Ontogenetic allometry reveals the imprint of myrmecophagy in the skull of the numbat, Myrmecobius fasciatus Waterhouse, 1836 (Marsupialia: Myrmecobiidae)

Abstract

Ontogenetic allometry, the covariation of shape with size over the course of development, represents a fundamental component of morphological diversification that can vary across species and in association with ecological factors. Ontogenetic allometry patterns for the skull have been described for species from several marsupial lineages, associated with common dietary niches, e.g., carnivory. We here conduct a novel detailed examination of cranial ontogeny in the numbat (Myrmecobius fasciatus), unique among marsupials in being myrmecophagous and adopting a strictly diurnal habit. We investigated ontogenetic allometry using multivariate analyses in a postnatal series (n = 28) of complete numbat skulls to estimate the rate of growth of its major constituent elements, described by 15 measurements. We find positive allometry for length of the nasals and palate, which relates directly to the remarkable rostral elongation in the species relative to other marsupials, in addition to several other ontogenetic changes. These comprise negative allometry of palate breadth, elongation of upper and lower toothrows that produces diastemata between weakly developed teeth, and decreasing muzzle height, all of which contribute to the development of a long, tapering, narrow rostrum, with limited mastication ability, as seen in the small temporal space and poor development of crests and processes associated with jaw musculature. Our specific allometric results and comparisons, together with qualitative observations, reveal a distinct imprint of myrmecophagy on skull ontogeny in the numbat and help explain the development and evolution of specialized feeding function of this species.

Norberto P. Giannini [[email protected]], CONICET Fundacion Miguel Lillo, Unidad Ejecutora Lillo, UEL, Tucumán, Argentina, Facultad de Ciencias Naturales, Instituto Miguel Lillo, Universidad Nacional de Tucumán, Tucumán, Argentina, Department of Mammalogy, American Museum of Natural History, New York, NY, USA;

Fernando Abdala [[email protected]], CONICET Fundacion Miguel Lillo, Unidad Ejecutora Lillo, UEL, Tucumán, Argentina, Evolutionary Studies Institute, University of the Witwatersrand, Johannesburg, South Africa;

David A. Flores [[email protected]], CONICET Fundacion Miguel Lillo, Unidad Ejecutora Lillo, UEL, Tucumán, Argentina, Instituto de Vertebrados, Fundación Miguel Lillo, Tucumán, Argentina;

Laura A. B. Wilson [[email protected]], School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia, School of Archaeology and Anthropology, The Australian National University, Canberra, ACT 2601, Australia.

Keywords:

• cranial ontogeny
• allometry
• postnatal growth
• Australidelphia
• morphology

Acknowledgements

The following institutions and people allowed us to study specimens under their care: Kenny Travouillon at the Western Australian Museum, Perth; Sandy Ingleby at the Australian Museum, Sydney; Robert Voss and Eileen Westwig at the American Museum of Natural History, New York; Darrin Lunde at the National Museum of Natural History, Washington DC; Bruce Patterson and Bill Stanley at the Field Museum of Natural History, Chicago. The authors thank Kenny Travouillon (Western Australian Museum) and Justin Adams (Monash University) for providing additional photographs, and Robin Beck for comments. NPG, FA and DAF thank the Unidad Ejecutora Lillo, CONICET, Fundación Miguel Lillo and the Universidad Nacional de Tucumán, Argentina, for support and funding. NPG thanks the American Museum of Natural History, New York.

Additional information

Funding

NPG acknowledges funding from CONICET PIP 2021-23 11220200102778CO. FA is supported by CONICET and National Research Foundation of South Africa. DAF acknowledges funding from PICT2020-02042 of the Agencia Nacional de Promoción Científica y Técnica and PIP 928 of the CONICET, Argentina. LABW is supported by an Australian Research Council Future Fellowship (FT200100822).