Two new Jurassic fossils yield conflicting reconstructions of the mammalian tree. These divergent genealogies have profoundly different implications for the origin and early diversification of mammals. See Article p.163 & Letter p.199
Fossil discoveries and molecular studies in recent decades have greatly advanced our understanding of mammalian relationships and diversification1. Yet major points of disagreement remain over some of the basal branches of the family tree. There is little doubt that mammals, strictly defined2, were widespread and ecologically diverse by the middle of the Jurassic period, about 165 million years ago3,4,5. But when did they originate? A major sticking point is the inclusion (or not) of certain poorly known early forms. Substantial information is now provided by two separate discoveries, reported in this issue by Zhou et al.6 (page 163) and Zheng et al.7 (page 199), of splendidly preserved fossils from China that date to between 160 million and 165 million years ago.
Both fossils, which include evidence of fur but lack complete skulls, have been assigned to the Haramiyida. This enigmatic group includes fossils dating back to the Late Triassic — that is, about 40 million to 50 million years before the appearance of undoubted mammals4,5. Intriguingly, haramiyidans have some rodent-like specializations of the teeth, and in this respect they vaguely resemble members of another extinct group, the Multituberculata, which is well known and has consistently been placed firmly within the mammalian tree2,3. Although the new fossils seem to be related, detailed examination reveals significant differences between the two animals — differences that lead the authors to contrasting views of mammalian relationships which imply quite different models for the origin and initial diversification of these iconic organisms.
Zheng and colleagues describe Arboroharamiya, a short-faced presumed omnivore or herbivore that was adapted for life in the trees (as implied by the name). Notably, the authors interpret the back of the jaw to be simple (although this is not clear from the photographs in the paper's Supplementary Information), indicating that the bones of the middle ear had assumed a typical mammalian configuration8. Surprisingly, the results of their analysis place haramiyidans (including Arboroharamiya) and multituberculates together within Mammalia (Fig. 1a). This implies that mammals originated at least 215 million years ago — a much earlier date than many palaeontologists would accept2,3, but one that is in agreement with a recent estimate based on a molecular-clock model9.
Megaconus, described by Zhou and colleagues, comes from the same geological formation as Arboroharamiya, but is slightly older. Apparently adapted for terrestrial life and a plant-based diet, Megaconus is primitive in many respects: the counterparts of the mammalian middle-ear bones remain attached to the jaw, for example, and the ankle resembles that of pre-mammalian forms. The authors' analysis places haramiyidans (including Megaconus) outside Mammalia and unrelated to multituberculates (Fig. 1b). Under this interpretation, the minimal age for mammalian origin is much younger, and is constrained by the appearance of multiple, uncontested types of mammal by about the Middle Jurassic3,4.
The evolutionary history of various mammalian groups has been characterized as 'long fuse', 'short fuse' and 'explosive'1,9,10, depending on the time elapsed between their origin and significant diversification. Zheng and colleagues' tree, which places the origin of mammals at 40 million to 50 million years before their Jurassic radiation, implies a long-fuse model for the group as a whole (Fig. 1a). Zhou and colleagues' competing hypothesis, on the other hand, is consistent with an explosive model (Fig. 1b). These alternative family trees and implied models of diversification have profound ramifications for interpreting the significance of key adaptations, ecological interactions and other events in the early history of mammals3,8,11.
Neither family tree perfectly explains all the data, of course. Indeed, each implies seemingly unlikely examples of independent evolution or reversal (homoplasy) of key characteristics. Zheng and colleagues' model, for instance, suggests that the complex, three-boned middle ear, long regarded as a diagnostic feature of mammals2, may have arisen independently at least three times — in the multituberculate–haramiyidan group, in monotremes (platypuses and echidnas) and in therians (marsupials and placentals). Similarly, the tree proposed by Zhou et al. requires independent evolution of strikingly similar tooth features — the presence of a single pair of enlarged, forward-facing incisors and complex cheek teeth with multiple rows of cusps, for example, that are presumably associated with herbivory — in haramiyidans and multituberculates.
These contrasting results, and their implications for mammalian evolution, should be considered in light of the underlying data. Of the two fossils, Megaconus is the more complete, with 41% of anatomical characters scored, compared with 23% for Arboroharamiya. In addition, the analysis by Zhou et al. is more comprehensive in its sampling of species and anatomy, particularly with respect to haramiyidans and multituberculates.
The obvious next step, of course, is to conduct analyses to synthesize and reconcile the data presented in these two contributions. Ultimately, however, more and better fossils, ideally including skulls, will be needed to refine knowledge of early mammalian radiations, both in terms of relationships and palaeobiology.
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Cifelli, R., Davis, B. Jurassic fossils and mammalian antiquity. Nature 500, 160–161 (2013). https://doi.org/10.1038/500160a
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