An Educational Three Dimensional Model to Describe the Masticatory Apparatus of the Phalangeroid Possum, Trichosurus vulpecula

A thesis submitted to Johns Hopkins University in conformity with the requirements for the degree of Master of Arts
Baltimore, Maryland 2017 | © Nicholas Reback 2017, All Rights Reserved

Abstract

(Last edited, Feb, 2019)

Phalangeroid possums of New Guinea and Australia possess many traits convergent with Malagasy Strepsirrhines and provide researchers with an opportunity to shed light on the order of 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.

A strong case has been made for in-depth study of phalangeroids. A logical place to start is with the jaw adductors and dentition as such information provides scientists with an important foundational understanding of mammal phylogeny, adaptive history, diet and trophic structure. Characterization of any mammalian masticatory apparatus, however, requires a thorough understanding of spatial relationships as well as biomechanical traits derived from muscle mass, fiber length, physiological cross-sectional area and location of attachments sites. This task is challenging enough for a single species and is increasingly difficult when comparing two or more taxa. Unfortunately, the tools most commonly employed to document the three dimensional nature of chewing muscles are limited to two-dimensional outputs such as 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.

This study aims to improve the ability of the scientist to draw meaningful comparisons between two taxa, Malagasy Strepsirrhines and phalangeroid possums, by introducing a refined set of visual communication tools including traditional flat illustration as well as a three dimensional reconstruction driven by volumetric data acquired from dissection of a single representative phalangeroid species, Trichosurus vulpecula. It is our hope that the contribution of these resources to the scientific community will help further the academic discussion regarding ecological context surrounding the evolutionary sequence of adaptations that define the order Primate.

How are Possums and Lemurs similar?

The Phalngeroid possum Trichosurus vulpecula
(photo credit: Gerald Cubit, Arkive Ditigal Media © 2011)

The Strepsirrhine primate Otolemur crassicaudatus
(photo credit: Gerald Cubit, Arkive Ditigal Media © 2011)

Introduction

All modern mammals, with the exception of monotremes, diverged from a common therian ancestor during the late Jurassic approximately 160 million years ago (Luo et al, 2011) (fig 1, below). Today, of the 5420 known mammal species, 335 are represented by metatherian (marsupial) radiations that primarily inhabit the forested regions of New Guinea and Australia, with a small number found in the New World ecosystems of South and Central America, plus a single species in North America (Vaughn, 2011; Mitchell, 2014).

Figure 1. Combined geologic timescale and Phylogenetic tree of mammal forms.

By contrast, the range of inhabited ecological niches and variety of adaptive traits possessed by eutherians (placentals) is far more diverse (Vaughn, 2011; Swindler, 2012). One such group, the Lemuriformes of Madagascar, of the suborder Strepsirrhini, are of considerable interest for their possession of primitive primatelike adaptations. Both extant and extinct taxa provide researchers with insights as to the order of adaptations that led to the radiation of arboreal primates from a pre-primate insectivore, presumably a plesiadapiform, sometime near the Paleocene-Eocene boundary (Cartmill, 1974; Bloch, 2001; Lou, 2011; Swindler, 2012). The fossil record for such antecedents, however, remains curiously absent (Swindler, 2012; Sussman, 1991). Evolutionary hypotheses about the ecological context surrounding the order of acquired traits leading to
Eocene prosimian varieties and, hence, modern primates are in a perpetual state of debate (Kay et al., 1990; Beard, 1993; Bloch and Silcox, 2001; Silcox, 2001).

More about possums and primates coming soon!

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