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Fossil Friday: Miocene Aardvarks and the Abrupt Origin of Tubulidentata

December 23, 2022, 6:28 AM
Photo: Amphiorycteropus gaudryi, Miocene Greece, modified after Koufos 2022 fig. 3, fair use.

This Fossil Friday we continue our series on the origins of placental mammal orders with the aardvark order Tubulidentata. Aardvarks are a strange group of insectivorous mammals with a single living relic species Orycteropus afer, which is endemic to subsaharan Africa and exclusively feeds on ants and termites. The order is named after the characteristic tube-shaped teeth that lack enamel. Aardvarks are often considered to be living fossils (Bennett 2017, Shoshani 2021, Anonymous 2022). So, what about their actual fossil history? Including the living species there were 5 genera and 17 valid species described in the order Tubulidentata (Lehmann 2009, Pickford 2019), but generally their fossil record is comparatively sparse until this day (Koufos 2022).

Miocene to Pleistocene

Fossil aardvark species have been described from the Miocene to Pleistocene of Africa, southern Europe, and South Asia (Patterson 1975, Lehmann 2006, 2009, Asher & Seiffert 2010). Until recently the oldest undisputed fossil aardvarks were the two species Orycteropus minutus and Myorycteropus africanus from Lower Miocene deposits in Kenya (MacInnes 1956, Patterson 1975, Pickford 1975, Lehmann 2006), which were dated to an age of 19.6 mya and 17.8 mya respectively (Lehmann 2007). Slightly older undescribed remains were mentioned by Pickford & Andrews (1981). Pickford (2019) described the new fossil aardvark genus Eteketoni from the Lower Miocene of Uganda, which may be closely related to Myorycteropus and now rivals Orycteropus afrianus as oldest fossil record of the order with an estimated age of 20-18.5 mya.

McKenna & Bell (1997) mentioned the two genera Archaeorycteropus and Palaeorycteropus from the Oligocene of Quercy in France (ca. 34 mya) as questionable tubulidentates, which would qualify as oldest fossil record of the order. Pickford (1975) had at least Palaeorycteropus listed as fossil Tubulidentata, and Shoshani (2001) also accepted this genus as Oligocene record of Tubulidentata. However, several earlier works had already strongly disputed that either of these two genera is a tubulidentate at all (Simpson 1931, Thenius & Hofer 1960, Patterson 1975, Thewissen 1985). Lehmann (2007, 2009) agreed that both genera are Eutheria of uncertain relationship. Therefore, Asher & Seiffert (2010) excluded both from their phylogenetic tree of Afrotheria.

Leptomanis edwardsi is another enigmatic fossil mammal from the Oligocene of Quercy, and was mostly regarded as a putative relative of pangolins. Simpson (1931) thought that it is “possibly an orycteropodid” but put this with a question mark. Thewissen (1985) re-described the material of Leptomanis and concluded that “the best option for Leptomanis seems to be that it is the oldest tubulidentate so far known.” Others remained unconvinced and considered Leptomanis to be of uncertain affinities (Patterson 1975, 1978, Lehmann 2007, 2009). Gaudin et al. (2009) listed Leptomanis as a synonym of Necromanis within the order Pholidota, thus as a fossil pangolin. Finally, Crochet et al. (2015) disputed the synonymy with Necromanis but affirmed the position in Pholidota.

It has been suggested by some experts that an extinct order of carnivorous African mammals called Ptolemaiida might be related to aardvarks (Simons & Gingerich 1974, Nishihara et al. 2005, Cote et al. 2007, Seiffert 2007). The most primitive but not the oldest member of Ptolemaiida is the enigmatic species Kelba quadeemae from the Early Miocene of East Africa about 18.3 mya (Savage 1965, Cote et al. 2007). The oldest fossil record of Ptolemaiida is material from the latest Eocene of Fayum in Egypt (Simons & Bown 1995), which therefore could also be the oldest putative stem group representatives of Tubulidentata. However, this relationship is far from established.

The Eocene genera Herodotius and Chambius, which were previously considered as stem macroscelideans, never clustered with Macroscelidea but were recovered as sister group of aardvarks in some of the trees by Seiffert (2007), which would put the oldest fossil record of tubulidentate lineage into the Eocene as well. However, this result was unstable and in some other trees of the same author these herodotiine genera rather clustered with pseudoungulates, paenungulates, or hyraxes.

A Real Conundrum

So, we are left with a real conundrum for Darwinists. On the one hand, Shoshani (2001) concluded about Tubulidentata:

Little evolution has taken place in the genus over almost 20 million years, this is a hallmark of living fossils … , in all probability, the origin of tubulidentate taxa might date to the beginning of the Cenozoic era (Palaeocene epoch, about 65 Ma), and perhaps earlier (in the Cretaceous epoch of the Mesozoic era, some 70 Ma).

On the other hand aardvarks and their fossil relatives only appear much later in the fossil record of the Miocene less than 20 million years ago. Indeed, they represent the youngest of the placental mammal orders and one of the very few exceptions that are not first recorded in a narrow window of time in the Paleocene/Eocene (also see Asher & Seiffert 2010: fig. 46.1).

It is also interesting to note that aardvarks were long believed to be closely related to the Xenarthra and Pholidota (pangolins) within a hypothetical group Edentata. Rare dissenters like Le Gros Clark & Sonntag (1926) were vindicated by modern phylogenomic studies, which demonstrated that all three orders are unrelated and aardvarks belong to the African mammal clade Afrotheria (Asher & Seiffert 2010), which is hardly supported by any anatomical similarities. So much about the congruence of anatomical and genetic similarity predicted by Darwin’s theory.

Next Fossil Friday we will look into another member of the Afrotheria, i.e., the order Macroscelidea that includes the uber cute elephant shrews.

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