The evolution and systematics of the Lagomorpha (pikas, rabbits, and hares) has a rich tradition. 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-
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-
A complementary question is how long ago did the lagomorphs become an independent lineage? Again, here we are assisted by both molecular and paleontological data, and the 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-
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-