100 years of primate paleontology.

Abstract

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 use. These major questions about the phylogenetic relationships of living primates are now resolved. With the rise of phylogenetic systematics (see Centennial Perspective by Cartmill in this issue) came the development of phylogenetic methods for reconstructing the branching of evolutionary trees. With extinct taxa, except in rare cases, the only information available is morphological—there is generally no soft-tissue or genetic information. Using morphological datasets, consisting of a number of taxa, characters, and character states, the “best” evolutionary tree is considered to be the one that minimizes the amount of homoplasy (convergent evolution and evolutionary reversals); this is the criterion of maximum parsimony. A number of phylogenetic analysis programs, such as PAUP (Swofford, 2002) or TNT (Goloboff, Farris, & Nixon, 2008), use algorithms to find the shortest possible tree length. Without other compelling reasons (see below), paleontologists tend to accept the most parsimonious as the likeliest phylogeny. For the paleontologist, a commonly encountered challenge to phylogenetic analysis is how to deal with incongruence between morphology-based trees that include living taxa when the latter have well-corroborated tree topologies based on molecular genetic data. In my view, now that the phylogeny of living primates is so well understood, any phylogenetic analysis that includes extinct and living primates should be constrained so as to be consistent with molecular phylogenies (Springer, Teeling, Madsen, Stanhope, & de Jong, 2001). For example, there is strong evidence that Primatomorpha (Dermoptera + Primates) is a monophyletic group and that a dichotomy exists among living primates between Haplorhini and Strepsirrhini. These well-established relationships should allow us to reject some phylogenetic proposals for extinct primate taxa. Even so, resolution of the position of the phylogenetic relationships among Euarchonta and the H-S dichotomy has done little to resolve questions about the relationships of fossil taxa vis-à-vis living Primatomorpha, or haplorhines. With respect to Euarchonta, it is now apparent that Plesiadapiformes is a paraphyletic group. But the further claim (Bloch, Silcox, Boyer, & Sargis, 2007) that tree shrews are sister to flying lemurs to the exclusion of primates is doubtful based on molecular evidence. But uncertainty remains: some workers recognize one plesiadapiform family (Microsyopidae) as a sister taxon to Dermoptera with dermopterans in turn sister to Primates (Ni et al., 2013). Others have argued that other plesiadapiform families (Carpolestidae or a carpolestid-plesiadapid clade) are more closely related to primates (Bloch & Boyer, 2002; Bloch et al., 2007; Silcox, Bloch, Boyer, Chester, & López-Torres, 2017). Either of these interpretations is consistent with the concept of Primatomorpha, so the established phylogeny of living primates, Dermoptera and Scandentia cannot be invoked to sort out which among these interpretations is more accurate. Practically universal acceptance of the haplorhine-strepsirrhine dichotomy would appear to rule out some hypotheses concerning Eocene primates. For example, it is not plausible to accept the phylogeny proposed by Franzen and colleagues (2009) wherein one Eocene adapiform Darwinius gave rise to anthropoids and another adapiform is related to crown strepsirrhines, whilst tarsiers evolved independently from an Eocene omomyiform. Another version of this hypothesis that now appears untenable is one first proposed by Gidley (1923) and resurrected by Gingerich (1975) wherein tarsiers evolved from a plesiadapiform ancestor independent of a second group including lorises, lemurs, and anthropoids. Even so, acceptance of the haplorhine-strepsirrhine dichotomy does little to resolve the question of where tarsiers and anthropoids fit phylogenetically with respect to Eocene omomyiforms (Godinot, 2015; Seiffert, Perry, Simons, & Boyer, 2009; Williams, Kay, Kirk, & Ross, 2010b). One commonly held view is that tarsiers arose from one or another omomyiform (making the latter paraphyletic with respect to Tarsiidae; Beard, Krishtalka, & Stucky, 1991; Rosenberger & Szalay, 1980). This hypothesis represents the tarsier-omomyiform clade as the sister group of earliest Anthropoidea (Eosimiidae; Beard, 2006; Ni et al., 2013). Another proposal is that Tarsius and Anthropoidea share a common stem that is sister to omomyiforms as a whole (Cartmill & Kay, 1978; Hoffstetter, 1977). Each of these scenarios is consistent with molecular-genetic trees because each upholds a tarsier-anthropoid sister relationship to the exclusion of strepsirrhines. My own preference is for the tarsier-anthropoid clade to the exclusion of omomyiforms (Figure 4) because such a tree is consistent with the two most salient, unique, and fossilizable adult cranial characteristics that Tarsius and Anthropoidea share to the exclusion of omomyiforms: both have a partial to complete bony separation of the orbital cavity from the temporal fossa by a process of the alisphenoid bone projecting from the lateral wall of the braincase to form a sutural contact with the frontal bone. And both have a separate part of the air-filled middle ear cavity called the anterior accessory chamber projecting from the auditory canal to produce a bony partition through which the internal carotid artery passes (MacPhee & Cartmill, 1986). Such an arrangement also obviates the need to assume that omomyiforms possessed a hemochorial placenta, that they had lost the a rhinarium and cleft in the upper lip, and lost a tapetum lucidum behind the neural layer of the retina. His scenario also suggests that they had acquired a fovea and macula in the retina, and were unable to synthesize vitamin C. Otherwise, some or all these characteristics evolved in parallel in tarsiers and anthropoids. A dendrogram of euprimate phylogeny with Eocene families represented. Paleocene through Miocene are proportionally scaled; post-Miocene time is not to scale. Three crowns represent crown Primates (=Euprimates), crown Haplorhini and crown Strepsirrhini. Adapiformes may or may not be paraphyletic; Omomyiformes are monophyletic. Redrawn after Williams et al. (2010a) Fossil remains often are fragmentary. How confident can we be of the phylogenetic placement of a fossil taxon known from just a single tooth, or a few teeth, especially when we know that levels of homoplasy are very high in all anatomical systems (Sanchez-Villagra & Williams, 1998). Pattinson and colleagues (2014) have examined this problem by simulating missing characters in a large character-taxon matrix of primates. They report that phylogenetic analyses including taxa with large amounts of missing data are prone to poorly resolved consensus trees caused by these taxa exhibiting “wildcard” behavior—that is a fossil taxon can “float” across an otherwise resolved tree, finding a number of parsimonious placements owing to poor sampling of its total morphology. They report that even with up to 30% complete morphological data, a phylogeny may be fully resolved but incorrect, that is the placement of a fragmentary taxon may be very different from a revised phylogeny when more data is available. Especially, they report that morphological data dominated by only one morphological partition (be it dental, cranial, or postcranial) tends to perform worse than simulations that sample several data partitions. Thus, it is important to bear in mind that phylogenetic interpretations of extinct taxa should always be viewed with caution when based on just a few characters—the characters may give a highly resolved but incorrect placement. Finally comes the ‘hopeful monster’ problem. We should be skeptical of the allocation of fragmentary remains to a particular taxon, especially when the resulting reinterpretation is profound—either because the proposed phylogeny is radically and incorrectly altered or because the difference implies unexpected convergent evolution in a major anatomical system. E. D. Cope incorrectly associated the foot bones of an Early Eocene ungulate found together with the teeth of Pelycodus, an adapiform primate, and proposed the order Mesodonta for this mixture of mammals (Cope, 1885; Wortman, 1903). There are several modern examples of possible unexpected convergent evolution as a consequence of the possible admixture of the bones of different taxa. Examples of potential mixing of higher-level taxa can be found among Eocene haplorhines. It is now generally accepted that the Eosimiidae of Asia and Africa are stem Anthropoidea (Figure 4). An isolated and unassociated petrosal bone from the Chinese Middle Eocene has been assigned (with query) to an eosimiid anthropoid (MacPhee, Beard, & Qi, 1995). If this assignment proves to be correct, the similarities of the otic anatomy of Tarsius and Anthropoidea would of necessity have evolved in parallel. To me, a more plausible scenario is that this isolated bone more likely belongs to a small omomyiform species, which it more nearly resembles. Another bone of contention is a frontal bone fragment from the Middle Eocene of Burma. Takai, Shigahara, Egi, and Tsubamoto (2003) assigned this frontal to Amphipithecus. If, as many believe, Amphipithecus is an anthropoid, the bone could provide evidence that postorbital closure had not occurred in stem anthropoids. Others challenge the attribution of this bone to Amphipithecus, or even to primates (Beard et al., 2005). In fact, we have no published material of any eosimiid that, with certainty, documents critical aspects of its ear region or orbit. With the advent of molecular genetic evidence, we have gained a much better understanding of the phylogeny of primates and their living relatives. Molecular evidence also holds promise for estimating the timing of cladogenic events. This in turn has greatly constrained the way paleoprimatologists evaluate the fossil record. In a revolutionary article, Zuckerkandl and Pauling (1965) noted that evolutionary change in amino acid sequences (and the underlying genetic code) should be approximately proportional to evolutionary time because most such changes have little or no effect on the functional properties of proteins, concluding that ‘There may thus exist a molecular evolutionary clock’ (page 148) for evolution. Zuckerkandl and Pauling recognized that the rate of evolutionary transformations at the molecular level must be calibrated with reference to the fossil record. Sarich and Wilson (1967) proposed such a calibration based on an Old World monkey-hominoid split at about 30 Ma, and concluded that humans and African apes shared a common ancestor 5 million years ago. Whilst this calibration has since been revised many times, it gave a shock to the system that paleoprimatologists could not ignore. We see this effect in the scientific literature from about 1970 onwards. Molecular clocks initially were met with resistance by many biologists who argued that the rate of molecular evolution must be actually quite variable and that the African ape-human clade, in particular, was subject to a slowdown. Morris Goodman (1981) noted a ‘profound deceleration’ of the rate of evolution of proteins in the human lineage. This slowdown also was established for the underlying genetic code, especially for regions that do not code for amino acids. Goodman's immunochemistry studies (Goodman, 1962, 1963) confirmed the hypotheses of Huxley, Darwin, and Gregory that African apes and humans form a clade, with orangutans more distantly related, a view that is now generally accepted among paleontologists. But when did the split occur between humans and African apes? Until the molecular revolution, paleoprimatologists were unconstrained about how far back in time to project the human-ape split or any of the other branch times for primate or human evolution. Simpson (1959) dated the African ape-human split to the Late Oligocene (∼32 Ma) and the Old World monkey-ape split to Middle Eocene (∼45 Ma; Figure 5). Pilbeam (1967) agreed, placing the African ape-human split at between 20 and 35 Ma. He further identified separate species of African Proconsul as the ancestors of gorilla and chimpanzee at about 17 Ma. Simons (1967) iden