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Fossil Friday: Golden Moles and the Abrupt Origin of Afrosoricida

January 6, 2023, 6:29 AM
Photo: Diamantochloris inconcessus, Eocene, Namibia, combined after Pickford 2018 figs. 1 and 3, fair use.

Last week we looked into the fossil history of elephant shrews. This first Fossil Friday in the new year we will move on in our series on placental mammal origins to another group of mainly insectivorous afrotherians: the order Afrosoricida, which comprises golden moles (Chrysochloridae), otter shrews (Potamogalidae), and the iconic tenrecs (Tenrecidae) from Madagascar. As in other small mammals their fossil record mostly consists of isolated jaw fragments and teeth, just like the featured fossil of the golden mole Diamantochloris inconsessus from the Eocene of Namibia (Pickford 2018).

Mainly based on molecular data (Springer et al. 2003) it has been suggested that the Afrosoricida originated 65 million years ago, right after the K/Pg impact event or even before (Tabuce et al. 2007: fig. 5, Poux et al. 2008: fig. 3, Everson et al. 2016: fig. 4). Of course, the fossil record does not at all support such a view (Sargis & Dagosto 2008: fig. 5.17, Asher 2010: fig. 9.1), so that some authors decided within a year to simply place the assumed origin 10 million years later (Tabuce et al. 2008: fig. 1). Isn’t evolutionary biology a wonderful science? Here is a brief list of the oldest known fossil genera in each group of Afrosoricida with their estimated stratigraphic range (based on PaleoDB and Seiffert 2010):

Afrosoricida (48.60–0 mya)

            Chrysochloridae (48.6–0 mya)

                        Damarachloris Pickford, 2019b (48.6–40.4 mya)

                        Diamantochloris Pickford, 2015a (48.6–40.4 mya, primitive Chrysochloridae acc. to Pickford 2018)

                        Namachloris Pickford, 2015c (40.4–37.2 mya)

                        Prochrysochloris Butler & Hopwood, 1957 (20.43–15.97 mya)

            Tenrecoidea (= Tenrecomorpha) (48.6–0 mya)

                        Dilambdogale Seiffert, 2010 (37.2–33.9 mya, rather about 37)

                        Eochrysochloris Seiffert et al. 2007 (33.9–28.4 mya, rather about 33)

                        Jawharia Seiffert et al. 2007 (33.9–28.4 mya, rather about 33)

                        Nanogale Pickford, 2019a (48.6-40.4 mya)

                        Plesiorycteropus Filhol, 1895 (0.012–0.0 mya)

                        Qatranilestes Seiffert, 2010 (33.9–28.4 mya, rather about 30)

                        Widanelfarasia Seiffert & Simons, 2000 (33.9–28.4 mya, rather 33.9)          

                        Potamogalidae (40.4–0 mya)

                                    Namagale Pickford, 2015b (40.4–37.2 mya)

                        Tenrecidae (40.4–0 mya)

                                    Arenagale Pickford, 2015b (40.4–37.2 mya)

                                    Erythrozootes Butler & Hopwood, 1957 (24-16 mya)

                                    Parageogale Butler, 1984 (= Butleriella) (24-16 mya)

                                    Protenrec Butler & Hopwood, 1957 (23.03–11.608 mya)

                                    Sperrgale Pickford, 2015b (40.4–37.2 mya)

Golden Moles (Chrysochloridae)

Golden moles are small, burrowing mammals endemic to sub-Saharan Africa with 21 living species (Asher et al. 2010). The oldest and most primitive fossil golden moles, Diamantochloris and Damarachloris, were discovered in Middle Eocene (Lutetian) sediments from Black Crow in Namibia, which are maximally 48.6 million years old (Pickford 2015a, 2018, 2019b). Together with the tenrecomorph Nanogale (see below) they also represent the earliest fossil record of Afrosoricida known so far. The best known but slightly younger genus is Namachloris, of which about 120 remains from almost the complete skeleton have been found (Pickford 2015c). They show that even these early representatives already had the burrowing adaptations of their living descendants. Another very old alleged chrysochlorid is Eochrysochloris from Fayum in Egypt (Seiffert et al. 2007, Seiffert 2010). However, Pickford (2015a, 2015c, 2018) suggested that Eochrysochloris “is probably not a chysrochlorid” but rather a tenrecid.

Tenrecoidea

The oldest representative of Tenrecoidea or Tenrecomorpha is Nanogale fragilis from the Eocene freshwater limestone of Black Crow in Namibia that can be dated to maximally 48.6 million years (Pickford 2019a). There is only a single mandible fragment known, which represents the smallest mammal from the fossil record in Africa. It has some characteristics resembling tenrecs and others rather resembling otter shrews, so that it may belong to their common ancestral lineage. Other very old tenrecoids were found in Eocene/Oligocene (37-30 mya) outcrops of the Jebel Qatrani Formation in the Fayum region of northern Egypt (Seiffert 2006), and include the genera Dilambdogale, Eochrysochloris, Jawharia, Widanelfarasia, and Qatranilestes (Seiffert & Simons 2000, Seiffert et al. 2007, Seiffert 2010).

Otter Shrews

Otter shrews (Potamogalidae) only include two living genera with three species of nocturnal and amphibious mammals, feeding off crustaceans and fish. They are believed to be closely related to the Malagasy tenrecs but only occur in Western and Central Africa. According to molecular clock studies their lineage should at least date to the Lower Eocene (Everson et al. 2016), but the possible fossil record is very sparse and controversial. Van Valen (1967) thought that the genera Erythrozootes and Protenrec might be fossil otter shrews, but most other and later workers rather attributed them to the genuine tenrecs. Seiffert (2007) again found the Miocene Protenrec as sister group of Potamogale instead on tenrecs, but subsequent studies did not accept this position. The only unequivocal fossil record of otter shrews is Namagale described by Pickford (2015b) from the Late Eocene (Bartonian) Eocliff in Namibia, which is 40.4-37.3 million years old.

Tenrecs

Living tenrecs are hedgehog-like mammals endemic to Madagascar with 31 living species classified in 8 genera and 3 subfamilies. Until recently the oldest fossil tenrecs were the three genera Erythrozootes, Parageogale, and Protenrec from the Miocene of Kenya and Namibia (Butler & Hopwood 1957, Butler 1984, Poduschka & Poduschka 1985, McKenna & Bell 1997, Mein & Pickford 2003, 2008, Asher & Hofreiter 2006, Seiffert et al. 2007, Pickford et al. 2008, Poux et al. 2008, Asher & Seiffert 2010). Strangely, these genera seem to be most closely related to the Malagasy tenrec genus Geogale (Asher & Hofreiter 2006). Poduschka & Poduschka (1985) disputed the relationship of Parageogale, which they had invalidly named Butleriella, with the  living genus Geogale, but this very relationship was vindicated by new material and further studies (see Asher & Seiffert 2010). This relationship arguably would imply a back dispersal event from Madagascar to Eastern Africa more than 267 miles across the Mozambique Channel of the Indian Ocean (Douady et al. 2002, Poux et al. 2008, Everson et al. 2016). This is a quite daring hypothesis to say the least (see Bechly 2018). Apart from this anomaly the general colonization of Madagascar by tenrecs has been dated with molecular evidence to have happened between 55 mya and 37 mya by Douady et al. (2002) or between 56-30 mya by Everson et al. (2016), which falls well within the range of the oldest African fossil stem tenrecs and well after the separation of Madagascar from mainland Africa about 165-120 mya. These oldest putative stem tenrecs are the two species Arenagale calcareus and Sperrgale minutus described by Pickford (2015b) from the Late Eocene (Bartonian) Eocliff in Namibia, dated to about 40 million years ago.

We should also briefly mention the Malagasy Aardvark Plesiorycteropus, which is known from two subfossil species. Apparently, these animals went extinct just a few hundred years ago, likely due to anthropogenic causes like overhunting and deforestation. They were long believed to be related to Xenarthra or Tubulidentata, and even assigned to its own distinct mammal order Bibymalagasia. However, a molecular study of ancient collagen revealed that they represent another major branch of Tenrecoidea (Buckley 2013), though no older fossils are known yet.

But, we have not yet exhausted the potential candidates for the oldest Afrosoricida. There are two obscure fossil mammals from the Late Paleocene of Morocco that may qualify: Todralestes variabilis was originally described as an insectivoran of the polyphyletic waste basket taxon “Proteutheria” (Gheerbrant 1991, 1994, Gheerbrant et al. 1998), while Afrodon chleuhi was described from the same outcrops as an insectivoran of the extinct family Adapisoriculidae (Gheerbrant 1988, Gheerbrant & Russell 1989, Gheerbrant et al. 1998). Both Todralestes and Afrodon share dental similarities with supposed early afrosoricids like Widanelfarasia, Protenrec, and Dilambdogale (Seiffert & Simons 2000, Seiffert et al. 2007). Asher & Seiffert (2010) and Seiffert (2010) even recovered these two genera as most basal stem Afrosoricida in their cladogram. If this should be correct, it would place the origin of the afrosoricid lineage into the Late Paleocene, right in the brief window of time when most of the placental mammal orders first appear in the fossil record. Of course, as always there is considerable disagreement about the phylogenetic affinities of these taxa: Tabuce et al. (2008: fig. 1) put a question mark at the alleged Late Paleocene occurrence of stem afrosoricids suggested by Seiffert et al. (2007), without explicitly listing the concerning genera. Pickford et al. (2008) considered Todralestes as a member of the unrelated extinct mammal order Cimolesta, and Afrodon still as an adapisoriculid insectivoran of the living mammal order Lipotyphla. The consensus tree of Halliday et al. (2015) did neither support a relationship of Todralestes nor of Widanelfarasia and Dilambdogale with Afrosoricida or even Afrotheria.

Like in all the other groups of afrotherian mammals, the modern consensus of attributing Afrosoricida to the African mammal clade Afrotheria is almost exclusively based on molecular data (Nishihara et al. 2005, Seiffert 2003, 2007, Tabuce et al. 2007, 2008, Asher & Seiffert 2010, Heritage et al. 2020), while anatomical similarities were universally interpreted as evidence for a relationship with insectivorans (e.g., Van Valen 1967, Butler 1984, Novacek et al. 1985, McKenna & Bell 1997) and even explicitly rejected an afrotherian clade (Asher 1999). Pickford (2019a) mentioned that “many recent phylogenetic analyses of Afrotheria seem to be incompatible with each other.”

A Repeating Pattern

Again and again we find the same pattern:

  • Darwinism predicts gradual accumulation of small changes over long periods of time but the empirical data of the fossil record point to rapid bursts of biological novelty.
  • Darwinism predicts that anatomical similarities should align with genetic similarities but the actual trees and/or classifications generated from these data conflict with each other.
  • Darwinism predicts that molecular clock estimates should agree with the stratigraphic appearance of taxa in the fossil record but they don’t.

Should we draw any conclusions from such consistent empirical failures of a theory? Maybe we should instead modify a popular dictum by the famous evolutionary biologist Theodosius Dobzhansky into “Nothing makes sense in biology in the light of (Darwinian) evolution” to align it better with reality.

Next Fossil Friday we will have a look at the early fossil record of hyraxes, another afrotherian group with an interesting history.

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