Years of Primate Paleontology

From the first growth of the tree, many a limb and branch has decayed and dropped off; and these lost branches of various sizes may represent those whole orders, families, and genera which have now no living representatives, and which are known to us only from having been found in a fossil state.... As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications. (p 129,130 in Darwin 1859). The remains of the fossil forms of the primates are unfortunately still few in number and very defective; nevertheless, they are being gradually augmented, and the hope seems justified that in the not far distant future forms will be recovered that will be of as acute interest to the student of man's origin as the known remains of some of his earlier representatives. An intensive systematic search for such remains in Africa, Asia, and Malaysia is one of the most urgent scientific necessities (Hrdlička, 1918). Members of the Association continue to recognize the need to document human ancestry and its roots. The mission statement of the Association states: "Physical anthropology is a biological science that deals with the adaptations, variability, and evolution of human beings and their living and fossil relatives." This begs the question as to how closely related to humans a primate needs to be for it to fall within the confines of physical anthropology. A narrow reading of the Association's mission could imply, for example, that the evolution of lemurs and lorises and their extinct relatives might well be outside the Association's mission. Fortunately, that has not been the case, as demonstrated in meeting abstracts and publications in the Journal. Moreover, the National Science Foundation, a primary source for paleoprimatology research funding, mentions nonhuman primate paleontology as an area it supports. Up until the 1960s, most of the work of primate paleontology was focused upon humans and their ancestors. Many studies of Miocene and earlier primates were undertaken by comparative anatomists and broadly-trained vertebrate paleontologists. Fossil primate studies were largely carried out by paleontologists interested in paleofaunas that contained primates, for example, the works of C. L. Gazin, J. W. Gidley, W. Granger, W. D. Matthew, M. Schlosser, G. G. Simpson, H. G. Stehlin, or F. Ameghino. Or fossils were incorporated into broader works by those with a primary research interest in human and comparative anatomy, for example by Wilfrid Le Gros Clark (1934, 1959). William K. Gregory was a notable exception. While he maintained a broad research scope including study of Recent and fossil fishes, reptiles, and mammals other than primates, he made seminal contributions to primate paleontology. In 1916, he published a study of the early stages of anthropoid evolution (Gregory, 1916). This work was followed by a landmark study of the anatomy of a well-preserved skeleton of the North American Eocene 'lemur' Notharctus (Gregory, 1920). Gregory's synthetic work on the evolution of the human dentition (1922) influenced every primate paleontologist going forward. It brought his broad expertise on the anatomy of living primates to bear on the early fossil record of primates. Notably also, he called to the attention of the North American paleontological community the work of European paleontologists and comparative anatomists. Paleoprimatology as a discipline really took off in the early 1960s with the establishment of primate-centered fossil studies at Yale University under the direction of Elwyn Simons (1930–2016). Having received his early training in vertebrate paleontology at Princeton University studying a group of Paleocene-Eocene ungulates, Simons moved on to Oxford University, working with Le Gros Clark. At first, Simons was best known for his revival of GE Lewis' (1934) claims for Ramapithecus as a mid-Miocene human ancestor. However, Simons's work under Le Gros Clark on Paleocene and Eocene primates from Europe also rekindled interest in Paleogene primates. In this way, and through Simons' efforts, there was a broadening of paleoanthropology to include the whole primate record. Like Gregory, Simons emphasized the importance of understanding human evolution within the framework of the evolution of primates and their relatives in the whole of the Cenozoic. Always dismissive of "armchair" paleontologists who primarily were interested in studying fossils that had already been collected, Simons placed an enormous emphasis on gathering new fossils. He and his students returned to fossil fields that had lain fallow for half a century or more in India, Pakistan, and Egypt. He undertook broad collaborative field programs to recover fossil primates from Madagascar and the Western United States. Simons trained and worked with several generations of Yale, and later Duke University students who embodied his notion of the central importance of assembling new fossil collections. Although similar programs were developing in parallel in Great Britain under the influence of John Napier and P. R. Davis and at a few other US institutions, for example at the American Museum of Natural History by the students of Malcolm McKenna (Fred Szalay, Eric Delson, and their students), Simons may fairly be described as the father of modern paleoprimatology. In what follows, I present a highly personal account of trends and threads that comprise paleoprimatology, emphasizing how developments in other fields that greatly influenced the way we think about fossil primates in time, in space, and in relation to the environments in which they lived. I evaluate the evidence for one of the most contested aspects of primate evolution. How and when did primate adaptations evolve? The fossil record of primates has grown enormously in the past 100 years but primate species are still relatively rare (Martin 1993). As of 2017, 336 extinct genera of primates were recognized on the Paleobiology Database (http://paleodb.org) and the 'list of Fossil Primates' at (https://en.wikipedia.org/wiki/List_of_fossil_primates). Just 43, or 13%, of these taxa had been described by the time of the founding of the American Journal of Physical Anthropology in 1918 (Figure 1). In spite of the vastly improved record since then, primate fossils remain woefully scarce. Consider that there are 79 genera of living primates (18 in Asia; 24 in Africa; 15 in Madagascar; 19 in South America; 4 in Central America) [including some overlaps] and more than 504 species (Estrada et al., 2017). Hypothetically, if the average primate genus persists 3 million years, then a minimum of 1300 genera and >10,000 species may have existed over the 65 million years of primate evolution. But even the 336 known fossil genera are mostly based on scant remains, mainly jaws and teeth, which in turn constrains our ability to test scenarios of primate evolution. Date of description of fossil primates (including Euprimates and Plesiadapiformes) since the first description of a primate fossil (Adapis Cuvier 1821). 13% of taxa recognized today were known at the time of the founding of the American Journal of Physical Anthropology Before 1918 the record of fossil primates was mainly from North America and Europe, outside the current tropical distribution of non-human primates. Just two genera were described from South America, one from Asia, four from continental Africa, and six from the Pleistocene-Recent of Madagascar. More regions of the globe are now sampled to some extent but critical gaps remain. For example, primates inhabited Africa since the Early Paleogene; but of 82 extinct genera thus far described, just five are more than 40 million years old and the record of subtropical and southern parts of the continent is virtually nil. And, although primates must have resided in Madagascar since the Eocene, the primate fossil record is entirely from the Pleistocene-to-Recent. In South America (including Central America and the Greater Antilles), just 26 extinct genera are recorded, which is barely more than the generic count of living taxa. Platyrrhines as old as 40 million years should be expected but none is older than 30 million years, and virtually no tropical or subtropical sites are known prior to 15 million years ago. When I began my graduate studies at Yale University with Elwyn Simons almost half a century ago, William K. Gregory's (1910) and George G. Simpson's (1945) influential works on the relations of primates to other mammals were canonical. Gregory recognized a monophyletic group, the Archonta, consisting of primates, tree shrews (Scandentia), flying lemurs (Dermoptera), bats (Chiroptera), and elephant shrews (Macroscelidea). Gregory, following many earlier workers, also recognized that Paleocene-Eocene Plesiadapiformes (sensu Silcox et al., 2017) belonged among Archonta (Gregory, 1927; Figure 2). There was some disagreement as to whether tree shrews should be classified as primates, but no one disagreed with the notion that the two were close relatives (Clark, 1926; Gregory, 1913; Simpson, 1959). The disagreement was more a question of whether it made sense to include tree shrews because of their great phenetic separation from modern primates (Martin, 1968). The same disagreements arose as to whether plesiadapiforms should be included among primates (Cartmill, 1972). Beyond that, there was little agreement about where primates might be nested among other Archonta or mammals in general. The fossil record of tree shrews, colugos, and elephant shrews was (and is) sparse and uninformative. "Tentative phylogeny of Primates" from W. K. Gregory (1927). This phylogram approaches the modern concept of the duration of the Cenozoic, but underestimates the length of the Miocene and overestimates the length of the Oligocene. Gregory's views are strikingly modern considering the state of knowledge in the 1920s, although his branch times are too ancient in many cases. Anthropoidea is represented by a single clade with roots in the Early Eocene. The anthropoid sister taxon is a group consisting of omomyiforms and tarsiers. Galagos, lorises, and lemurs are shown to be descendants of European and North American adapiforms. Tree shrews are depicted as the sister taxon of euprimates, with plesiadapiforms as sister to tree shrews and euprimates The advent of molecular studies based on proteins and the genetic code has resolved many of the questions about the nearest relatives of primates and about primate cladogenesis. Zuckerkandl and Pauling (1965) noted that the pattern of branching of molecular phylogenetic trees of living taxa should be identifiable in terms of molecular information alone. In the ensuing years a vast new store of genetic data has accumulated that ratifies their observations and greatly clarifies the deeper branches of primate history (Esselstyn, Oliveros, Swanson, & Faircloth, 2017; Janecka et al., 2007; Kriegs et al., 2006; Mason et al., 2016; Perelman et al., 2011). From molecular studies, bats and elephant shrews were cast out of Archonta and a new taxon, Euarchonta, was erected to denote this more restrictive clade. More broadly, molecular genetic evidence indicates that euarchontans are related to the rodents and lagomorphs (rabbits, pikas, and hares) in the larger clade Euarchontoglires (also called Supraprimates by Kriegs et al., 2006). Current evidence supports a northern continental origin for Euarchontoglires; the clade links with the Laurasiatheria, including hedgehogs, moles, bats, even- and odd-toed ungulates, carnivorans, and pangolins. Much has been made of the biogeographic implications of this arrangement. For example, tree shrews and dermopterans are restricted to south Asia today, so it often is supposed that primates must have originated in that region. This interpretation may prove to be correct, but it is well to remember that we have more than 65 million years to play with and that early primates have been documented from Europe, North America, Asia, and the Indian subcontinent, and possibly Africa by the earliest Eocene. Plesiadapiforms also were widely dispersed and some of them may be dermopteran relatives (Ni, Hu, Wang, & Li, 2005). On the other hand, a laurasiathere root for Euprimates1, "primates of modern aspect" as Simons called them, all but rules out the possibility of primate origins from India as suggested by Kraus and Maas (1990). Since 1970 we have seen the gradual erosion and virtual extinction of the systematic concept 'Prosimii', a taxon proposed to include lemurs, lorises, tarsiers and, for some, the extinct plesiadapiforms. In its place, we recognize two extant groups, the Haplorhini for tarsiers and anthropoids and the Strepsirrhini for lemurs and lorises. This classificatory change embodies two factors. The first factor was a change in view about factors to consider in classification. Simpson (1945, 1959) and Mayr (1969), among others, subscribed to what was grandly called Evolutionary Systematics, a melding of phylogeny and evolutionary grades within classification. By these principles, primates were classified on the basis of a combination of phylogenetic relationship (shared descent from a last common ancestor), and also the degree of evolutionary change. For practitioners of evolutionary systematics, there were two kinds of primates—advanced Anthropoidea and "prosimians" that had not attained this simian grade (Simpson, 1959; Figure 3). By virtue of its supposed phenetic and behavioral resemblance to lemurs and lorises, Simpson (1945) placed south Asian Tarsius within Prosimii without regard to whether the genus is more closely related to anthropoids or to lemurs and lorises. As Simpson saw it, even if Tarsius proved to be the sister-group to anthropoids, it would still be acceptable to assign it to a paraphyletic taxon Prosimii (a group that does not include all the descendants of its last common ancestor). G. G. Simpson's concept of progressive evolutionary grades in primate evolution. In this view 'pongids' are accepted as paraphyletic, that is, African apes are more closely related to humans than are orangutans. Old and New World Anthropoidea (a and b in the figure) reached the 'simian' grade independently from separate Old and New World stocks of prosimian grade. (Redrawn and modified from Simpson 1959) In the 1970s, primatologists began to take a different view of systematics that embraced the views of Hennig (1966). For Hennigian, or phylogenetic systematics, the phylogeny dictates the classification and paraphyletic taxa are unacceptable. Paleoprimatologists were torn (and continue to be torn) between these currents. Many counted themselves as evolutionary systematists and eschewed a purely phylogenetic classification. The actual phylogenetic position of living, let alone fossil, taxa, they argued, was uncertain so the use of "wastebasket" paraphyletic taxa is a useful convenience. "Evolutionary" classifications also were viewed as being more stable—not needing to change with increased information about phylogeny. Furthermore, such classifications had the appeal of containing more phenetic information and for labelling adaptive shifts that seemed significant. Simpson suggested that the "simian" grade had been reached independently in the Old and New Worlds and placed the term "monkeys" in quotes as a "purely vernacular" term (p. 269 in Simpson, 1959)—so, Simpson might have said, "When I say monkey, you can picture a monkey in your mind's eye". That has nothing to do with whether that last common ancestor of monkeys looked like a modern monkey, or not. One can see the push and pull of these schools throughout in the 1970s. For example, Delson and Andrews (1975) in the same paper offer both a phylogenetic and an evolutionary classification of Old World monkeys. And, to this day, most practicing paleoprimatologists continue to recognize the a Paleocene-Eocene Plesiadapiformes, although many consider it to be a paraphyletic taxon. By 1980, phylogenetic classification had largely triumphed in zoology (Wiley 1981). It is now established that Tarsius is the sister taxon to Anthropoidea, a contention long ago made on the basis of placentation and adult anatomy (reviewed in Cartmill and Kay, 1978; Luckett, 1978). Nucleotide data (Perelman et al., 2011) and transpositions of Alu sequences (Schmitz, Ohme, & Zischler, 2001) unambiguously support the monophyly of the Haplorhini (Anthropoidea and Tarsius). Strepsirrhini (lemurs and lorises) is the sister group of Haplorhini. Until the 1970s, paleontologists and comparative anatomists were divided in their opinion about which living ape taxa were more closely related to humans. Gregory (1916) opined that we were more closely related to the African apes, as proposed by Huxley and Darwin (Darwin, 1871; Huxley, 1863). W. E. Le Gros Clark, H. F. Osborn (Clark, 1936; Osborn, 1927) and many others considered humans to be sister to all extant apes–orangutans, gorillas, and chimpanzees, and even gibbons. Still others argued for an even more separate ancestry, with monkeys or even tarsiers (Straus Jr, 1949; Wood Jones, 1916); for a detailed review, see Fleagle and Jungers (1982). Now it is well established from molecular genetic data that Homo is more closely related to African apes, and specifically to chimpanzees, than to orangutans, so the family Pongidae, sensu Simpson, is paraphyletic and no longer in questions about the phylogenetic of living primates are now the rise of phylogenetic systematics by Cartmill in this the of phylogenetic for the branching of evolutionary extinct taxa, in rare the only information is is no or genetic consisting of a number of taxa, and the evolutionary tree is considered to be the one that the of evolution and evolutionary this is the of A number of phylogenetic such as or & use to the tree other paleontologists to the most as the phylogeny. For the a to phylogenetic is how to with between trees that include living taxa when the have tree based on molecular genetic In my now that the phylogeny of living primates is so well phylogenetic that extinct and living primates should be so as to be with molecular & For example, there is evidence that is a monophyletic group and that a among living primates between Haplorhini and should us to some phylogenetic for extinct primate taxa. of the position of the phylogenetic among and the has little to questions about the of fossil taxa living or to Euarchonta, it is now that Plesiadapiformes is a paraphyletic But the & that tree shrews are sister to flying lemurs to the of primates is based on molecular But some recognize one family as a sister taxon to with dermopterans in turn sister to et al., have argued that other or a are more closely related to primates & et al., 2007; & 2017). of these is with the concept of so the established phylogeny of living primates, and be to out which among these is more of the would to out some Eocene primates. For example, it is not to the phylogeny proposed by and one Eocene rise to anthropoids and is related to tarsiers independently from an Eocene of this that now is one first proposed by and by (1975) tarsiers from a ancestor of a group including lorises, lemurs, and of the does little to the question of where tarsiers and anthropoids with to Eocene omomyiforms & Kay, & One view is that tarsiers arose from one or the paraphyletic with to & & Szalay, This the clade as the sister group of earliest Anthropoidea 2006; et al., is that Tarsius and Anthropoidea a common that is sister to omomyiforms as a whole & Kay, 1978; of these scenarios is with trees because a sister relationship to the of is for the clade to the of omomyiforms (Figure because such a tree is with the two most and adult that Tarsius and Anthropoidea to the of both have a to separation of the from the by a of the from the of the to a with the both have a separate of the called the from the to a through which the & an also the need to that omomyiforms a that they had lost the a and in the and lost a the of the also that they had a and in the and were to C. some or all these in parallel in tarsiers and A of phylogeny with Eocene Paleocene through Miocene are time is not to represent Haplorhini and may or may not be are et Fossil remains often are How can we be of the phylogenetic of a fossil taxon known from just a single or a few teeth, when we that of are very in all & and have this by in a of primates. that phylogenetic including taxa with of data are to resolved trees by these taxa is a fossil taxon can an resolved tree, a number of to of its that even with to a phylogeny may be resolved but that is the of a taxon may be very different from a phylogeny when more data is they that data by only one it or to than that several data it is to bear in that phylogenetic of extinct taxa should be viewed with when based on just a few may give a highly resolved but the should be of the of remains to a taxon, when the is because the proposed phylogeny is and or because the evolution in a E. D. the of an Early Eocene found with the of an and proposed the for this of mammals There are several modern of evolution as a of the of the of different taxa. of of taxa can be found among Eocene It is now accepted that the of Asia and Africa are Anthropoidea (Figure An and from the Eocene has been to an anthropoid & this to be correct, the of the anatomy of Tarsius and Anthropoidea would of have in a more is that this more to a which it more of contention is a from the Eocene of and this to as many is an the could evidence that had not in the of this to or even to primates et al., 2005). In we have no published of that, with critical aspects of its or the advent of molecular genetic we have a understanding of the phylogeny of primates and their living evidence also for the of This in turn has greatly the way paleoprimatologists evaluate the fossil record. In a Zuckerkandl and Pauling (1965) noted that evolutionary change in sequences (and the genetic should be to evolutionary time because most such have little or no on the of that may thus a molecular evolutionary for evolution. Zuckerkandl and Pauling recognized that the of evolutionary at the molecular must be with to the fossil record. and proposed such a based on an Old World at about 30 and that humans and African apes a common ancestor million years ago. this has since been many it a to the that paleoprimatologists could not see this in the scientific from about 1970 were with by many who argued that the of molecular evolution must be and that the African in was to a noted a of the of evolution of proteins in the human This also was established for the genetic for regions that do not code for studies the of Huxley, and Gregory that African apes and humans a with more a view that is now accepted among paleontologists. But when did the between humans and African Until the molecular paleoprimatologists were about how far in time to the or of the other branch times for primate or human evolution. Simpson the African to the and the Old World to Eocene Figure the African at between and He separate species of African as the of and at about Simons

Biology · Evolutionary biology · Geography · Primate · Zoology · Bat Biology and Ecology Studies · Evolution and Paleontology Studies · Paleontology · Primate Behavior and Ecology

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• Significance of primate petrosal from Middle Eocene fissure-fillings at Shanghuang, Jiangsu Province, People's Republic of China

  Open Access•R D E Macphee, K Christopher Beard et al.•Journal of Human Evolution•1995

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• Cranial morphology and adaptations of Palaechthon nacimienti and other paromomyidae (Plesiadapoidea, ? primates), with a description of a new genus and species

  Open Access•R F Kay, M Cartmill•Journal of Human Evolution•1977

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• Primates

  Open Access•J Wible, John R Wible et al.•Journal of Human Evolution•1987

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• The eosimiid primates (Anthropoidea) of the Heti Formation, Yuanqu Basin, Shanxi and Henan Provinces, People's Republic of China

  Open Access•K Christopher Beard, Jingwen Wang•Journal of Human Evolution•2004

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• Molecular systematics of the New World monkeys

  Open Access•John E Cronin, Vincent M Sarich•Journal of Human Evolution•1975

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• A comparative analysis of infraorbital foramen size in Paleogene euarchontans

  Open Access•Magdalena N Muchlinski, E Christopher Kirk•Journal of Human Evolution•2017

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• A comparative analysis of vibrissa count and infraorbital foramen area in primates and other mammals

  Open Access•Magdalena N Muchlinski•Journal of Human Evolution•2010

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• New postcranial elements for the earliest Eocene fossil primate Teilhardina belgica

  Open Access•Daniel L Gebo, Thierry Smith et al.•Journal of Human Evolution•2012

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• Endocasts of Microsyops (Microsyopidae, Primates) and the evolution of the brain in primitive primates

  Open Access•M T Silcox, Ashleigh E Benham et al.•Journal of Human Evolution•2010

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• Estimating body size in early primates

  Open Access•Marian Dagosto, Daniel L Gebo et al.•Journal of Human Evolution•2018

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• Osteological evidence for the evolution of activity pattern and visual acuity in primates

  Open Access•R F Kay, E Christopher Kirk•American Journal of Physical…•2000

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• Phylogénie des Primates. Confrontation des résultats obtenus par les diverses voies d'approche du problème

  Open Access•Robert Hoffstetter•Cahiers du Centre de recherches…•1977

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• Ecology and Behaviour of Nocturnal Primates

  Claud A Bramblett, Pierre Charles-Dominique et al.•Man•1978

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• Towards a New Definition of Primates

  R D Martins, R D Martin•Man•1968

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• Evolutionary Parallelism and Human Phylogeny

  W E Le Gros Clark•Man•1936

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• Recent Finds and Interpretations of Miocene Hominoids

  David Pilbeam•Annual Review of Anthropology•1979

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