This Fossil Friday we continue our series on the origins of the various modern placental mammal orders. Our subject today is the order Xenarthra, which is endemic to the New World and divided into two suborders, Cingulata (armadillos and their extinct relatives, pampatheres and glyptodonts) and Pilosa (sloths and anteaters). Both suborders include examples of the extinct Pleistocene megafauna of America. The suborder Pilosa includes the giant ground sloths like the genus Megatherium, and the suborder Cingulata includes the car-sized giant armadillos of the genus Glyptodon. A complete skeleton of Glyptodon asper from the Pleistocene of Argentina is featured at the top of this article. This specimen is on display at the Natural History Museum of Vienna. With an age of only 1.5 million years it is very far from being the oldest representative of this order. A recent review article by De Iuliis (2018) commented that:
These three clades and the differences between them reflect deep histories that reach back at least to the Paleocene and their fossil representation (of cingulates and sloths, but not vermilinguans) is exceptionally rich and diverse.
So, let’s have a closer look at this rich fossil record of xenarthrans. Gaudin & Croft (2015: fig. 2) provided an excellent chart of the stratigraphic distribution of the xenarthran fossil record with up-to-date chronology of the South American Land Mammal Ages (SALMA). They concluded that ”no unambiguous Paleocene fossil xenarthran remains are known.” So what are the oldest fossils of this order known to science?
Oldest Fossils of the Order
The more primitive suborder Cingulata is indeed well represented in the Paleogene fossil record from South America (Sedor et al. 2022). The earliest known cingulate and arguably oldest fossil representative of Xenarthra is Riostegotherium yanei described by Oliveira & Bergqvist (1998) from the Itaboraí Basin in the Rio de Janeiro State of Brazil. This material was already mentioned by Scillato-Yané (1976) under the incorrect name Prostegotherium. There is also at least one other undescribed armadillo-like species from this locality and age (Cifelli 1983, Bergqvist et al. 2004, 2019). A thorough revision of the material by Bergqvist et al. (2004) confirmed the identification of Riostegotherium as oldest xenarthran remains (also see Bergqvist et al. 2019).
Interestingly, Bergqvist et al. (2004) also observed that “shared similarities to palaeanodonts add support to the proposal that Palaeanodonta may have been ancestral to, or is the sister-taxon of Xenarthra”. Why is this interesting? Because the extinct Palaeanodonta a few years later turned out to be stem pangolins and both turned out to not be related to Xenarthra at all (e.g., Gaudin et al. 2009). This shows how the alleged connection between morphological similarity and phylogenetic relationship is rather a Darwinist myth that is upheld as convenient tool to promote the theory.
Anyway, there are also some problems with the dating of Riostegotherium: The deposits where it was found belong to the Itaboraian SALMA, which was was originally dated by Marshall (1985) to the middle Paleocene (63.6-61 mya), but this was indirectly inferred in the absence of radioisotopic or magnetostratigraphic datings. Bond et al. (1995) instead suggested a Paleocene/Eocene age of 60-57 mya, which was also accepted by Bergqvist et al. (2004, 2019). Gelfo et al. (2009) provisionally re-dated the Itaboraian SALMA to an Early Eocene age of 55.5-53.5 mya (Lower Ypresian). The seminal work of Woodburne et al. (2014) moved the dating even a bit younger to the Middle Ypresian about 53-50 mya, contemporaneous with the Early Eocene Climatic Optimum (EECO). Similarly, Gaudin & Croft (2015) suggested an age of 54-52 mya in their stratigraphic chart. Nevertheless, Bergqvist et al. (2019) mentioned that “study in progress by the senior author challenges this latter interpretation, suggesting that at least part of the fissures was formed and filled during the late Paleocene.” Therefore they just gave a vague Paleocene/Eocene age. Apparently the study in progress is not yet published to this day. Therefore, it cannot yet be excluded that Riostegotherium might be of Late Paleocene age after all.
Prior to the discovery of Riostegotherium, the oldest xenarthrans were astegotheriine armadillos from the Barrancan subage of the Casamayoran SALMA of Argentina, which was believed to be about 7 million years younger (Scillato-Yané 1976). According to Kay et al. (1999) “the Casamayoran SALMA was at least 18–20 m.y. younger than previously assumed”. Therefore, Dunn et al. (2013) and Ciancio et al. (2016) dated the Barrancan to a Middle Eocene (Lutetian-Bartonian) age of 41.7-39.0 mya, which was basically confirmed by Woodburne et al. (2014) with an estimate of 42-38.5 mya (also see Gaudin & Croft 2015).
Remains of the astegotheriine armadillos Stegosimpsonia sp. and Astegotherium dichotomus were recovered from Cañadón Vaca in Patagonia. The respective layers date to the older Vacan subage (as opposed to the younger Barrancan subage) of the Casamayoran SALMA from the early Middle Eocene about 46-44 mya (Cifelli 1985, Gaudin & Croft 2015, Ciancio et al. 2018, 2019).
The extinct armadillo taxa Peltephilidae and Astegotheriini are also known from the Riochican SALMA of Patagonia (Simpson 1948, McKenna & Bell 1997, Oliveira & Bergqvist 1998, Gelfo et al. 2010). The Riochican SALMA is interspersed between the older Itaboraian SALMA and the younger Casamayoran SALMA. It had been dated to a Late Paleocene age of 57-55.5 mya by Bond et al. (1995), but was later re-dated to be only about 49-48.5 mya (Ypresian, Early Eocene) according to Woodburne et al. (2014), and to 51-49.5 by Gaudin & Croft (2015).
Armadillo remains of Pucatherium parvum and Noatherium emilioi have recently been described from the lower Lumbrera Formation in Northwestern Argentina (Fernicola et al. 2021), which the latter authors re-dated to the Early Eocene Climatic Optimum (Ypresian). This is older than previously believed, so that these armadillos could be contemporaneous with Riostegotherium if the re-dating of the Itaburaian by Woodburne et al. (2014) is correct (see above).
Considerable Scientific Controversy
Another recent discovery is a xenarthran metacarpal bone from the Late Eocene of Seymour Island in Antarctica (Davis et al. 2020). It was found in the Cucullaea I Allomember (TELM4) of the La Meseta Formation. The age of this formation is a matter of considerable scientific controversy, which I will discuss in great detail in a forthcoming technical paper on the waiting time problem in the origin of whales (Bechly et al. in prep.). Here it must be sufficient to note that an Early Eocene (Ypresian) age of 52-48 mya seems most likely and represents the consensus of most experts based on different lines of evidence (strontium isotopes, magnetostratigraphy, eustatic lowstands, dinoflagellate and mammalian biostratigraphy), while some dissenters are considering a younger Middle Eocene (Bartonian) age of about 40 mya mainly based on some dinoflagellate cysts.
The earliest fossil record for the suborder Pilosa is Pseudoglyptodon from the Priabonian-Bartonian (37.8 mya) of Argentina (Gaudin & Croft 2015). Within Pilosa there are two major subgroups: sloths (Folivora) and anteaters (Vermilingua). Ameghino (1895) described an astragalus of Proplatyarthrus longipes from the early Late Eocene (Mustersan SALMA), which could be the oldest folivoran, but unfortunately this specimen seems to be lost according to Pujos et al. (2021). The oldest other alleged sloth remains were from the Middle Eocene of Antarctica (Vizcaíano & Scillato-Yané 1995), but were disputed and considered to be an undetermined mammal by MacPhee & Reguero (2010). Pseudoglyptodon sallaensiswas described by Engelmann (1987), based on a mandible fragment from the Oligocene Salla Beds (Deseadan SALMA) of Bolivia, which also yielded other armadillo fossils (Billet et al. 2011). As already mentioned before, Pseudoglyptodon is commonly considered to be the oldest folivoran, but its possibly not a sloth in the strict sense (Pujos & De Iuliis 2007). Due to its peculiar characteristics and fragmentary preservation it remains enigmatic (Pujos & De Iuliis 2007, Pujos et al. 2021). Engelmann (1987) gave an age of 25-28 mya, which agrees with modern datings of the Deseadan SALMA to 29.4-24.2 mya (Dunn et al. 2013). McKenna et al. (2006) described new material of Pseudoglyptodon from the Late Eocene-Early Oligocene (Tinguirirican SALMA, 33.6-31.3 mya, Dunn et al. 2013) of Chile and the Late Eocene of Cerro Blanco in Argentina (Mustersan SALMA), and recognized this genus as the sister group of all other sloths. Nevertheless, Varela et al. (2019) commented: “However, as Pseudoglyptodon presents many convergences with cingulates, we cannot rule out its position to be an artifact of our limited knowledge of this taxon.” The Mustersan SALMA was traditionally considered to be of Middle Eocene age, then considered to be postdating 35-36 mya (Kay et al. 1999); but ultimately recognized as early Late Eocene with an age estimates of 38.2-38 mya (Dunn et al. 2013), and 37.9-36.5 mya (Woodburne et al. 2014, Gaudin & Croft 2015). Consequently, the Argentine material of Pseudoglyptodon indeed represents the oldest known fossil pilosans and likely the oldest sloths as well.
Some other very early fossil remains that are unequivocally referable to sloths are a Megalonychidae from the Early Oligocene (ca. 35 mya) of Puerto Rico (MacPhee & Iturralde Vinent 1995) and Patagonia (Carlini & Scillato-Yané_2004).
What About Anteaters?
The oldest fossil record of anteaters is an undescribed vermilinguan from the Early Miocene Colhuehuapian SALMA (20 mya) of Patagonia (Carlini et al. 1992, Gaudin & Branham 1998, Gaudin & Croft 2015). This implies a 30 million year ghost lineage of undocumented existence (Delsuc et al. 2001). The oldest described vermilinguan is Protamandua rothi from the Early to Middle Miocene Santacruzian of Patagonia (Ameghino 1904, Hirschfeld 1976, Patterson et al. 1992, Gaudin & Branham 1998). According to Gaudin & Croft (2015) it is about 18 million years old, but PaleoDB gives a range of 17.5-11.608 mya. We see that in most cases even the most modern radiometric methods provided quite different and fuzzy datings of the fossil outcrops. This does not mean that all the datings are completely wrong, but it shows that we should not place too much confidence in the current consensus concerning precise dates and possible evolutionary scenarios that are based upon them.
This also holds for so-called molecular clock datings, which place the origin of armadillos around the K/Pg boundary (Delsuc et al. 2001, 2004, Presslee et al. 2019). For Darwinists this would necessarily require an even earlier origin for the order Xernarthra in the Late Cretaceous. Too bad, this is of course strongly contradicted by the fossil record and the total absence of any Cretaceous crown group placental mammals. Should we dare to consider the possibility that something is wrong with the Darwinist assumptions? Heaven forbid!
An Abrupt Appearance
Anyway, we can conclude from the discussion above that the order Xenarthra appeared abruptly in the Lower Eocene about 53 million years ago, only 3 million years after the oldest fossils of our own order Primates, which we discussed last week. Carlini & Scillato-Yané (2004) made a revealing admission:
The absence of most major groups of Xenarthra in South American mammal-bearing sediments of Late Cretaceous and Early Paleocene age is the greatest enigma in the study of this Superorder.
I totally agree and can only note that this of course only emphasizes the abruptness of their appearance in the fossil record.
Xenarthra is one of the four major clades of placental mammals recognized in modern phylogenetic systematics. The three others are Afrotheria, Euarchontoglires, and Laurasiatheria. Next Fossil Friday we will look at a member of the African mammal clade Afrotheria, i.e., the aardvark order Tubulidentata and its fossil relatives.
P.S.: I have already discussed the misidentified alleged European anteater Eurotamandua from the Eocene Messel Pit in a previous Fossil Friday article (Bechly 2022).
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