Evolution and systematics

The evolution and systematics of the Lagomorpha (pikas, rabbits, and hares) has a rich tradition of controversy, prompting one paleontologist to author a paper several decades ago entitled, "What, if anything, is a rabbit?" The problem, however, is not defining the limits of what constitutes a lagomorph (expanding "rabbit" to include pikas and hares), but rather determining the position of lagomorphs within the mammals. Lagomorphs represent a well-defined grouping, and although they were originally classified within the Order Rodentia, even in this alignment lagomorphs were separated into the Duplicendentata whereas the "true" rodents were classified as Simplicendentata. This distinction was based on the second small peg-like upper incisors that sit behind the primary incisors in all lagomorphs, while rodents possess only a single upper incisor.

In 1912, Gidley proposed that the duplicendentates be raised to a separate order, the Lagomorpha. The existence of the Lagomorpha and Rodentia orders has been adopted as standard terminology, but unfortunately this separation did little to solve the problem of relationship of lagomorphs with other mammals. For example, Gidley proposed the independence of the lagomorphs because he thought they showed significant differences from the rodents (above and beyond dentition) and believed they showed more affinities with ungulates, perhaps the Artiodactyla. Simpson, however, maintained that the lagomorphs and rodents should be recognized together and coined the term Glires to represent a clade made up of the two orders. According to this view, lagomorphs and rodents share a common ancestor, although Simpson was the first to admit that the relationship implied by Glires was "permitted by our ignorance, rather than sustained by our knowledge." Through the years the issue of whether or not lagomorphs belong in the Glires with the rodents was hotly debated, and lagomorphs were variously linked with a variety of other mammalian taxa.

We are now fortunate to have molecular techniques to complement an increased knowledge of fossil lagomorphs, and there has been a flurry of investigations in the past 10 years (many since 2000) that specifically address the evolutionary placement of the lagomorphs. Unfortunately, the results of these studies are as contradictory as in earlier times when we had fewer data. A majority of studies using molecular sequence data significantly support the Glires clade, incorporating the Lagomorpha and Rodentia. In these cases, Glires is apparently a sister taxon to Primates, Dermoptera (flying lemurs), and Scandentia (tree shrews). In addition, morphological data analyzed in a similar manner define Glires with 100% support. However, other molecular studies specifically reject the grouping of Lagomorpha and Rodentia, while others are ambiguous on the issue. One investigator put it this way: "The rabbit wanders about in the mitochondrial protein tree, undecided whether to join the carnivore-perrisodactyle-cet-artiodactyle clade, the primate branch, or neither." Other studies link lagomorphs with the tree shrews or the Xenarthra (armadillos). It is likely that the lack of support for the Glires clade in some molecular studies reflects poor taxon sampling within the lagomorphs and rodents. While the jury is out until there can be resolution among these various molecular approaches, a parsimonious conclusion is that the lagomorphs are indeed linked with rodents—as Glires—in their evolutionary history.

A complementary question is how long ago did the lago-morphs become an independent lineage? Again, here we are assisted by both molecular and paleontological data, and the

A collared pika (Ochotona collaris) sunning in Denali National Park, Alaska, USA. (Photo by John Hyde. Bruce Coleman, Inc. Reproduced by permission.)

results are surprising. Until recently it was assumed that the major lineages of mammals diversified in the early Tertiary. Now, there is strong evidence from molecular data that lagomorphs were independent as long ago as the Cretaceous. Some reports using molecular sequence data indicate that the lagomorphs split from the rodents 64.5 million years ago (mya), others push the date back to over 100 mya. The variability in these molecular approaches, however, stems from their use of different genes, sampling, and methods, such that an unambiguous time of divergence between lagomorphs and rodents is not presently possible. In addition, a form with lagomorph characteristics, Alymlestes kielanae, has been uncovered in central Asia and dated to nearly 85 mya, thus pushing back the paleontological clock for lagomorphs. Thus, lagomorphs apparently became independent far earlier than had previously been assumed, and this independence likely occurred during the Cretaceous.

A significant fossil record of two rodentiform taxa (called eurymylids and mimotonids) is found in the Paleocene. The mimotonids appear to be primitive lagomorphs, whereas the eurymylids are linked with ancestral rodents (although in earlier treatments they were often classified as lagomorphs). These forms appear to be too advanced along their respective specialized lines to be ancestors, thus confirming that the Glires separated sometime during the Cretaceous. The most primitive representative of the mimotonids was Mimotoma, a rabbit-like animal similar to Alymlestes, but with a rabbit dental formula. Like rabbits it had two upper incisors, although the second incisor was still large and functional, while in modern-day rabbits it is small. Mimotoma was the likely ancestor to Mimolagus, an Eocene form that possessed many more lagomorph-like characteristics. Alymlestes, Mimotoma, and Mimolagus are all Asian forms, thus confirming an Asian center of origin for lagomorphs.

The first true rabbits (Leporidae) appear in the lower Eocene in Asia (Lushilagus, Shamolagus) and slightly later in

North America (Mytonolagus). Lushilagus possesses teeth very similar to Mimotoma, and modern lagomorph dentition could easily have been derived from these teeth. These early forms lacked the limb proportions that characterize modern rabbits; they more closely resembled modern pikas. From this point it is possible to derive the lineages leading to both of the modern-day families of lagomorph, the Leporidae (rabbits and hares) and the Ochotonidae (pikas). The split into these two families occurred about 37 mya, or near the Eocene-Oligocene transition. By the middle Oligocene the first pika, represented by Desmatolagus gobiensis, was found in Asia, and other pikas soon appeared in Europe. Pikas would eventually flourish and reach their zenith in the Miocene. Geographically most pika evolution was restricted to Asia, Europe, and North America, but some forms reached Africa in the Miocene. Today, pikas are represented by a single genus, Ochotona, distributed in Asia and North America. Twenty-five extinct genera of ochotonids have been described, one of which, Prolagus, occupied Europe until historical times before becoming extinct.

By the early Oligocene a variety of true rabbits were found in Asia and North America, and the family would eventually spread throughout most of the world. Thirty-one extinct genera of leporids have been described, and today the family is represented by 11 extant genera.

The chapters on the Leporidae and Ochotonidae detail the evolution of these families, respectively. However, one important observation highlights the evolution of the lago-morphs. There are relatively few lagomorph species compared with the closely related rodents (over 2,000 rodents versus only 91 lagomorphs), and this discrepancy has never been adequately explained. Furthermore, morphologically there has been significantly less innovation (fewer specialized adaptations) in lagomorphs than in rodents. The term lagomorph means "hare shaped" and this description adequately serves to portray all species in the order.

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