Subclass Prototheria
egg-laying mammals

The subclass Prototheria contains the egg-laying mammals, which are the most ancestral forms in the class Mammalia. There are only three extant species grouped into two families and a single order, the Monotremata. Despite bearing fewer species than most mammalian genera, the prototherians are so unique among mammals that there is little question that they represent a distinct and ancient branch of the mammmalian family tree. However, it is not clear how monotremes are related to the two other major lineages of mammals, marsupials (Metatheria) and placentals (Eutheria). Some evidence supports the hypothesis that prototherians form a clade with the marsupials, while other evidence suggests that prototherians are sister to a clade containing both marsupials and placentals. (Heckner, 1990; Janke et al., 1996; Janke, Xu, and Arnason, 1997; Killian et al., 2001; Nowak, 1991; Vaughan, Ryan, and Czaplewski, 2000)

Prototherians probably split from the lineage leading to other mammals sometime in the Mesozoic. They retain many characters of their therapsid ancestors (for example, a complex pectoral girdle, laying of eggs rather than bearing live young, limbs oriented with humerus and femur held lateral to body, and a cloaca). The skulls of monotremes are almost birdlike in appearance, with a long rostrum and smooth external appearance. Modern monotremes lack teeth as adults; sutures are hard to see; the rostrum is elongate, beak-like, and covered by a leathery sheath; and lacrimal bones are absent. Monotremes have several important mammalian characters, however, including fur (but they lack vibrissae), a four chambered heart, a single dentary bone, three middle ear bones, and the ability to lactate. (Heckner, 1990; Nowak, 1991; Vaughan, Ryan, and Czaplewski, 2000)

Monotremes are restricted to Australia and New Guinea. Their fossil record is very poor; the earliest fossil attributed to this group is from the early Cretaceous. A fossil from Argentina suggests that the monotremes were more widely distributed early in their history. (Heckner, 1990; Nowak, 1991; Vaughan, Ryan, and Czaplewski, 2000)

Prototherians are either terrestrial (Tachyglossidae) or primarily aquatic (Ornithorhynchidae). Their terrestrial habitats include deserts, sandy plains, rocky areas, and forests in both lowlands and mountains. Platypuses inhabit lakes, ponds and streams; they shelter in burrows along the banks and spend much of their time foraging in the water. (Heckner, 1990; Nowak, 1991)

There is no doubt that the three extant prototherians, as well as extinct species known from the fossil record, form a monophyletic clade. A suite of morphological, developmental, reproductive and genetic characters strongly support this phylogenetic hypothesis. Prototheria contains a single order (Monotremata), two families (Ornithorhynchidae and Tachyglossidae) and three monotypic genera (Tachyglossus, Zaglossus and Ornithorhynchus). (Heckner, 1990; Nowak, 1991; Vaughan, Ryan, and Czaplewski, 2000)

The real debate about prototherian systematics involves its relationship with the two other mammalian subclasses, the Metatheria (marsupials) and Eutheria (placentals). Mophological, reproductive, and developmental evidence overwhelmingly supports a sister relationship between marsupials and placentals to the exclusion of the more primitive monotremes. Recently, comparisons of mitochondrial DNA sequences among mammals have supported the alternative hypothesis that monotremes and and marsupials form a clade exclusive of the placentals. Cladistic analyses using large segments of nuclear DNA have supported the more traditional view, however. Relationships among higher mammalian taxa (including both inter-subclass and inter-ordinal relationships) are currently very much in question and are the subject of exciting scientific debate among mammalian systematists. (Janke et al., 1996; Janke, Xu, and Arnason, 1997; Killian et al., 2001; Vaughan, Ryan, and Czaplewski, 2000)

Besides the absence of teeth, lacrimals, and obvious sutures, prototherians share a number of skeletal characteristics. On the skulls, the jugals are reduced or absent, the dentary is a slender bone with only a vestige of a coronoid process, the angle of the dentary is not inflected medially (unlike that of marsupials), auditory bullae are missing (part of the middle ear is enclosed by tympanic rings), and much of the wall of the braincase is made up by the petrosal rather than the alisphenoid (unlike all other modern mammals). Postcranially, the skeleton of prototherians is also unique among mammals. It is a fascinating mosaic of primitive characteristics inherited from therapsids but found in no other living mammals, and modifications probably related to the burrowing habits of modern prototherians. Their shoulder girdles are complex, including the standard components of modern mammals ( scapula and clavicle), but also additional elements including coracoid, epicoracoid, and interclavicle. The scapula, however, is simplified, lacking a supraspinous fossa. The shoulder girdle is much more rigidly attached to the axillary skeleton than in other mammals. Femur and humerus are held roughly parallel to the ground when the animal walks, more in the fashion of therapsids and most modern reptiles than like modern mammals. Ribs are found on the neck (cervical) vertebrae as well as the chest (thoracic) vertebrae; in all other modern mammals, they are restricted to the thoracic region. (Heckner, 1990; Nowak, 1991; Vaughan, Ryan, and Czaplewski, 2000)

Another interesting skeletal characteristic of prototherians is the large epipubic bones in the pelvic region. Epipubic bones were originally thought to be related to having a pouch, but they are found in both males and females. They also occur in all species of marsupials, whether a pouch is present or not (not all marsupials have a pouch). It is now thought that epipubic bones are a vestige of the skeleton of therapsids, providing members of that group with extra attachments for abdominal muscles to support the weight of the hindquarters. (Heckner, 1990; Nowak, 1991)

Prototherians are endothermic, but they have unusually low metabolic rates and maintain a body temperature that is lower than that of most other mammals. (Heckner, 1990; Nowak, 1991)

All male prototherians have spurs on their ankles that are presumed to be used in fighting and in defense. In one family (Ornithorhynchidae), a groove along the spur carries poison secreted by adjacent glands. (Heckner, 1990; Nowak, 1991)

Little is known regarding the natural lifespan of prototherians; however, they can live several decades in captivity. In at least one case, a short-nosed echidna lived 50 years. (Heckner, 1990; Nowak, 1991)

Prototherians are primarily solitary animals and at least one species (Tachyglossus aculeatus) is territorial. Activity patterns vary among species, and even among populations of T. aculeatus; prototherians may be diurnal, crepuscular or nocturnal. Echidnas are fully terrestrial and eat mainly ants, termites, and worms whereas platypuses spend much of their time foraging in the water for a wider variety of invertebrates. All three species are exceptional diggers, using powerful limbs to dig shelters or to quickly escape from predators. In addition to digging their way out of trouble, echidnas can roll up and erect their spines as a defense mechanism. If insufficient food is available, prototherians may enter temporary torpor or more prolonged periods of hibernation when food is scarce in the winter. (Heckner, 1990; Nowak, 1991; Vaughan, Ryan, and Czaplewski, 2000)

Hearing, olfaction, touch, and vision are all important to some degree in prototherians. Hearing and sight are well developed in platypuses and moderately well-developed in echidnas. The sense of touch is perhaps most important to a platypus that is searching at the bottom of a stream for food or an echida that is rooting through the earth for termites or worms. Platypus bills and echidna snouts are extremely sensitive organs that are essential to effective foraging. Platypuses may even use electrical stimuli to locate prey. Olfaction is well-developed in echidnas and may be used in individual recognition. Prototherians occasionally produce some simple vocalizations, but their function is unknown. (Heckner, 1990; Nowak, 1991)

All prototherians are carnivorous, with their diets consisting of various invertebrates. Platypuses forage in the benthos of lakes and streams, using their sensitive bills to find prey. They are generalist predators, whereas echidnas specialize on either ants and termites (Tachyglossus) or worms (Zaglossus). Both species of echidna are powerful diggers and use their claws and snouts to root through the earth to find food. (Heckner, 1990; Nowak, 1991)

Little is known about the predators of prototherians or how predation impacts prototherian populations. With their robust spines, echidnas are certainly well-protected from threats. To deter potential predators, echidnas erect their spines, roll into protected balls, or rapidly dig a hole or enter a crevice, exposing only their spines. (Heckner, 1990; Nowak, 1991)

Prototherians may significantly impact populations of their prey; this may be more true for echidnas because they specialize on only a few prey types rather than eating a little bit of many different species. Because they are adept diggers, prototherians create and modify habitat for other organisms. Platypuses in particular can exavate extensive burrows on the banks of freshwater lakes and streams. Prototherians are hosts of various parasites (e.g. trypanosomes in platypuses and hepatozoans in echidnas). (Clark, Holz, and Spratt, 2005; Noyes et al., 1999)

Prototherians have no known negative impact on people, except perhaps for the pain a platypus can cause if it delivers venom with its spur. If unmolested, platypuses will not attack humans. (Nowak, 1991)

One of the three species within Prototheria (Zaglossus bruinji) is eaten by the indigenous people of New Guinea. Hunting pressure has been so great that this species is now threatened with extinction. (Nowak, 1991)

At least two of the three species of prototherians are threatened with extinction or were at some point in the recent past. Long-nosed echidnas are currently listed as endangered by the International Union for the Conservation of Nature (IUCN) redlist (www.redlist.org). Habitat destruction and heavy hunting pressure have severely limited populations of this species. In the past, platypuses were declining rapidly due to their overexploitation in the fur trade, but recent conservation efforts have helped populations rebound considerably. (Groombridge, 1994; Nowak, 1991)