Sunday, October 8, 2017

[Paleontology • 2017] Exceptional Soft Tissues Preservation in A Mummified Frog-eating Eocene Salamander, Phosphotriton sigei


Fossil specimen of Phosphotriton sigei, an exceptionally preserved 40-35 million years old salamander (left part) and its internal organs conserved within it (right part). The skeleton, in grey, is perfectly preserved, as well as several soft organs such as the gut and lung. Within the stomach, the last meal of the animal is also preserved. Surprisingly, it fed on a frog, an extremely rare kind of prey for salamanders. Accessing the internal anatomy of this fossil without destroying it could only be achieved through modern synchrotron technology.

Abstract

Fossils are almost always represented by hard tissues but we present here the exceptional case of a three-dimensionally preserved specimen that was ‘mummified’ (likely between 40 and 34 million years ago) in a terrestrial karstic environment. This fossil is the incomplete body of a salamander, Phosphotriton sigei, whose skeleton and external morphology are well preserved, as revealed by phase-contrast synchrotron X-ray microtomography. In addition, internal structures composed of soft tissues preserved in three dimensions are now identified: a lung, the spinal cord, a lumbosacral plexus, the digestive tract, muscles and urogenital organs that may be cloacal glands. These are among the oldest known cases of three-dimensional preservation of these organs in vertebrates and shed light on the ecology of this salamander. Indeed, the digestive tract contains remains of a frog, which represents the only known case of an extinct salamander that fed on a frog, an extremely rare type of predation in extant salamanders. These new data improve our scarce knowledge on soft tissue anatomy of early urodeles and should prove useful for future biologists and palaeontologists working on urodele evolutionary biology. We also suggest that the presence of bat guano and carcasses represented a close source of phosphorus, favouring preservation of soft tissues. Bone microanatomy indicates that P. sigei was likely amphibious or terrestrial, and was probably not neotenic.

 Synchrotron tomography permitted access to the inside of the skeleton of Phosphotriton sigei. The skeleton and several organs are perfectly preserved.
photo: Jérémy Tissier  

Figure 1: Specimen MNHN.F.QU17755, holotype of Phosphotriton sigei. (A and B) Fossil in dorsal and ventral views. Some characteristics of urodeles, such as costal grooves or scaleless skin, are observable on the external aspect of the specimen. The cloaca and vertebral column are visible. The dotted line represents the position of the tomogram illustrated in Fig. 1C. (C) Tomogram of the tail part of the animal showing the muscles, in green, ventral and lateral to the vertebrae, and the spinal cord preserved inside the neural canal of a vertebra. Bony material is characterized by a dark grey shade, because of its light density, compared to the mineral matrix (grey or white) and void (black). Soft-tissues are also mostly darker than the mineral matrix, but are mainly recognizable by their structure and shape, on tomograms or in 3D. (D) 3D reconstruction of undetermined tail muscles, in green, which could attach to the ischium or femur. Dotted line represents the position of the tomogram illustrated in Fig. 1C.

Discussion
Ecology. The presence of anuran bones in the digestive tract of the fossil (Figs. 3C–3E) is evidence of a type of predation that is very rare in urodeles. Preying on frogs was reported in Amphiuma (Montaña, Ceneviva-Bastos & Schalk, 2014), a large and especially voracious extant urodele. Another voracious urodele, Necturus, has been reported (Hamilton, 1932) to have eaten other urodeles (Desmognatus and Eurycea), but not frogs. P. sigei was relatively small and the swallowed anuran, although small, was likely a metamorphosed individual, as shown by the well-shaped humeral condyle, but not a fully grown adult, as shown by the broad neural canal, assuming that the vertebrae belong to the same individual as the humerus. The straight diaphysis of the humerus and the position of the humeral condyle in line with the diaphysis suggest that the prey was a ranoid. Ranoids were already reported from the Phosphorites (Rage, 1984; Rage, 2016). The length of the humerus (five mm) suggests that the individual measured about 18–20 mm in snout-vent length.

To further investigate the ecology of the animal, we studied the microanatomy of the femur, through a transverse virtual section of the diaphysis on tomograms, and calculated its compactness profile with the software Bone Profiler (Girondot & Laurin, 2003). Without much surprise, both inference models (based on backward elimination and forward selection procedures, respectively) presented by Laurin, Canoville & Quilhac (2009) suggest an amphibious or terrestrial lifestyle (see Supplemental Information). This would suggest that P. sigei was not neotenic because all extant neotenic urodeles are strictly aquatic.


Conclusions: 
The only specimen of Phosphotriton sigei represents a peculiar case of exceptional preservation, in which several organs are preserved in three dimensions, in addition to the skeleton: lung, spinal cord, lumbosacral plexus, digestive tract, muscles, and an unidentified urogenital organ. In addition, the alimentary tract contains skeletal remains of a frog, which is a very rare prey for salamanders. Contrary to the above-cited case of arthropods (Schwermann et al., 2016a), we do not believe that the new data on soft anatomy will revolutionize our understanding of lissamphibian evolution, particularly because such characters have played a modest role in phylogenetic studies of lissamphibians. However, these data, such as the presence of a lung, proved critical to place the mummy in the phylogeny, and these data document the oldest known occurrence of anurophagy in urodeles.


Jérémy Tissier​, Jean-Claude Rage and Michel Laurin. 2017. Exceptional Soft Tissues Preservation in A Mummified Frog-eating Eocene Salamander.   PeerJ. 5:e3861.  DOI:  10.7717/peerj.3861
Jérémy Tissier, Jean-Claude Rage, Renaud Boistel, Vincent Fernandez, Nicolas Pollet, Géraldine Garcia and Michel Laurin. 2016. Synchrotron Analysis of A ‘Mummified’ Salamander (Vertebrata: Caudata) from the Eocene of Quercy, France. Zoological Journal of the Linnean Society. 177(1); 147–164.   DOI:   10.1111/zoj.12341

Ancient petrified salamander reveals its last meal
phy.so/426241628 via @physorg_com

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