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Dickson's Thylacine - Nimbacinus dicksoni
Topic Started: Apr 10 2014, 01:41 PM (1,809 Views)
Taipan
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Dickson's Thylacine - Nimbacinus dicksoni

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Temporal range: Upper Oligocene - Lower Miocene (23-16 MYA)

Scientific classification
Kingdom:Animalia
Phylum:Chordata
Class:Mammalia
Infraclass:Marsupialia
Order:Dasyuromorphia
Family:†Thylacinidae
Genus:†Nimbacinus
Species:Nimbacinus dicksoni

Nimbacinus dicksoni was an ancient relative of the modern but extinct Thylacine. It lived approximately 23-16 million years ago in the Miocene period. Nimbacinus dicksoni was about 1.6 ft (50 cm) long. Being a predator, it probably ate birds, small mammals, and reptiles. Like the modern Thylacine, it may have been an awkward runner and used stamina to catch prey rather than speed. Fossils have been found in Australia at Riversleigh in north-western Queensland and Bullock Creek in the Northern Territory. The fossils are very well preserved.




Tiny Killer: Mini 'Tasmanian Tiger' Took Down Large Prey

Charles Q Choi, LiveScience Contributor | April 09, 2014 05:00pm ET

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The skull of an ancient thylacinid from the Riversleigh World Heritage Area in Australia.

An extinct marsupial hunter only the size of a fox may have hunted prey larger than itself, researchers say.

This predatory ability makes the ancient creature different from its most recent living relative, the also-extinct thylacine, or "Tasmanian tiger." The last known wild thylacine was shot in 1930, and the last captive member of the species died in a zoo in 1936.

Hunting apparently helped drive the species to extinction. People targeted the dog-like Tasmanian tigers because they believed that the animals killed sheep; in fact, a 2011 study published in the Journal of Zoology found that the creatures' jaws were too weak to take down large prey, and that they would have only killed animals smaller than themselves.

The new study analyzed an exceptionally well-preserved whole skeleton of an extinct relative of these last thylacines, known as Nimbacinus dicksoni; the specimen dates to about 11.6 million to 16 million years old.

"The discovery of an entire skeleton of Nimbacinus was a truly amazing finding, particularly as it is was in such good condition," said study author Stephen Wroe, a zoologist and paleontologist at the University of New England in Australia.

Tiny lions and carnivorous kangaroos

The marsupial carnivore was about the size of a very large housecat or a small fox, weighing about 11 pounds (5 kilograms). "Its face looked like a cross between a cat and an opossum," said study lead author Marie Attard, a zoologist at the University of New England in Australia.

The modern thylacine (Thylacinus cynocephalus)was larger, comparable in size to a medium-sized or large dog. Modern thylacines weighed in at between 40 and 70 lbs. (20 to 30 kg).

Paleontologists discovered the fossil in the mid-1990s in the Riversleigh World Heritage Area in Australia. In ancient times, warm, humid, lowland rainforests covered this region — then, about 10 million to 15 million years ago, it became progressively cooler and drier, transforming into dry open woodlands and grasslands.

Nimbacinus belonged to an extinct family of marsupial carnivores known as the thylacinids, consisting of at least 12 known species. Nimbacinus may have lived in ancient Riversleigh with several other thylacinid species, along with marsupial lions smaller than a housecat and small carnivorous kangaroos, potentially competing with them all for prey.

"As a medium-sized carnivore, Nimbacinus was likely hunted by larger meat-eaters, including snakes, ground-dwelling crocodiles and larger species of marsupial lions," Wroe told Live Science.

Aside from studies of the recently extinct thylacine, most knowledge about thylacinids comes from skull fragments, limiting what scientists could deduce about the animals. The newly unearthed Nimbacinus skull, however, helped Attard and her colleagues reconstruct how this creature may have lived.

Modeling a marsupial

The researchers created a 3D computer model of the Nimbacinus skull to realistically simulate how the skull may have behaved. Digitally reconstructing the whole skull posed a challenge, as the top of its cranium had been slightly crushed and only half of its lower jaw, or mandible, was intact. "It was like opening a jigsaw puzzle box, only to find crucial missing pieces," Attard told Live Science.

The scientists then compared the mechanical performance of the Nimbacinus skull with that of the extinct thylacine. They also compared its performance to that of living marsupial carnivores such as the Tasmanian devil, spotted-tailed quoll and northern quoll. These belong to a different and diverse family of marsupial carnivores, the dasyurids.

In a surprise, the researchers discovered the mechanical performance of the Nimbacinus skull was far more similar to the spotted-tailed quoll, a member of a different family of marsupial carnivores, than to the Nimbacinus' closer relative, the thylacine.

These findings suggest Nimbacinus had a powerful bite for its size, was mostly carnivorous and was probably capable of hunting prey larger than itself.

"Our biomechanical analysis of the skull of Nimbacinusrevealed that it was likely an opportunistic hunter of the rainforest and had a broadly similar way of life to that of larger living dasyurids such as the spotted-tailed quoll," Attard said. "It likely preyed upon small- to medium-sized birds, frogs, lizards and snakes, as well as a wide range of marsupials, including possums, bandicoots, dasyurids, ancient ancestors of koalas, small wallabies, thingodontans [extinct marsupials with boomerang-shaped molars], marsupial moles and wombats. This suggests possible convergent evolution between Nimbacinus and the spotted-tailed quoll, meaning that these two species independently evolved similar adaptations to similar environments."

In contrast, the recently extinct Tasmanian tiger was considerably more specialized in what it could eat than Nimbacinus and large living dasyurids. This likely made the Tasmanian tiger more restricted in the range of prey it could hunt, "and more vulnerable to extinction," Attard said.

Reconstructing past communities and the ecologies of the species that contribute to them "is pivotal if we are to map out and understand change over time," Wroe told Live Science in an email. "Trying to understand how these animals lived and what they ate is also fun!"

Future analysis of the Nimbacinus skeleton could reveal if it was partially tree dwelling like the spotted-tailed quoll, which could help explain the similarities the researchers have noted so far between the two marsupial species.

The scientists detailed their findings online April 9 in the journal PLOS ONE.

http://www.livescience.com/44714-thylacine-extinct-marsupial-hunter.html




Comparative Bite Force/Stress Analysis

Virtual Reconstruction and Prey Size Preference in the Mid Cenozoic Thylacinid, Nimbacinus dicksoni (Thylacinidae, Marsupialia)

Marie R. G. Attard, William C. H. Parr, Laura A. B. Wilson, Michael Archer, Suzanne J. Hand, Tracey L. Rogers, Stephen Wroe
Published: April 09, 2014
DOI: 10.1371/journal.pone.0093088

Abstract
Thylacinidae is an extinct family of Australian and New Guinean marsupial carnivores, comprizing 12 known species, the oldest of which are late Oligocene (~24 Ma) in age. Except for the recently extinct thylacine (Thylacinus cynocephalus), most are known from fragmentary craniodental material only, limiting the scope of biomechanical and ecological studies. However, a particularly well-preserved skull of the fossil species Nimbacinus dicksoni, has been recovered from middle Miocene (~16-11.6 Ma) deposits in the Riversleigh World Heritage Area, northwestern Queensland. Here, we ask whether N. dicksoni was more similar to its recently extinct relative or to several large living marsupials in a key aspect of feeding ecology, i.e., was N. dicksoni a relatively small or large prey specialist. To address this question we have digitally reconstructed its skull and applied three-dimensional Finite Element Analysis to compare its mechanical performance with that of three extant marsupial carnivores and T. cynocephalus. Under loadings adjusted for differences in size that simulated forces generated by both jaw closing musculature and struggling prey, we found that stress distributions and magnitudes in the skull of N. dicksoni were more similar to those of the living spotted-tailed quoll (Dasyurus maculatus) than to its recently extinct relative. Considering the Finite Element Analysis results and dental morphology, we predict that N. dicksoni likely occupied a broadly similar ecological niche to that of D. maculatus, and was likely capable of hunting vertebrate prey that may have exceeded its own body mass.

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TABLE 1: Predicted body mass and masticatory muscle forces for modeled dasyuromorphians.

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FIGURE 3: Von Mises stress under a bilateral canine bite in lateral view.
The models are subjected to a load applied to both canines, with bite force scaled based on theoretical body mass. Species modeled were (A) Dasyurus hallucatus, (B) Dasyurus maculatus, (C) Sarcophilus harrisii, (D) Nimbacinus dicksoni and (E) Thylacinus cynocephalus. White colored regions of the skull represent VM stress above 10 MPa. (F) Distribution of von Mises stress was measured from anterior to posterior along the mandible.

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FIGURE 4: Von Mises stress under a bilateral canine bite in dorsal view.
The models are subjected to a load applied to both canines, with bite force scaled based on theoretical body mass. Species modeled were (A) Dasyurus hallucatus, (B) Dasyurus maculatus, (C) Sarcophilus harrisii, (D) Nimbacinus dicksoni and (E) Thylacinus cynocephalus. White colored regions of the skull represent VM stress above 10 MPa. (F) Distribution of von Mises stress was measured from anterior to posterior along the mid-sagittal plane.

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FIGURE 5: Von Mises stress under extrinsic loads in lateral view.
The models are subjected to various loads applied to the canines, including a (A, E, I, M, Q) lateral shake, (B, F, J, N, R) axial twist, (C, G, K, O, S) pullback and (D, H, L, P, T) dorsoventral. The force applied was equivalent to 100 times the animal's estimated body mass for an axial twist, and 10 times the animal's estimated body mass for a lateral shake, pullback and dorsoventral shake. Species compared were (A–D) Dasyurus hallucatus, (E–H) Dasyurus maculatus, (I–L) Sarcophilus harrisii, (M–P) Nimbacinus dicksoni and (Q–T) Thylacinus cynocephalus. White colored regions of the skull represent VM stress above 10 MPa. Distribution of von Mises (VM) stress was measured from anterior to posterior along the mandible for a (U) lateral shake, (V) axial twist, (W) pullback and (X) dorsoventral.

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FIGURE 6: Von Mises stress under extrinsic loads in dorsal view.
The models are subjected to various loads applied to the canines, including a (A, E, I, M, Q) lateral shake, (B, F, J, N, R) axial twist, (C, G, K, O, S) pullback and (D, H, L, P, T) dorsoventral. The force applied was equivalent to 100 times the animal's estimated body mass for an axial twist, and 10 times the animal's estimated body mass for a lateral shake, pullback and dorsoventral shake. Species compared were (A–D) Dasyurus hallucatus, (E–H) Dasyurus maculatus, (I–L) Sarcophilus harrisii, (M–P) Nimbacinus dicksoni and (Q–T) Thylacinus cynocephalus. White colored regions of the skull represent VM stress above 10 MPa. Distribution of von Mises (VM) stress was measured from anterior to posterior along the mid-sagittal plane for a (U) lateral shake, (V) axial twist, (W) pullback and (X) dorsoventral.

Differences in biomechanical performance between the three extant dasyurids included in this study appear consistent with their respective known feeding behaviors. Dasyurus hallucatus showed comparatively higher levels of stress in most simulations than S. harrisii and D. maculatus. Dasyurus hallucatus eats invertebrates and other relatively small prey, which may not require adaptation to sustain the full range of extrinsic loads simulated here. This species shows particularly high VM stress in axial twisting, especially in contrast to S. harrisii However, it performs relatively well under pull-back loading, which may be linked to a capacity for pulling invertebrates from the ground. Observational studies on wild D. hallucatus will be required to confirm the functional role of their skull in prey acquisition. Future work on the comparative musculoskeletal anatomy and collection of in vivo or ex vivo biomechanical data of the extant species would likely improve the predictive power of current bite force and muscle force estimations. Overall consistencies found between known prey size and biomechanical performance for extant dasyuromorphians underscore the potential value of projections based on comparative FEA for extinct/fossil taxa.

Our comparative biomechanical modeling of dasyuromorphian skulls suggests considerable differences in predatory behaviors between the two thylacinids considered here. Our 3D based results indicate that the Oligocene to Miocene N. dicksoni had a high bite force for its size, comparable to that of extant dasyurids known to take relatively large prey, D. maculatus and S. harrisii . In light of similar levels of ‘carnassialization’ (development of relatively long, high amplitude vertical shearing crests) in the cheektooth dentition with D. maculatus, and a lack of obvious dental specialization consistent with regular bone-cracking, our results suggest a predominantly carnivorous diet for N. dicksoni that may have included relatively large prey. Dasyurus maculatus are opportunistic hunters, varying their diet in response to environmental disturbances and short-term fluctuations in prey abundance. They will prey on vertebrate species up to and sometimes exceeding their own body mass. Prey includes bandicoots, smaller dasyurids, possums, smaller macropodoids, snakes, lizards, birds and frogs, as well as invertebrates. Potential prey for a fox-sized thylacinid living in the closed forest communities of Riversleigh likely included many small to medium-size birds, frogs, lizards and snakes, as well as a wide range of marsupials, including bandicoots (peramelemorphians), dasyurids (dasyuromorphians), kangaroos (macropodoids), thingodontans (yalkiparidontians), marsupial moles (notoryctemorphians) and wombats (vombatoids).

Although our FEA results for N. dicksoni suggest a capacity to kill prey approaching or exceeding its own body mass, its prey range may have been limited by competition with sympatric carnivores. The extent of niche overlap and competition within this ancient, medium-large sized carnivore community may have been partially alleviated by occupying different habitats and specializing in different hunting strategies. The recovery of a near complete skeleton of N. dicksoni will provide further information on the locomotion and predatory behavior based on postcranial material; for example, was N. dicksoni as arboreal as the extant D. maculatus?

Differences in mechanical performance suggest that T. cynocephalus is unusual relative to other dasyuromorphians, including, N. dicksoni, as indicated by distinctly higher VM stresses than all other species in response to each loading case. Thylacinus cynocephalus, in contrast to N. dicksoni, has completely lost the metaconid on the lower molars and has a proportionately much larger postmetacrista on the upper molars. On the basis of traditional beam theory we predicted that taxa with longer rostra would exhibit higher stress , as evident in the long-snouted T. cynocephalus relative to shorter-snouted dasyuromorphians. Differences between T. cynocephalus and other species were also significant for three out of five simulations examined after conservative Bonferroni correction for multiple testing. These results further support the contention by Attard et al. that niche breadth in T. cynocephalus may have been more limited and that it likely preyed on relatively small to medium-sized vertebrates such as wallabies, possums and bandicoots.

Although measures of skull performance in response to forces imposed by struggling prey revealed closer similarity between the fossil thylacinid N. dicksoni and large extant carnivorous dasyurids, than with T. cynocephalus, there were differences. Our reconstruction suggests that the TMJ was more elevated in N. dicksoni than in D. maculatus, and higher relative to the height of the cheektooth row. The TMJ is a complex joint and is important for occlusion and mastication. The position of the TMJ can influence bite strength and muscle activation . The position of the TMJ along the anterior-posterior axis tends to lie closer to the plane of the tooth row in carnivorous taxa. Conclusive determination of the precise position and morphology of the TMJ in N. dicksoni must await the discovery of more complete cranial material.

Morphological evidence from past studies further demonstrates diversity within this family. The smallest thylacinid, Muribacinus gadiyuli, is believed to have fed on relatively small vertebrates and invertebrates because it lacks a number of dental features present in large prey specialists (e.g., robust protoconids and brachycephalization) such as similarly sized D. maculatus. The variety of feeding behaviors among thylacinids may have helped facilitate their co-existence within different ecological niches that were later filled by diversifying carnivorous dasyurids.


http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0093088;jsessionid=BD11A1EFAC62DC25EA00D48A6F32A50A
Edited by Taipan, Sep 28 2017, 05:44 PM.
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