In an attempt to bring some equality to synapsids, and talk about something novel, for today's Tuesday Tetrapod, I bring Ornithorhynchus anatinus, or the platypus.
O. anatinus. Flickr image by kookr.
The platypus is a mammal known as a prototherian, or monotreme. "Prototheria" ("First" or "early" beasts) is in contrast to "Metatheria", or marsupials ("middle beasts") and "Eutheria", or placental mammals ("true beasts"). It is the sole member of its family, ornithorhynchidae, and the other four species of protetherians all belong to tachyglossidae (the echidnas). They have the common synapomorphies of all mammals, with a jaw composed only of the dentary, hair, and the production of milk; however, various other traits commonly associated with mammals are in a different form or absent in prototherians. For example, milk is produced, but it is not excreted from a nipple. Instead, it is excreted directly from glands on the skin. In addition, the live birth that is commonly associated with mammals is only a synapomorphy of the [ metatheria + eutheria ] node, and prototherians, in fact, lay eggs. While extant monotremes lack teeth, their ancestors had tribosphenic molars (though there is debate on common ancestry or independent evolution in the lineage).
Flickr image by ccdoh1.
The platypus in particular is unique in a few other ways. Its bill is electrosensitive, similar to the electrosensitivity of a shark. This works by detecting the electrical potentials generated by the nervous system of animals, in conjunction wit the largely saline solution that the animal is composed of moving with respect to any general ion gradients in the water (though the platypus lives in freshwater). By emitting a low strength E-field from its bill, specialized cells can "expect" a certain field measurement adjacent to it, based on properties of electric fields, and other living organisms distort this field, which then differs by this "expectation" in a characteristic manner. Additionally, the platypus is one of very few venomous mammals. The male platypus possess a spur on its hind feet with a hollow tube for venom delivery.The venom is produced in the crural gland, and the proteins involved are non-necrotic / non-lethal, and instead are designed to produce incapacitating pain and shock in their targets, rather than death.The indirect effects of oedema and shock may be enough to kill smaller animals, however.< !-- More from Benton -->
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