Preserved gastrointestinal tracts in Actinopterygian Fish from the Middle Triassic Monte San Giorgio Lagerstätte of Switzerland.

In Fish as in other animals an increased gut length  is associated with an increased amount of vegetable food in the diet, with omnivores having longer guts than carnivores, and herbivores having longer guts than omnivores. The spiral valve is a spiraled area of gut, which increases the total surface available for absorption of food, without greatly increasing the overall volume occupied by the gut. Preserved gastrointestinal tracts are rare in the fossil record, making it difficult to assess the origin of modern structures, particularly those with unusual distributions within different different taxonomic groups. A spiral valve is found in modern Sharks, Lampreys, non-Teleost Actinopterygian Fish (Bichirs, Reedfish, Sturgeon, Paddlefish, Gar and Bowfin - a total of fifty one species in four distantly related lineages) and Lungfish, while Coelocanths have a scroll valve (thought to be derived from a spiral valve). However Teleosts, Tetrapods (terrestrial Vertebrates) and Hagfish have no trace of any such structure. 

In a paper published in the journal Nature: Scientific Reports on 6 January 2015, Thodoris Argyriou of the Paleontological Institute and Museum at the University of Zurich, Marcus Clauss of the Clinic for Zoo Animals, Exotic Pets and Wildlife at the University of Zurich, Erin Maxwell of the Stuttgart State Museum of Natural History and Heinz Furrer and Marcelo Sánchez-Villagra, also of the Paleontological Institute and Museum at the University of Zurich, describe the preserved gastrointestinal tracts of three early Actinopterygian Fish from the Middle Triassic Monte San Giorgio Lagerstätte of the Tricino Canton in Switzerland. All three Fish belong to the genus Saurichthys, an extinct lineage of Pike-like Fish distantly related to Sturgeon and, Paddlefish. 

The first specimen examined is placed in the species Saurichthys costasquamosus. The specimen is almost complete, lacking only the tip of the snout. The specimen is slightly over 30 cm in length, with the largest known specimen of the species being 87 cm. Almost the complete length of the gastrointestinal tract is preserved, with about 40% of this occupied by a smaller fish of the genus Luganoia, swallowed head-first. The rear of the gut appears to include a section of spiral intestine interpreted as a spiral valve with more than 17 turns.

(a) Saurichthys costasquamosus with undigested Actinopterygian prey (cf. Luganoia) followed by a three dimensional spiral cololite. The area of interest is delineated by a box. (b) Interpretative drawing of the area of interest of the previous specimen. Scales of the midlateral row were omitted. Abbreviations are as follows: ant.int.: anterior intestine; C.F.: caudal fin of the contained prey; mv.: medioventral scale row; N.: neurocranium of the contained prey; n.a.: neural arch-like elements; vl: ventrolateral scale row. Scale bars equal 1 cm. Argyriou et al. (2015).

The second specimen is assigned to the species Saurichthys macrocephalus. It is coiled into an 'S' shape with the head detached and has an estimated body length of 24 cm, compared to 66 cm in the largest known specimen of the species. The gastrointestinal tract is preserved as a white ribbon running through the body cavity, divided into a number of segments interpreted as a stomach, a short anterior intestine and a spiral intestine with approximately 17 turns.

(c) Saurichthys macrocephalus, photographed under UV light, with a two dimensional cololite present, extending from the stomach to the spiral intestine. The area of interest is delineated by a box; (d) interpretative drawing of the area of interest around the cololite. Abbreviations are as follows: ant.int.: anterior intestine; mv.: medioventral scale row; n.a.: neural arch-like elements; vl: ventrolateral scale row. Scale bars equal 1 cm. Argyriou et al. (2015).

The third specimen described is assigned to the species Saurichthys paucitrichus. It is has an estimated length of 21 cm, with the post-gastrilc portion of the gastrointestinal tract preserved in three dimensions. The spiral section of the intestine has about 30 turns, and spans approximately 14 vertebral segments.

(a) Saurichthys paucitrichus with a three dimensional intestinal cololite preserved in situ, the area of interest is delineated by a box; (b) Detail of the area of interest containing the spiral cololite in the previous specimen; (c) Interpretative drawing of the spiral cololite of the previous specimen. Abbreviations are as follows: ant.int.: anterior intestine; mv.: medioventral scale row; n.a.: neural arch-like elements; plv.: pelvic bone; vl: ventrolateral scale row. All scale bars equal 1 cm. Argyriou et al. (2015).

The spiral glands of all three specimens are exceptionally long compared to those of modern species of similar size. A longer spiral gland could be associated with a herbivorous diet, but nothing about the morphology of Saurichthys suggests such a habit; one of the examined specimens even has a large prey item in its foregut. Alternatively the elongate spiral gland could be an adaptation to a very high energy lifestyle, such as seen in pelagic Fish which actively chase down their prey. However Saurichthys  does not appear to have been adapted to such a lifestyle either; they are similar in shape to modern Pike, which are ambush predators, and evidence has been found that some species attacked Pterosaurs, flying animals which no Fish could have chased down but which could have been targeted by an ambush predator.

Phylogenetic framework of gastrointestinal tract morphology and spiral valve turn counts of Actinopterygians, including Saurichthys paucitrichus. All drawings depict the gastrointestinal tracts in ventral view with foregut to the left and hindgut to the right. Argyriou et al. (2015).

The Saurichthyiformes (the group to which Saurichthys belonged) first appeared in the Permian, and where for the most part quite large Fish. Saurichthys is thought to have thought to have been descended from larger species. Argyrion et al. therefore suggest that the long spiral gland may have been a trait retained from this larger ancestor, rather then an adaptation to its lifestyle.

See also...

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Ionoscopiform Fish from the Middle Triassic of Guizhou Province, China.             Halecomorphs are Neopterygid Fish (Ray-finned Fish) related to Ginglymodians (Gars) and Teleosts (almost all modern Ray-finned Fish). They are split into three groups, the extant Amiiformes, which contain a single living species, the Bowfin, Amia calva...

Reconstructing cranial endocasts of Palaeozoic Ray-finned Fish.                  Palaeontologists have been interested in the endocasts of vertebrate skulls (moulds of the interior of the skull made by sediment) since at least the nineteenth century, due to the possibility that these can...

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