MASTERS THESIS

ABSTRACT

Phalangeroid possums of New Guinea and Australia possess many traits convergent with Malagasy Strepsirrhines. Possums therefore provide researchers with an opportunity to shed light on the ecological context and order adaptive traits that lead to the appearance of the first primate.

Like Strepsirrhines, phalangeroids are thought to have evolved in geographic isolation. They are well adapted to arboreal life through traits such as grasping hands and feet with reduced claws and an opposable first digit on the hind foot, increased orbital convergence for better depth perception and an overall large brain to body size ratio.

While Strepsirrhines have been well studied from several scientific perspectives, data on phalangeroids is limited to morpho-ecological observations and a strong case has been made for their continued study. A logical place to start is with the jaw adductors and dentition. Such information provides scientists with an important foundational understanding of mammal phylogeny, adaptive history, diet and trophic structure.

Unfortunately, the tools most commonly employed to document the three-dimensional nature of chewing muscles are limited to two-dimensional outputs including text, quantitative data displays, black and white line drawings and photography. To make matters worse, the efficacy of such tools is compromised by poor quality of execution. 

 

PROJECT GOALS

Goal #1: To document the masticatory apparatus of the phalangeroid possum, Trichosurus vulpecula, using an improved set of visual communication tools:

  1. traditional flat illustration
  2. a volumetrically accurate three dimensional digital reconstruction
  3. A three dimensionally printed physical model

Goal #2: Provide a step by step workflow of the reconstruction process to allow third parties to replicate the results with respect to other taxa. 

Goal #3: To improve the ability of the scientist to draw meaningful comparisons between Malagasy Strepsirrhines and Phalangeroid possums and contribute to the discussion regarding the ecological context of the evolutionary sequence of adaptations that define the order Primate. 


 

PROBLEM

 

Primates appeared approximately 65 million years ago, but a fossil for ancestral primate forms has yet to be discovered (). The order of acquired traits that led to the appearance of the first primate, and the ecological context surrounding this event, are in a perpetual state of debate ().

 

 

OBSTACLES

1. Evolutionary time exceeds that of the researcher.

2. There are many interacting variables that make evolution inherently difficult to study.

OPPORTUNITY

Convergent evolution can help provide answers. This is the process whereby organisms not closely related (not monophyletic), independently evolve similar traits as a result of having to adapt to similar environments or ecological niches. This is also referred to as Parallelism.


PARALLELISM AMONG POSSUMS AND PRIMATES

The order Primate is defined by following traits:

  1. grasping hands and feet with opposable first digits and nails instead of claws
  2. hind limb locomotor dominance
  3. late onset sexual maturity
  4. stereoscopic vision and reduced olfaction
  5. large brain to body size ratios
  6. and complex social behavior

The Phalangeroid possums of New Guinea and Australia possess many of these. Convergent cranial morphology is clearly visible between the Phalangeroid possum Trichosurus and the Strepsirrhine primate Otolemur. 

 Trichosurus vulpecula

aka: Common Brushtail Possum

Family Phalangeridea

Marsupial

New Guinea & Australia

(photo credit: © Andrew Trevor Jones, www.redbubble.com)

 

Otolemur Crassicaudatus

aka: Brown Greater Galago

Family Galagidae

Placental

East Africa

 (photo credit: © Gerald Cubitt / www.photoshot.com)


POSSUM-PRIMATE PHYLOGENY

Marsupial and placental mammals are separated by nearly 160 million years of evolution. Their last common ancestor is known as a Therian.

 Between 65-45 mya, both flaura (plants) and fauna (animals) underwent significant diversification. This is referred to as a period of diffuse co-evolution.

 It was during this time that the new radiations of the orders Diprotodonita and Primate appeared as well.


ANATOMICAL LANDMARKS & MUSCLE ATTACMENTS


VIRTUAL RECONSTRUCTION 

The masticatory apparatus of a Trichosurus vulpecula was digitally reconstructed. Hard tissue structures were acquired through segmentation of CT data. Musculature was sculpted using ZBrush. The mass of each layer was verified in Solidworks using volumetric data acquired from dissection.

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