Primate origins: A reappraisal of historical data favoring tupaiid affinities

Abstract

The origin of primates remains a fascinating question. In spite of many anatomical and molecular studies, the identification of the living sister group of primates is not clearly settled. The lack of consensus about primate origins is the result of the great antiquity of the events that marked primate differentiation. Morphological as well as molecular signals are masked by the amount of subsequent evolution in primates and in potential sister groups, and by the extinction of some critical intermediates. In addition, the Paleocene fossil record of mammals is still particularly poor in Africa and in the southern tropical regions of Asia, where some of the important steps presumably took place. What makes the problem especially puzzling is the realization that the increase in and the enhanced quality of the anatomical and molecular data sets extracted from the living forms did not result in any increased consensus. On the contrary, the cladistic treatment of a large morphological data set concerning archontan p logeny, assembled by Simmons (1993) from previous studies, did not detect a strong signal concerning the sister group of primates. This constitutes a challenge for phylogeneticists. Molecular data sets, which also did not yield a consensus in 1993, continue to expand. It seems now thata strong molecular signal favors a close relationship between Primates, Scandentians, and Dermopterans (Liu et al., 2001; Madsen et al., 2001; Murphy et al., 2001; see Springer, this volume). During the first half of the 20th century, tree shrews (family Tupaiidae) were often considered as the most primitive representatives of the order Primates. Carlsson (1922) compared many features in Tupaia, Lemuriformes, Macroscelididae, and Lipotyphla. She found many similarities between Tupaia and lemuriforms, including the presence of a postorbital bar, and she was the first to formally include tupaiids in primates. Her view was strongly supported by the work of Le Gros Clark (1959, 1971), and was adopted by Simpson (1945) and Saban (19 3) among others. However, a remarkable book on "Comparative Biology and Evolutionary Relationships of Tree Shrews" (Luckett, 1980a) placed the long-standing belief in a close tupaiid-primate relationship into question, with the rigor of cladistic methodology. Many similarities between the two groups appeared as primitive retentions or convergences, and tree shrews were left in their own order Scandentia without any clear affinity within the mammals. Another attempt at putting "primates and their relatives in phylogenetic perspective" led to the contributions assembled by MacPhee (1993a). No consensus emerged from this attempt, but it did lead to "the rehabilitation of scandentians as being at least reasonably close relatives of primates (and colugos)" (MacPhee, 1993b). A group of Early Cenozoic fossil mammals, including the Plesiadapidae and Paromomyidae, were described as primates by most early paleontologists. They show some general dental similarities with primates, and there are detailed similarities betw een some molars of Plesiadapis and Cantius. However, it has been long recognized that these must be convergences because they do not exist in primitive plesiadapids and primitive adapids such as Donrussellia. Furthermore, Plesiadapis was believed to have a petrosal bulla, which is a hallmark of primates (Szalay, 1969). Hence, plesiadapids and related families, which are usually assembled in the taxon Plesiadapiformes (Simons, 1972), were considered as representatives of an early radiation of the primates by many (e.g., Gingerich, 1976; Romer, 1966; Simons, 1972; Szalay and Delson, 1979). At the same time, other scholars were working toward a better definition of primates based on primarily the derived characters shared by the living forms (Cartmill, 1972; Martin, 1968, 1985). This research generated a new understanding of the adaptive significance of primate characteristics (Cartmill, 1972, 1974a). In fact, our understanding of primate characteristics has probably progressed much further during the past decad than our understanding of archontan phylogeny (e.g., Dagosto, 1988; Rasmussen, 1990; Ravosa et al., 2000; see other contributions in this volume). The series of derived characters shared by virtually all living primates-petrosal bulla, a complete postorbital bar encircling large forward-oriented orbits, an opposable hallux, and nails instead of claws (Cartmill, 1972; expanded in Martin, 1986)-is widely accepted. However, the taxonomic consequences of this understanding are not treated in the same way by all authors. Those who maintain a broader view of the order Primates and their close relationships to the plesiadapiforms use the term 'Euprimates' (Hoffstetter, 1977), to include the living ones and their close relatives, or the informal "primates of modern aspect" of Simons (1972). Moreover, they consider the Plesiadapiformes a suborder of the Primates (Fleagle, 1988; Gunnell, 1989; Szalay et al., 1987; Van Valen, 1994). Others restrict the order Primates to the euprimates and consider the Plesiadapiformes as a separate order of mammals-an opinion that is becoming more widely accepted (e.g., Fleagle, 1999). This choice is adopted here because it gives the taxon primates adaptive significance and also because the sister taxon of the modern primates is unresolved. Between 1990 and 1993, a quite different hypothesis emerged from the study of new fossil material. Postcranial studies of the plesiadapiform family Paromomyidae seemed to favor a close affinity of this family with Dermoptera, leading to the concept of Primatomorpha-a mirorder including primates and dermopterans (the plesiadapiforms being included within dermopterans; Beard, 1993a,b). Similar relationships are accepted by McKenna and Bell (1997). These new hypotheses are accompanied by radical changes in classifications (e.g., the inclusion of the Eudermoptera in the Plesiadapiformes). Much worse, the inclusion of Dermoptera as a suborder of the order Primates (McKenna and Bell, 1997) simply destroys all previous constructions of the taxon primates. More over, all these dramatic changes reflect a hypothesis which is very questionable. This is scrutinized in the next section. A more recent treatment of the problem by Silcox (2001) again nests modern primates within plesiadapiform groups. As the quantity of information has enormously increased and the current hypotheses are so numerous and contradictory, it has become a challenge toreconcile the multiple lines of evidence and to expose the most likely hypothesis. Here the author proposes such a view, that of a paleontologist, inclined to favor that part of the evidence for which there is some historical insight. The primary reliance on cranioskeletal characters is based on the conviction that there are lessons to be learned from traditional systematics and from the paleontological record. Of course, such a view needs to be confronted with insights inferred from the study of living forms (e.g., molecules, neural, and reproductive traits, etc.). © Springer Science+Business Media, LLC 2007.