Letting the 'cat' out of the bag: pouch young development of the extinct Tasmanian tiger revealed by X-ray computed tomography.

Axel H Newton,Frantisek Spoutil,Kathryn Medlock,Jan Prochazka,Robert N Paddle,Christy A Hipsley,Jay R Black,Marketa Knitlova,Andrew J Pask

doi:10.1098/rsos.171914

Abstract

The Tasmanian tiger or thylacine (Thylacinus cynocephalus) was an iconic Australian marsupial predator that was hunted to extinction in the early 1900s. Despite sharing striking similarities with canids, they failed to evolve many of the specialized anatomical features that characterize carnivorous placental mammals. These evolutionary limitations are thought to arise from functional constraints associated with the marsupial mode of reproduction, in which otherwise highly altricial young use their well-developed forelimbs to climb to the pouch and mouth to suckle. Here we present the first three-dimensional digital developmental series of the thylacine throughout its pouch life using X-ray computed tomography on all known ethanol-preserved specimens. Based on detailed skeletal measurements, we refine the species growth curve to improve age estimates for the individuals. Comparison of allometric growth trends in the appendicular skeleton (fore- and hindlimbs) with that of other placental and marsupial mammals revealed that despite their unique adult morphologies, thylacines retained a generalized early marsupial ontogeny. Our approach also revealed mislabelled specimens that possessed large epipubic bones (vestigial in thylacine) and differing vertebral numbers. All of our generated CT models are publicly available, preserving their developmental morphology and providing a novel digital resource for future studies of this unique marsupial.

Highlights

The thylacine (Thylacinus cynocephalus, Harris 1808) was a large Australian marsupial mammal known from the island state of Tasmania, commonly referred to as the Tasmanian tiger or marsupial wolf due to its striped lower back and dog-like appearance
A remnant thylacine population became isolated on Tasmania before they were hunted to extinction in the early twentieth century, with the last known individual dying in captivity in Hobart Zoo in 1936 [6]
To determine the ages of the unknown specimens, we reclassified the complete series of known pouch young using crown–rump length (CRL) and head length (HL) measurements to establish a growth trajectory

Summary

Introduction

The thylacine (Thylacinus cynocephalus, Harris 1808) was a large Australian marsupial mammal known from the island state of Tasmania, commonly referred to as the Tasmanian tiger or marsupial wolf due to its striped lower back and dog-like appearance (figure 1). Its overall appearance displayed several independently evolved similarities with placental canids (dogs and wolves), despite the two groups last sharing a common ancestor approximately 160 million years ago [8] These homologies were especially evident in the thylacine skull, which exhibited numerous convergent adaptions to a carnivorous ecology [9,10,11], especially when compared to its closest living relative the insectivorous numbat (Myrmecobius fasciatus) and other dasyurids, e.g. Tasmanian devil [12]. These include an elongated dog-like snout, long canine teeth and shearing premolars, and a pronounced sagittal crest for muscle attachment [5]. Recent comparisons of three-dimensional cranial shape between the thylacine and other fossil and living mammals showed that this degree of morphological similarity is similar to that found in other textbook examples of phenotypic convergence, making the thylacine-canid comparison an exceptional model of convergent evolution among distantly related taxa [12]

Methods

Results

Conclusion