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. 2016 Jan 6:6:18758.
doi: 10.1038/srep18758.

Exceptional preservation reveals gastrointestinal anatomy and evolution in early actinopterygian fishes

Thodoris Argyriou  1 , Marcus Clauss  2 , Erin E Maxwell  3 , Heinz Furrer  1 , Marcelo R Sánchez-Villagra  1
Affiliations

Exceptional preservation reveals gastrointestinal anatomy and evolution in early actinopterygian fishes

Thodoris Argyriou et al. Sci Rep. .

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.

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Figures

Figure 1
Figure 1. †Saurichthys specimens with preserved GIT casts.
(a) †Saurichthys costasquamosus (MCSN 5696) 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; (c) †Saurichthys macrocephalus (PIMUZ T 3916), 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; 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. All scale bars equal 1 cm.
Figure 2
Figure 2. †Saurichthys paucitrichus with preserved GIT cast.
(a) †Saurichthys paucitrichus (PIMUZ T 59) 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.
Figure 3
Figure 3. Phylogenetic framework of GIT morphology and spiral valve turn counts of actinopterygians, including †Saurichthys paucitrichus.
Phylogenetic hypothesis based on refs ,. Interpretative drawings of GITs of Polypterus, Polyodon spathula, Lepisosteus osseus and the teleost Alosa redrawn and modified from ref . The interpretative drawing of the Amia calva GIT is redrawn and modified from ref . The drawings of the Acipenser baerii and †S. paucitrichus GITs are based on our observations. All drawings depict the GIT in ventral view with foregut to the left and hindgut to the right.
Figure 4
Figure 4. Relationship between maximum body size (logTL) and maximum spiral valve turn counts.
Data and references in ST1. Different fish groups (“orders”) are color coded. Elasmobranchs: The general trend of turn increase with body size is evident. However, the constancy or decreased variability of turn counts within groups is also marked. †Saurichthys paucitrichus (PIMUZ T 59, thinner black outline) and †Asthenocormus titanius (thicker black outline) plot as outliers, exhibiting a much higher turn count than all extant osteichthyans and elasmobranchs of similar size. It should be noted that extant osteichthyans tend to exhibit fewer turns than most elasmobranchs despite achieving moderate body sizes.
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