversity of GIT features and signs of biological factors that influence its morphology. A phylogenetically informed analysis of a dataset containing 134 taxa suggests that body size and phylogeny are important factors affecting the spiral valve turn counts. The high number of turns in the spiral valve of †Saurichthys and some recent sharks and rays reflect both energetically demanding lifestyles and the evolutionary histories of the groups.
Highlights
Most paleontological perspectives on the vertebrate gastrointestinal tract (GIT) are based on indirect evidence from fossilized faeces[11] because the corresponding soft tissues are highly susceptible to decay and early loss during fossilization[12]
We review the distribution of different morphologies of the GIT across extant and extinct fishes in order to trace factors including body size, diet, lifestyle and phylogeny that may correlate with different GIT morphologies
A spirally coiled portion indicating the presence of a spiral valve with a very high turn count is a remarkable feature of †Saurichthys cololites
Summary
A spirally coiled portion indicating the presence of a spiral valve with a very high turn count is a remarkable feature of †Saurichthys cololites. Fishes with a spiral valve have shorter overall intestinal lengths than other species[35], conserving space in the abdominal cavity for other purposes (such as developing embryos)[36] Phylogeny plays an important role in understanding the variation in spiral valve counts in fishes, with closely related species and genera tending to exhibit similar turn counts[39,46]. This applies generally to extant taxa despite fluctuations in size and different trophic niches, rendering functional interpretations questionable if not controlled for relatedness. We emphasize the importance of investigating gastrointestinal contents in fossils, because they often reflect the morphology of the surrounding soft tissue and provide information on palaeobiology and phylogeny that would otherwise remain elusive
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