Friday, 27 January 2023

Where success is an orphan.

The fossil record continues to troll Darwinism.

Fossil Friday: Elephants and the Abrupt Origin of Proboscidea

Günter Bechly

This Fossil Friday features the iconic Deinotherium (16.9-0.78 mya) since today we look into the last order of afrotherian mammals, the Proboscidea and their putative fossil relatives. Proboscideans include the living and fossil elephants such as mammoths, mastodons, gomphotheres, stegodonts, and deinotheres. The fossil record of proboscideans is very extensive with about 50 genera with 185 species. It includes isolated teeth as well as complete skeletons with numerous early representatives from the Paleogene (Tassy 1990, McKenna & Bell 1997, Shoshani & Tassy 2005, Sanders et al. 2010). These fossils certainly provide significant information about the history and development of proboscideans (Tassy & Shoshani 1988, Tassy 1990, Shoshani & Tassy 1996, 2005, Shoshani 1998, 2001, Gheerbrant 2009, Gheerbrant & Tassy 2009, Begum 2021, Cantalapiedra et al. 2021). For our purpose we will ignore all of the later radiations of elephant-like mammals and will focus on the earliest representatives of the first radiation. So, when do proboscideans first appearance in the fossil record?

Oldest and Most Primitive

The oldest and most primitive assumed proboscidean is Eritherium azzouzorum from Sidi Chennane in the Ouled Abdoun Basin of Morocco (Gheerbrant 2009, Gheerbrant et al. 2012, Schmitt & Gheerbrant 2016). This outcrop is of Middle Paleocene (Selandian, 61.7-58.7 mya) age of almost 60 million years, which makes it the oldest known placental mammal locality in Africa. Eritherium was a relatively small animal, only about the size of a fox, and without a trunk or tusks. It did not really look like an elephant at all, and indeed there is a problem with its alleged proboscidean affinity: in some more recent phylogenetic studies, Eritherium is resolved outside of Proboscidea as a more basal paenungulate (Cooper et al. 2014, Rose et al. 2019), but it was confirmed as basal proboscidean by Gheerbrant et al. (2014, 2016) and Schmitt & Gheerbrant (2016)

Pretty much undisputed is Phosphatotherium escuilliei from the Early Eocene (earliest Ypresian, 55.8-48.6 mya) of the Ouled Abdoun Basin in Morocco (Gheerbrant et al. 1996, 2005a, Shoshany & Tassy 2005, Sanders et al. 2010), which represents the next oldest and second most primitive proboscidean (Gheerbrant et al. 2005a, Schmitt & Gheerbrant 2016). There was one little problem, though, which is the fact that the precise location of its discovery is unknown, because the holotype fossil was acquired from a commercial fossil dealer. The original description by Gheerbrant et al. (1996) admits this but says that an “unambiguous Thanetian age” is documented by the attached phosphatic matrix and the associated shark teeth, which are index fossils for an Upper Paleocene (Thanetian) layer that lacks Ypresian taxa. This “unambiguous” evidence apparently was not so unambiguous after all, because these very phosphatite layers were later recognized as earliest Ypresian by Gheerbrant et al. (2001, 2003), and the Ypresian age was also confirmed by later discovered new material of Phosphatotherium (Gheerbrant et al. 2005a).

Daouitherium rebouli is another primitive proboscidean from the Early Eocene (earliest Ypresian, 55.8-48.6 mya) of the Ouled Abdoun Basin in Morocco (Gheerbrant et al. 2002, Shoshany & Tassy 2005, Sanders et al. 2010). The original describers mentioned that the discovery of Daouitherium documents “an unexpected early diversity of proboscideans and of the old origin of the order” (Gheerbrant et al. 2002).

Numidotherium koholense was described from the Early Eocene (Ypresian) from El Kohol in Algeria (Mahboubi et al. 1984, Court 1995, Shoshani & Tassy 1996, Sanders eat al. 2010). A second species N. savagei was described by Court (1995) from the Upper Eocene (Bartonian, 40.4-33.9 mya) of southern Libya.

Last but not least, there is Khamsaconus bulbous, which was described from a single molar tooth from the Ypresian (55.8-48.6 Mya) of south Morocco. It was initially attributed to the louisinine “condylarths” thus in the possible relationship of the elephant shrew order Macroscelidea (Sudre et al. 1993; also see Bechly 2022c). Most later works rather considered this taxon as a very primitive and small early proboscidean (Gheerbrant et al. 1998, 2002, 2005a, 2012, Gheerbrant 2009, Sanders et al. 2010), but sometimes only with a question mark because the taxon is very poorly known.

One of the best-known early proboscideans is the somewhat younger genus Moeritherium, of which six species have been described. The earliest species is M. chehbeurameuri from the Bartonian of Algeria, which is 40.4-33.9 million years old (Delmer et al. 2006). Moeritheriumis quite remarkable for its very elongate body shape, which is certainly a derived trait that excludes Moeritherium from the direct ancestry of later proboscideans. The similarities between Moeritherium and the “walking sea cow” Protosiren led Andrews (1906) to first suggest a close relationship of elephants and sea cows, which later became generally recognized as the Tethytheria clade (McKenna 1975, Asher et al. 2003, Nishihara et al. 2005, Seiffert 2007, Tabuce et al. 2007, 2008, Asher & Seiffert 2010, O’Leary et al. 2013). Similar adaptations were considered by some experts as indicating a common semi-aquatic ancestor for these two mammal orders (Gaeth et al. 1999, Thewissen et al. 2000, Shoshani & Tassy 2005, Asher & Seiffert 2010).

Barytherium grave is the youngest of the primitive proboscideans and was discovered at the Eocene/Oligocene Fayum Depression in Egypt (33.9-28.4 mya) (Andrews 1906, Shoshani & Tassy 1996, Sanders et al. 2010), which also yielded other early proboscideans like Moeritherium, Phiomia, and Palaeomastodon.

None of these early proboscideans shows the strange phenomenon of horizontal tooth displacement that is found in more modern elephants and independently originated three times within Tethytheria (Shoshani 2001; also see Bechly 2022a). It is also worth noting that the phylogenetic relationships and classification within Proboscidea proved to be a quite controversial issue (Tassy 1988, Tassy & Shoshani 1988, Court 1995), which is hardly a big surprise as it seems to apply to most issues of higher phylogeny in all groups of organisms.

Embrithopoda

Embrithopoda is an enigmatic extinct group of large mammals from the Paleogene of the Old World, of which the best-known member is the iconic Arsinoitherium from the Late Eocene and Early Oligocene of Northern Africa. Even though Arsinoitherium resembled a rhino, most experts considered Embrithopoda as tethytherians within Afrotheria (see Erdal et al. 2016: tab. 2), and maybe even close relatives of proboscideans (Benoit et al. 2013, Avilla & Mothé 2021). However, the most recent studies rather suggest that embrithopods occupy a more basal position as sister group to all other Tethytheria (Erdal et al. 2016, Gheerbrant et al. 2018, 2021), so that the derived similarities with proboscideans have to be considered as convergences (also see Gheerbrant et al. 2005a and Benoit et al. 2013). The allegedly oldest fossil record for this group is Stylophus minor from the Early Eocene (Ypresian, 55.8-48.6 mya) Grand Daoui area in Morocco (Gheerbrant et al. 2018).

Anthracobunia

The Anthracobunia is a group of primitive perissodactyl-like mammals and includes the extinct families Cambaytheriidae and Anthracobunidae from the Early Eocene (55.8-48.6 Mya) of India and Pakistan (Wells & Gingerich 1983, Rose et al. 2006, 2014, 2019; also see Dunn 2020), and according to some workers may\ even include the extinct Desmostylia we discussed last week (Cooper et al. 2014, Rose et al. 2019, Gheerbrant et al. 2021; also see Bechly 2023b). Some scientists considered anthracobunids as tethytherians (Wells & Gingerich 1983, Ginsburg et al. 1999, Ducrocq et al. 2000, Rose et al. 2006, Tabuce et al. 2007, and Gheerbrant et al. 2014) and some other authors even considered them as basal proboscideans (Gingerich et al. 1990, Shoshani et al. 1996, Thewissen et al. 2000, Gheerbrant et al. 2005b, Asher & Seiffert 2010, Erdal et al. 2016), which would make them contemporaneous with some of the earliest proboscideans in Africa (Asher et al. 2003). Shoshany & Tassy (2005) preferred to exclude anthracobunids from Proboscidea until additional evidence becomes available, and Tabuce et al. (2008) mentioned that “the hypothesis that some extinct taxa (desmostylians, embrithopods and anthracobunids) are included in tethytheres is less supported, because the characters used to include them within tethytheres are homoplastic and/or of ambiguous distribution.” Indeed, more recent studies by Cooper et al. (2014), Gheerbrant et al. (2016), and Rose et al. (2014, 2019) unambiguously placed them outside Afrotheria in the stem group of odd-toed ungulates (Perissodactyla). This also makes much more sense from a paleobiogeographic point of view, as all anthracobunids were found in East Asia.

As I discussed in my article on fossil sea cows (Bechly 2023b), Ishatherium subathuensiswas described by Sahni & Kumar (1980) and Sahni et al. (1980) from the early Eocene (Ypresian, 55.8-48.6 mya) Subhatu Formation in the Himalayas. It was described as the oldest known sirenian but only a few authors concurred with this interpretation (Sereno 1982). Domning et al. (1982) questioned the sirenian affinity as well as the dating. Indeed, most other experts think that it could as well be an anthracobunid perissodactyl or a moeritheriid proboscidean (Wells & Gingerich 1983, Domning et al. 1986, Zalmout et al. 2003, Rose et al. 2019), but Cooper et al. (2014) excluded Ishatherium from Anthracobunidae.

Radinskya and Phenacolophus

Both genera have been considered as putative Paenungulata and Tethytheria related to Embrithopoda (McKenna & Manning 1977, McKenna & Bell 1997). Asher et al. (2003)mentioned that “if the hypothesized relation between Arsinoitherium and phenacolophids in the Embrithopoda is correct (McKenna and Manning, 1977), then crown afrotheres are also represented by Paleocene taxa from Central Asia,” which of course would be problematic. The cladistic studies by Gheerbrant et al. (2005a, 2014) confirmed Phenacolophus as a sister group to Embrithopoda in Paenungulata, but resolved Radinskya as sister group to Perissodactyla. More recent studies (Gheerbrant et al. 2016, 2021) even excluded both genera from Afrotheria and instead placed them in the stem group of odd-toed ungulates (Perissodactyla), just like anthracobunids and possibly desmostylians, which makes much more sense biogeographically. But here is the more general question: Isn’t it strange that two totally unrelated groups like elephants and odd-toed ungulates, which belong to different supergroups like Afrotheria and Laurasiatheria, seem to have so many similarities, that it is difficult to place some extinct groups and even some recent groups like hyraxes (Bechly 2023a) closer to one or the other? Is that what Darwinism would predict? Of course not! Is it instead what intelligent design theory would predict? Indeed it is. Just add two and two together yourself.

Ocepeia and Abdounodus

There are two more taxa that are often discussed in the context of proboscidean phylogeny and evolution, which are the two species Abdounodus hamdii and Ocepeia daouinensisdescribed by Gheerbrant et al. (2001) from the Middle Paleocene (Selandian, 61.7-58.7 mya) phosphatic beds of Ouled Abdoun in Morocco, a fossil locality that also produced some of the earliest fossil proboscideans (see above). They were considered to be the first condylarth-like mammals from Africa and related to Perissodactyla and Paenungulata, which now are attributed to different supergroups of placental mammals. Later studies by the same authors suggested a possibly affinity with aardvarks or basal paenungulates (Gheerbrant et al. 2014, 2016). Abdounodus and Ocepeia have meanwhile been confirmed as basal Paenungulata by several phylogenetic studies (Gheerbrant et al. 2018, Avilla & Mothé 2021).

We can conclude that, apart from the usual mess of incongruent phylogenies, it is an undeniable fact that Proboscidea appeared abruptly in the Late Paleocene / Early Eocene with a surprising early diversity (Gheerbrant et al. 2002). Moreover, there are anatomical discontinuities between the distinct radiations of early lophodont proboscideans and later elephant-like forms (Tassy 1988, 1990, Begum 2021, Cantalapiedra et al. 2021). Herewith we have completed our review of the afrotherian mammal orders and can move on to the next mammalian supergroup, called Euarchontoglires, which includes primates, tree shrews, colugos, hares, rabbits, and pikas. Since we have already dealt with the origins of primates (Bechly 2022b), we will look into the fossil history of tree shrews next Fossil Friday.