Virtual reconstruction and prey size preference in the mid Cenozoic thylacinid, Nimbacinus dicksoni (Thylacinidae, Marsupialia)

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.

Figure 1. Digital reconstruction of Nimbacinus dicksoni.

Original (grey) and reconstructed 3D (yellow) in (A) lateral view and (B) dorsal view. (C) Pre-processed Finite Element model of N. dicksoni, showing jaw musculature represented by trusses.

Figure 2. Position of nodes selected on each model to measure von Mises stress.

Nodes were selected at equidistant points along the (A) mid-sagittal plane, (B) zygomatic arch and (C) mandible to measure the distribution of von Mises stress for each loading case.

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.

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.

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.

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.

Figure 7. Principal components one and two of von Mises stress along mid-sagittal plane.

Results of principal components analysis to compare stress among species along mid-sagittal nodes for each loading case, including (A) bilateral canine bite, (B) lateral shake, (C) axial twist, (D) pullback and (E) dorsoventral. Key to symbols: (pink square) Thylacinus cynocephalus; (red triangle) Nimbicinus dicksoni; (orange diamond) Sarcophilus harrisii; (green circle) Dasyurus maculatus; (purple triangle) Dasyurus hallucatus.