Fossil Friday: Purgatorius and the Abrupt Origin of Primates
With this Fossil Friday I introduce a new series of articles on the age of origin of the various modern placental mammal orders. These orders are the higher categories of mammal systematics, which for example include groups like bats, rodents, primates, carnivores, sirenians, elephants, even-toed and odd-toed ungulates etc. This issue is quite relevant and important for ID research as it establishes the abrupt origin of all these groups after the end-Cretaceous mass extinction caused by the global consequences of the Chicxulub impact. This week we open this series with the origin of our very own order, Primates.
The bestselling author Stephen Baxter (2003) mused in the first chapter of his novel Evolution what it was like to be “Purga,” one of our remote primate ancestors, on the last day of the reign of the dinosaurs before the asteroid impact. He might have borrowed this idea from the popular Disney animation movie Dinosaur (2000), which featured lemur-like primates frolicking with dinosaurs until the cosmic cataclysm ended the edenic scenery. Is this just Hollywood fantasy like The Flintstones or is there some scientific support for such a picture? Indeed, a recent study was celebrated in media reports as suggesting that primates walked with dinosaurs (McKeever 2021). Let’s have a look.
The fossils featured today are dental remains of the small Paleogene mammal Purgatorius, which is generally considered to belong to the extinct order Plesiadapiformes as their oldest and most primitive representative (Van Valen 1994, Clemens 2004, Rose 2006, Fox & Scott 2011). Plesiadapiformes is a very diverse group of more than 140 named species in 50 genera and 11-12 families that lived between the Early Paleocene and the Late Eocene (Fleagle 2013, Silcox 2014, Silcox et al. 2017). The genus Purgatorius was named after the Purgatory Hill locality in Montana, where it was first described by Van Valen & Sloan (1965), but has also been found in contemporaneous outcrops in Canada (Fox & Scott 2011). Most of the finds are of Puercan age, which is a brief period of only 1 million years in the Early Paleocene (Lofgren et al. 2004). These tiny animals presumably resembled squirrels and certainly had an arboreal way of life (Fiegl 2012, Chester et al. 2015, 2017, 2019). They are not yet known by skeleton finds, but only by isolated teeth or jaw fragments and some ankle bones, which is a very common situation in fossil mammals and paleoanthropology.
A Single Tooth
Anyway, until recently the oldest fossil record of Purgatorius was from the Early Paleocene of North America. A single tooth from the Hell Creek Formation of Harbicht Hill in Montana was initially considered to be of Late Cretaceous origin, because it was found in the same deposit as Triceratops dinosaur remains (Van Valen & Sloan 1965), but it was later shown to be a Tertiary intrusion in sediments of mixed origin (Clemens 2004). Other alleged Cretaceous material from the Ravenscrag Formation and the Bug Creek Group was later re-dated to a Paleocene age (Lillegraven et al. 1979). Unfortunately, the long-refuted attribution to the Late Cetaceous is still considered in some more recent textbooks such as Fleagle (2013: 215).
Clemens (2004) suggested that the abundance of Purgatorius in the Puercan 2-3 of Montana is explained by a dispersal in the Early Paleocene 64.75-64.11 million years ago. Chester et al. (2015) described tarsal bones attributed to Purgatorius from late Puercan (65 mya) of Montana. A new study by Wilson Mantilla et al. (2021) described three new species of Purgatorius from this locality, which has been more precisely dated to be of Earliest Paleocene age (65.921 mya), only about 105-139 thousand years after the K/Pg boundary (McKeever 2021, Sanders 2021). They concluded that these oldest plesiadapiform fossils suggest “purgatoriids and, by extension, Pan-Primates, Euarchonta and Placentalia probably originated by the Late Cretaceous”. That would be a reasonable conclusion if and only if Purgatorius was a placental mammal and a stem primate. So, was the Disney movie history after all? Sorry, to disappoint any fans, but here comes the fly in the ointment.
Initially, plesiadapiforms and Purgatorius were indeed considered as early primates (Van Valen & Sloan 1965, Clemens 1974, Szalay & Delson 1979, Van Valen 1994), mainly because they share certain arboreal adaptations (e.g., long fingers) and some similar dental features, even though they differed by still having claws and smaller lateral orbits. Furthermore, the dental similarities are only developed in some plesiadapiforms (Rose et al. 1994), so that Rose (1994) already found that “the detailed dental similarity must be convergent” and generally concluded:
“Although considerable evidence has been adduced to ally plesiadapiforms with primates, adapids with strepsirhines and anthropoids, and omomyids with tarsiers and haplorhines,much of it is based on either superficial resemblance, symplesiomorphy, or obvious convergence. Controversy persists because compelling evidence of these relationships, in the form of clear and significant synapomorphies, is still wanting. Opinion is increasing that Plesiadapiformes are not primates …”
Of course, a potential solution for this incongruent distribution of similarities could be a nesting of primates within a paraphyletic plesiadapiform grade, so that some plesiadapiform genera could be more closely related to primates than others. This was indeed suggested by the study of Bloch et al. (2007). However, as emphasized by Fleagle (2013):
“The scenario in which crown primates are nested within plesiadapiforms as the sister taxon of plesiadapoids involves an evolutionary reversal of increasing tooth number and dramatic reduction of the procumbent incisors that seems unlikely” (also see Godinot 2017).
Concerning the arboreal adaptations, more recent studies of living tree shrews have suggested that these adaptations already belonged to the archontan ground plan (Archonta are the supposed clade including tree shews, colugos, and primates) and thus do not suggest a uniquely primate relationship (Godinot 2017).
Considerable Scientific Debate
The significance of the similarities between plesiadapiforms and primates is still matter of considerable scientific debate to this day (e.g., Silcox et al. 2007 vs Soligo & Martin 2007). Already in the 1960-80s the consensus more and more shifted towards an exclusion of Plesiadapiformes from the order Primates (Hartwig 2002, Fleagle 2013). Rose (2006) is one of the few exceptions and tentatively accepted plesiadapiforms as early stem primates, but he readily admitted that:
“Plesiadapiforms, as well as euprimates, are usually traced back to early Paleocene Purgatorius, although no transitional forms leading to euprimates have been identified, and the source of Purgatorius itself is completely unknown …”
Most other authors instead considered Plesiadapiformes to be a separate order, which is either the sister group of primates, or of colugos (Dermoptera), or of both together (Kemp 2005, Silcox 2014, Godinot 2017). Kay et al. (1992) proposed a close relationship of plesiadapiforms and colugos based on the shared reduction of the internal carotid artery. Beard (1990, 1993) and McKenna & Bell (1997) came to the same result based on skeletal characters, and a recent cladistic analysis confirmed this position (Morse et al. 2019). Kemp (2005) commented in his standard textbook on The Origin & Evolution of Mammals that “Purgatorius is the earliest member of a diverse group, Plesiadapiformes, which many authors no longer believe to be primates”. But then a cladistic study by Bloch et al. (2007, 2016; also see Silcox 2001 and Bloch & Silcox 2006) indeed placed plesiadapiforms with primates and found no evidence supporting a dermopteran relationship. They suggested a divergence of the two groups about 62 million years ago, which they admitted to imply an euprimate ghost lineage of 7 million years. Chester et al. (2015, 2017, 2019) also recovered Purgatorius and other plesiadapiforms as early stem group representatives of primates. On the other hand, Ni et al. (2013, 2016) recovered plesiadapiforms as sister group to Dermoptera+Primates in their parsimony analyses and concluded “Plesiadapiforms, traditionally regarded as archaic primates, are not even stem primates, corroborating the now common practice of excluding plesiadapiforms from the order Primates”. Therefore, Godinot (2017) agreed with Hartwig (2002) that “there is no clear evidence that Plesiadapiformes are the closest sister group of the Euprimates”. In the most recent phylogenetic study, Plesiadapiformes and Purgatoriidae have been corroborated to be successively more closely related to a clade of colugos and primates (Seiffert et al. 2020), and excluded from the primate lineage with a 100% posterior probability in the Bayesian analysis.
Thus, neither plesiadapiforms in general nor Purgatorius in particular can be considered to be well-established fossil stem primates. The weak indirect evidence for a contemporaneous occurrence of early primates and dinosaurs evaporates into a mist of evolutionist speculation and storytelling. That said: in theory the evidence indeed could be consistent with the possibility that primate-like ancestors of the clade of colugos and primates may have lived in the latest stages of the age of dinosaurs close to the extinction event. Of course, there exists not a single fossil to empirically prove this speculation.
But It Gets Worse
There is substantial anatomical evidence, which suggests that Purgatoriusnot only has to be excluded from primates, but may not even be a crown group placental mammal. It rather seems to be related to enigmatic stem mammals like the “condylarthran” genus Protungulatum (Wible et al. 2007, 2009, Goswami et al. 2011, Halliday et al. 2015; also see Davies et al. 2017). Chester et al. (2015) mentioned this fact, but considered it as an artifact of taxon sampling, because their data matrix supported a primate affinity within placental mammals. This was mainly based on the ankle characters, so that it is surprising that the authors do not even consider the possibility that similarities between Purgatorius and primates could rather be based on convergent adaptation to an arboreal life. Apparently, Chester et al. were unaware of the concurrent study by Halliday et al. (2015), which they do not cite, while the Halliday paper included the evidence from Chester’s. The studies by Chester et al. (2017, 2019) and by Wilson Mantilla et al. (2021) can hardly be given such a benefit of doubt when they simply ignored this conflicting evidence and the crucial study by Halliday et al., which arguably represents the most important work on the affinities of enigmatic Paleocene mammals. Finding Purgatorius outside of placental mammals would have rendered Wilson Mantilla’s sensational story dead in the water. Honi soit qui mal y pense.
Since neither Purgatorius nor Plesiadapiformes seem to be stem primates, all the other plesiadapiform taxa, which have been suggested by some authors to rank among the earliest primates, also have to rejected as such. These include for example the following Paleocene genera and species (age ranges are based on the PaleoDB database at fossilworks.org):
- Carpolestes twelvemilensis (61.7-56.8 mya)
- Draconotus apertus (63.3-61.7 mya)
- Dryomomys millenius (61.7-56.8 mya)
- Elphidotarsius florencae (63.3-61.7 mya)
- Micromomys silvercouleei (61.7-50.3 mya)
- Nannodectes spp. (61.7-56.8 mya)
- Pronothodectes jepi and P. matthewi (63.3-61.7 mya)
- Russellodon haininense (66.043-61.7 mya) (De Bast & Smith 2017)
- Saxonella crepaturae (61.7-58.7 mya)
- Torrejonia wilsoni 62.4 mya (Chester et al. 2019)
But even if these plesiadapiform taxa would represent early stem primates, what they likely do not (see above), they would only document the presence of primates in the Early Paleocene but not prior to the dinosaur mass extinction at the K/Pg-boundary.
If Purgatorius and plesiadapiforms are not the earliest fossil primates, then which are the better supported candidates for this illustrious position?
A few teeth and a mandible fragment from the Late Paleocene of Morocco (about 58.7-55.8 mya) were described by Sigé et al. (1990) as Altiatlasius koulchii. The describers and many subsequent authors considered it to be the earliest euprimate (e.g., Rose 1994, Tabuce et al. 2004, Bloch et al. 2007, Godinot 2017), and maybe an omomyoid stem tarsier (Fleagle 2013) or even a simian (Godinot 1994, Beard 2006, Bajpai et al. 2008). However, the precise systematic position of Altiatlasius is very much disputed (Hartwig 2002, Fleagle 2013). Even a plesiadapiform relationship has been suggested (Hooker et al. 1999, Hartwig 2002) and has recently been supported by a cladistic analysis (Morse et al. 2019), which recovered Altiatlasius in a non-primate clade of plesiadapiforms and colugos. As in so many cases, phylogenetics proves to be a kind of guessing game rather than a hard science.
Another very old group of primitive primates is the extinct order Adapiformes (not to be confused with Plesiadapiformes), which is believed to be closer related to lemurs than to simians. Some of the oldest representatives are the notharctid genera Notharctus (50.3-40.4 mya), Cantius (50.3-40.4 mya), and Donrussellia (55.8-48.6 mya) from the Early Eocene of North America and France (Gingerich 1986, Hartwig 2002), as well as Marcgodinotius from the early Eocene (52 mya) of India (Bajpai et al. 2005). These early adapiforms arguably establish an appearance of the lemur lineage 55 million years ago.
Archicebus achilles was described by Ni et al. (2013) from the Earliest Eocene (55.8-54.8 mya) of Hubei Province in central China. It is a relatively complete skeleton of a tiny, tree-dwelling mammal, which was even smaller than the smallest living primate but had a monkey-like habitus and anatomy (Chen 2013, ESRF 2013). It is 7 million years older than all previously found early primate skeletons and was thoroughly studied with X-ray synchrotron tomography. It proved to be a very basal tarsiiform and thus closer related to simians (“monkeys”) than to lemurs (Fleagle 2013).
The Eocene genus Teilhardina, which is known in several different species from North America and Eurasia, belongs to the extinct family Omomyidae (Bown 1976, Rose 2006). This family is generally attributed to the stem group of tarsiers (Kemp 2005), or more rarely to the common stem of tarsiers and simians (Fleagle 2013, Morse et al. 2019) suggesting an Asian origin of this clade (Ni et al. 2005). These animals may have looked similar to modern bush babies. Teilhardina asiatica was discovered in the Hunan Province of China and dated to an earliest Eocene age of 54.97 mya (Ni et al. 2004). The fossil is a partial skull with complete dentition, which is a quite rare condition for early primate fossils. The closely related species Teilhardina magnoliana was found in the earliest Eocene Tuscahoma Formation from the Gulf Coastal Plain in North America (Beard 2008). Beard suggested that its ancestor must have crossed the land bridge connecting Siberia and Alaska more than 55.8 million years ago (Nickerson 2008), thus somewhat earlier than believed before (Smithet al. 2006). New material of Teilhardina brandti, which was originally described by Gingerich (1993) as oldest omomyid from North America, also is about 55.8 million years old (Rose et al. 2011, Boyer et al. 2018, Hoose 2018).
Teilhardina and Archicebus are the oldest well-dated primates, while all others suffer from a relatively wide range of uncertainty in their dating that could make them significantly younger. Therefore, these two genera are best supported by the evidence as earliest fossil record of primates and also establish the presence of the tarsier lineage about 55 million years ago.
Some of those other contenders, especially for the position as oldest simians (Anthropoidea), are the following:
The genera Azibius and Algeripithecus are small-bodied primates from the Eocene of Algeria. Azibius was originally described by Sudre (1975) and considered to be a plesiadapiform (“paromomyid”), then recognized as adapid primate by Gingerich (1976), again transferred to Plesiadapiformes by Tabuce et al. (2004) which was questioned by Rose (2006), and yet again reinstated as a lemuriform primate (Tabuce et al. 2009). Algeripithecuswas originally described by Godinot & Mahboubi (1992) as earliest known simian, which was widely accepted and considered as support for an African origin of simians (Godinot 1994). A more recent evaluation of the age of the Glib Zegdou Formation in Algeria suggests an early-middle Eocene age of 49-45 mya (Coster et al. 2012), which would support this ranking. Rose (2006) commented on Algeripithecus that its “significance will remain moot until more complete evidence is found.” A few years later, Marivaux et al. (2011) studied the talus bone and concluded that Algeripithecus is not a simian at all but was closely related to Azibius and belonged to the strepsirrhine branch of primates that also includes lemurs. This was strongly confirmed by the more recent study of Tabuce et al. (2009), who also emphasized that this strongly challenges the role of Africa as the ancestral homeland for simians.
This fate reminds of the case of Darwinius marsillae from the Middle Eocene oil shale (ca. 47 mya according to Franzen 2005) of the Messel pit in Germany. See my previous Fossil Friday article (Bechly 2022) about the remarkable story of this fossil, which was nick-named “Ida” and heavily overhyped as one of the oldest simian fossils and an important “missing link”, only to be quickly revealed to be just another early relative of lemurs.
The currently oldest fossil record of simians is the extinct family Eosimiidae. It is based on the fragmentary remains of Eosimias sinensis described by Beard et al. (1994) from the Middle Eocene of Jiangsu Province in China. Its age was estimated to be about 45 mya, but unfortunately no radiometric dates were available for this deposit. The simian and even primate affinity of Eosimias was disputed by several other experts (e.g., Godinot 1994), but the discovery of better material of a second species of Eosimias confirmed its simian relationship (Beard et al. 1996, Beard & Wang 2004, Rose 2006), even though some experts still remain skeptical (e.g., Godinot 2017). The Middle Eocene age would be about 10 million years younger than the oldest fossil record for stem tarsiers. Evolutionists had to explain away this inconsistency with a so-called ghost lineage of undocumented existence, because simians of course have to be as old as their sister group tarsiers. However, unlike many other such cases this particular problem got solved by the new discovery of another genus of Eosimiidae, which was described as Anthrasimias by Bajpai et al. (2008) from the Early Eocene (55-54 mya) of India. Thus the first appearance of the simian lineage can also be dated to about 55 million years ago.
An Abrupt Appearance
We can conclude from all the mentioned up-to-date research that the placental mammal order of Primates appears abruptly in the fossil record of the Eocene about 55-56 million years ago, during the hothouse climate of the Paleocene-Eocene Thermal Maximum (PETM). This just confirms what Rose (1994) already had concluded in his seminal review article on the earliest primates:
“Undisputed primates appear suddenly in the Holarctic fossil record at the beginning of the Eocene, approximately 55 million years ago.”
The fact that almost thirty years of great progress in primate paleontology did not change this result provides some confidence that it is not an artifact of an incomplete fossil record, but is here to stay. Primates not only appeared suddenly, but their different subgroups of lemurs, tarsier, and simians all appeared at about the same time. Primates never shared the planet with dinosaurs, even if Hollywood and National Geographic want to sell you a different story. We will see in subsequent articles in this series that such an abrupt appearance in a narrow window of time of the Paleogene period represents a consistent pattern found in all the placental mammal orders. Such a saltational pattern contradicts Darwinian gradualist expectations and is better explained by pulses of new information infused into the system. Even Darwinists implicitly admit this when they say that “eutherians experienced elevated evolutionary rates in the immediate aftermath of the Cretaceous–Palaeogene mass extinction” (Halliday et al. 2016). Unlike ID theorists, they have no plausible explanation of how a meteorite impact produced the genetic information for an explosive diversification of placental mammals.
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