Tachyglossus, the other egg-laying mammal

Figure 1. The echidna (genus: Tachyglossus) in life. This slow-moving spine-covered anteater has digging claws.

Figure 1. The echidna (genus: Tachyglossus) in life. This slow-moving spine-covered anteater has digging claws.

Tachyglossus aculeatus (Shaw 1792) is the echidna and the only other genus of egg-laying mammal. It protects itself with sharp spines and has a long, ant-catching tongue. The hands and feet are adapted to digging with short, almost immobile proximal elements (Fig. 3) and long claws. Prepubic bones precede the pubis. A proximal process sits atop the fibula. The leathery snout without whiskers is sensitive to vibrations.

Figure 2. The skull of Tachyglossus is largely fused together, lacks teeth and has no lateral temporal fenestra (because the jaws don't move much in this anteater.

Figure 2. The skull of Tachyglossus is largely fused together, lacks teeth and has no lateral temporal fenestra (because the jaws don’t move much in this anteater. Hard to find sutures here. Let me know if you have better data to make corrections.

Distinct for its sister,
Ornithorhynchus, and many other mammals, the acetabulum is perforated. The lateral temporal fenestra is absent. So are the teeth. Like the hedgehog, the echidna can roll itself into a ball for protection.

Figure 3. Tachyglossus skeleton, manus and x-rays. Note the perforated pelvis.

Figure 3. Tachyglossus skeleton, manus and x-rays. Note the perforated pelvis.

There are those
who say characters define a taxon. We have to get away from that hypothesis. Here a perforated acetabulum would make Tachyglossus a dinosaur, to the late Larry Martin’s delight. Tachyglossus has no temporal fenestra. So, does that make it an anapsid? No. The only thing that tells us what a taxon is… is its placement on a wide gamut cladogram that tests hundreds of candidate sister taxa and hundreds of traits. Testing a suite of several hundred traits in a wide gamut study is the only way to confidently determine taxonomy and avoid the pitfalls of convergence and taxon exclusion that plague smaller studies that too often fail to minimize false positives and ‘by default’ nestings. And some DNA studies cannot be validated, except by morphological studies.

References
Shaw G 1792. Musei Leveriani explicatio, anglica et latina.

wiki/Tachyglossus

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