Fossil Echidnas (Megalibgwilia) Family Tachyglossidae
Specimens of fossil echidnas Megalibgwilia
Fossil material for Megalibgwilia ramsayi includes 4 intact skulls. The fossil skull Z2031.1 (Fig. 1) is held by the Tasmanian Museum and Art Gallery and is originally from the Montagu Cave in North-western Tasmania (Murray, 1978a, b). It may have an age of about 13,000 years (Goede and Bada, 1985; Griffiths et al., 1991). The other three specimens are P20488, P23144, P22811 from the Museum of South Australia. All three are from Henschke Fossil Cave at Naracoorte (Griffiths et al., 1991). Dating of those cave deposits yielded a minimum age of 16,700 and a maximum age of 90,000 to 120,000 years (Ayliffe and Veeh, 1988; Griffiths et al., 1991).
Figure 1 shows dorsal (a) and ventral (b) views of Z2031.1. Parts of the skull have been labelled. The optic canal transmits the optic nerve (cranial nerve 2). The foramen rotundum transmits the maxillary division of the trigeminal nerve (5max) to the sensitive beak, and the foramen ovale transmits the mandibular division of the trigeminal nerve (5mand) to the mandible. The small fenestra vestibuli is covered in life by the foot-plate of the columellar stapes, transmitting vibrations to the inner ear. The internal choana is the posterior end of the long nasal cavity, the jugular foramen contains the internal jugular vein and cranial nerves 9, 10, and 11, and the foramen magnum transmits the spinal cord and vertebral arteries.
Figure 2 shows dorsal (a) and ventral (b) views of P20488. Parts of the skull have been labelled. Please see Figure 1 legend for explanation of foramina.
Figure 3 shows ventral views of the specimen and P22811 (a) and P23144 (b). Parts of the skull have been labelled.
Z2031.1 (See Figure 1 and 4) was scanned in a medical scanner (Siemens Somatom) at the Calvary Hospital in Hobart. Two of the three South Australian specimens (P20488, P23144) were scanned with a Sky-Scan in vivo microCT system, located at Adelaide Microscopy, The University of Adelaide. Specimen P22811 was too wide for the 7 cm bore of the Adelaide microCT scanner.
Features on the endocranium of fossil echidnas
Figures 4 and 5 show 3D reconstructions from CT scans of the skulls of Z2031.1 (Fig. 4) and P23144 (Fig. 5). In all specimens studied by CT, impressions of cortical sulci (indicated by Greek letters) are visible on the skull interior. Sulci alpha, beta and zeta are of particular functional significance, because they define boundaries between functional areas identified by electrophysiology (Lende, 1964, Krubitzer et al., 1995). The motor or manipulation field lies between the alpha and beta sulci and the primary somatosensory cortex (S1) lies posterior to the alpha sulcus on the lateral cortical surface. Most of the primary visual cortex (V1) lies between the alpha and zeta sulci on the dorsal surface of the brain (see short and long-beaked echidna brains). Both the pattern and length of sulci appear identical in living and extinct echidnas (i.e. short and long-beaked) suggesting that functional cortical topography and gyrification have not changed significantly over the last 40,000 years (Ashwell et al., 2014).
Figure 4 shows several views of a 3D reconstruction from the CT scan of Z2031.1. Dorsal, ventral, left lateral and posterior views are shown in a) to d), respectively. Panels e), f) and g) show the features on the skull interior (endocranium). The impressions of cortical sulci on the interior of the cranium are indicated by Greek letters (Fig. 4e, f, g).
Figure 5 shows several views of a 3D reconstruction from a CT scan of P23144. All images are of the skull interior (endocranium). Panel a) shows the interior of the right half of the skull. Panel b) shows the interior of the skull base and panel c) shows the interior of the dorsum of the skull. The impressions of cortical sulci on the interior of the cranium are indicated by Greek letters in panels a) to c).
References
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Goede A, Bada JL (1985) Electron spin resonance of Quaternary bone material from Tasmanian caves – a comparison with ages determined by aspartic acid racemization and C14. Australian Journal of Earth Science 32, 155-162.
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Lende RA (1964) Representation in the cerebral cortex of a primitive mammal, sensorimotor, visual, and auditory fields in the echidna (Tachyglossus aculeatus). Journal of Neurophysiology 27, 37-48.
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