The Platypus, Part 2

Skeleton of a platypus as seen from the underside. It is not mentioned in the text, but note how the rib cage is separate from the hips. That shows that the platypus has a diaphragm, a structure that is unique to mammals. Image modified from Griffiths, Mervyn (1988) “The Platypus” Scientific American 258(5): 84-91.

To begin describing the uniqueness of the platypus, it is helpful to first put it in context of its classification. The living platypus has been given the scientific name Ornithorhynchus anatinus, which means “duck-like animal with a bird’s snout.”[1] Even its superficial similarity to a duck worked its way into its name. In case you are wondering where the name “platypus” comes from, its scientific name used to be Platypus anatinus, which means “flatfooted duck-like animal.”  The name was changed because the genus Platypus had already been applied to the ambrosia beetle. The common name platypus, however, stuck with the mammal.

The platypus belongs the family Ornithorhynchidae. It contains one living species today. There are a few more members of Ornithorhynchidae known from fossils, but we will get to those in a later post. The family Ornithorhynchidae belongs to the order Monotremata. Usually, the word “monotreme” is used to describe the mammals that belong to the order Monotremata.

Aside from the platypus, there are four other living species of monotremes. These four species are all various types of spiny anteaters, also known as echidnas. The echidnas belong to the family Tachyglossidae. To reiterate, the mammal class Monotremata has only two living families: Ornithorhynchidae (one species of platypus) and Tachyglassidae (four species of echidnas). Again, there are a few more species and families known from the fossil record.

To emphasize how distinct the monotremes are, it should be pointed out that mammals are divided into three large groups. First, there are the placental mammals. These are the “normal” mammals. They are characterized by their females bearing live young and giving birth to young that are relatively highly developed. Again, just think of normal mammals, such as dogs, cats, and cows, and how their young are born pretty recognizable as a dog, cat, or cow. The second group of mammals are the marsupials. These mammals also give birth to live young, but do so at an earlier stage of development relative to placentals. These embryonic young are then nursed in a pouch until they develop further. Finally, the last group of mammals are the monotremes, which are the only mammals that lay eggs.

Right away, it becomes apparent that the platypus is unusual among mammals because it, and the four species of echidna, have to be classified in their own separate group of mammals. While the uniqueness of monotremes is best explained by noting that they lay eggs, that is not the only characteristic that separates them from other mammals. Far from it. There are in fact a host of traits that separate montremes from the other mammals. We will focus on two of them in this post.

The first unique trait of monotremes we will talk about it the one that lends itself to the name “Monotremata.” Monotremata means “one hole” and it refers to the cloaca. The cloaca is a common chamber with a single exit. Three separate organ systems, the urinary system, the digestive system, and the reproductive system, all empty into the cloaca and use it as a common exit.

Now, we are unfamiliar with a cloaca, because we do not have one. Aside from the monotemes, no mammals have a cloaca.[2] Instead, the urinary, digestive, and reproductive systems have separate exits (in females) or the urinary and reproductive systems have a common exit and the digestive system has a separate exit (males). While cloaca are not found in most mammals, they are found in the other tetrapods. “Tetrapod” refers to the non-fish vertebrates, so basically, a cloaca is found in birds, reptiles, and amphibians.

It is tempting to think that the cloaca is needed for egg laying, since the cloaca is found in those animals that lay eggs (amphibians, reptiles, birds, and monotremes) while it is absent in those that give live birth (marsupials and placentals). However, the shell of a platypus egg is actually created while the embryo is in the fallopian tube and uterus,[3] both of which lie outside of the cloaca. Frankly, I am not sure why monotremes have a cloaca, aside from that is simply the way God decided to make them.

The next feature of monotremes that set them apart from the other mammals is the shoulder girdle. While most mammals have two pairs of bones in their shoulder girdle, monotremes have four pairs of bones and an additional ninth bone. Again, the significance of this may not seem obvious, so let us begin by talking about the normal mammal shoulder.

Reach one arm across your body on over onto your back to feel your shoulder blade. The shoulder blade represents the first bone in your shoulder girdle: the scapula. There are several protrusions and extensions of the scapula, but basically, the scapula is a flat blade with a socket for the arm on the top and outside. The socket is on the top and outside in humans, anyway. In mammals that hold their bodies horizontally, the socket is located in the front and underneath the body.

Now, feel for your collar bone. There is a pair of these and they lie on your front, right at the base of the neck. The collar bone is also called the clavicle. As you feel around your clavicles, note that one end of the clavicle joins to the scapula and the other end joins with the sternum, the bone running down the front of your chest. The purpose of the clavicle is that it helps hold the scapula in place. The scapula is free to slide over your rib cage. That is why we can shrug our shoulders: our scapula slide up along our backs. While the scapula has free movement, the clavicle acts as a brace that prevents the scapula from sliding around too much.

The sliding motion of the scapula gives additional flexibility to our already flexible forelimbs. Such is not the case with the monotremes, however. In addition to the scapula and clavicle, they also have a pair of coracoids (these fuse with the scapula, basically extending the scapula to underside of the body), an interclavicle (this connects to the clavicles and the sternum), and a pair of epicoracoids (these connect the coracoids to the interclavicle). All of these additional bones extend the shoulder girdle and help lock it in place. Unlike most other mammals, monotremes cannot freely move their scapula.

Generally, the arrangement of the shoulder girdle in monotemes is similar to that of a reptile. Naturally, there are variations found among the reptiles, but they have shoulder girdles with at least three pairs of bones and they are typically locked in place so that the girdle cannot move around freely.

The sturdy, inflexible shoulder girdle of monotremes actually provides them advantages for their ways of life. In echidnas, the front feet are used for burrowing, so a sturdy girdle helps with that. As far as platypuses go, they do most of their locomotion with their front feet. More specifically, to swim, a platypus rows with its front feet and use its hind feet as rudders. Note that this is a unique swimming motion. Most other mammals swim with either all four feet, hind feet, tail, or a combination of tail and hind feet. In addition to swimming, platypuses also burrow, and they do this burrowing with their forefeet as well.[4] They burrow into riverbanks to make their homes and nests.[5] They actually spend most of the day in their burrows, typically coming out at night. Thus, a strong, sturdy shoulder girdle is beneficial to a platypus both in its manner of swimming and in its habit of burrowing.

As an aside, I want to also mention that the forefeet of platypuses are also designed for both swimming and digging. The front feet are webbed, and webbed far more than the hind feet. The webbing of the forefeet actually extends beyond the edge of the animal’s claws. However, this extension of the webbing can be folded back to allow the claws to protrude, as which point the claws can then be used for digging.

The forefoot of a platypus. The image on the left shows it with the full webbing. The image on the right shows what it would look with the webbing folded under the foot. Now the claws are exposed for digging. Modified from Griffiths, Mervyn (1988) “The Platypus” Scientific American 258(5): 84-91.

We have looked at some features of the monotremes, and the platypus specifically, that highlight their uniqueness among mammals. Structures, such as the cloaca and shoulder girdle, resemble features found in reptiles. However, note that these are just two features found in monotremes. I can name several features that monotremes possess that unite them with mammals. For example, they have a single bone in their lower jaw, three ossicles in their ear, and hips like those of other mammals. Plus, monotremes have two features that are often treated as defining characteristics of mammals: fur and mammary glands. Thus, despite their unique characteristics, there is even reason to consider them as mammals using our definition of what a mammal is.

As a brief aside, I want to quickly talk about the hips of a platypus. As was just mentioned, they are actually quite similar to the hips of other mammals. In fact, they are very similar to the hips of a marsupial, since the hips of a platypus have epipubic bones, which are found in marsupials but not placentals. Now, I believe that the epipubic bones have cause some confusion about the nature of platypuses. Epipubic bones are often associated with the presence of a pouch, since they are found in marsupials. Thus, some have claimed that a platypus has a pouch. It does not. The truth is, the exact purpose of epipubic bones is unknown. All we can say for sure is that it is absent in placentals and present in marsupials, monotremes, and a selection of fossil mammals.

One more final final thing to say, there is actually one feature about the platypus hips that make them different from other mammals: the hip sockets are direct outwards, meaning that the hind legs sprawl. The shoulder girdles are designed so that the forelimbs sprawl too. This differs from most mammals who hold their legs under their bodies. However, aside from that difference, the hips of platypuses are distinctly mammalian.

There is more to be said about the platypus, but we will get to that in the next post.

Thoughts from Steven

[1]Griffiths, Mervyn (1988) “The Platypus” Scientific American 258(5): 84-91

[2]Quick side note: I know I am making it sound like humans are mammals. I do not believe humans are animals: God created us after His image and did not do so with animals. However, our physical bodies mirror the body of a mammal, so as far as anatomy goes, we have mammalian bodies.

[3]Griffiths, Mervyn (1988) “The Platypus” Scientific American 258(5): 84-91

[4]Gregory, William King (1947) “The Monotemes and the Palimpsest Theory” Bulletin of the American Museum of Natural History 88(1): 1-52

[5]Griffiths, Mervyn (1988) “The Platypus” Scientific American 258(5): 84-91