Once upon a time, (like pre-nineties) it was thought mammal orders were neatly compartmentalized groupings with clearly distinct ancestries. For example, it was common scientific belief all anteater-like mammals were related, all carnivorous mammals were a lineage, all hoofed mammals were another and so on, with not many complications to this pattern. However, new findings from evolutionary genetics have thrown a massive monkey wrench to the neat and tidy traditionalist taxonomic frame of Linnean Classification. These molecular studies sometimes conflict with morphology perspectives, as in the case of whale evolution, but overall provide crucial insights where gaps in the fossil record exist. Afrotheria is one such revolutionary idea emerging from this flurry of new evolutionary research, but with increasing data in support, becomes more solidified in the standard evolutionary framework.
Afrotheria: What Moles, Shrews, Elephants and Elephant Shrews Share
Afrotheria is an unique and ancient superorder of placental mammals first proposed in the late 1990's (Stanhope et al). It is also one of the oldest lineages of mammals to diverge. Chronological divergence estimates by Nishihara et.al using DNA transposable elements data indicate the three major supercohorts (Afrotheria, Boreotheria and Xenarthra) diverged nearly simultaneously in the Middle Cretaceous, right in the time of the non-avian dinosaurs.
Afrotheria consists of the basal members Ocepeia and Hyopsodontidae, and two clades, Afroinsectiphilia and Paengulata. Ocepeia is a hyrax-like mammal recently discovered in North Africa, and thought to be close to the ancestor of all Afrotherians while hyopsodontids were once part of that legendary “waste-basket” taxon Condylarthra. Hyopsodontids are thought to have been generalist early hoofed-mammals and thought to be omnivorous. Some like Hyopsodus were partially fossorial and one study even proposes a use of terrestrial echolocation by this taxon.
Afroinsectiphilia carries the orders Macroscelidae (Elephant shrews), Afosoricida(moles, tenrecs and "regular" shrews) and Tubulidentata (Aardvarks), with possible inclusion of the extinct orders Bibymalagasia (Plesiorycteropus, enigmatic Malagasy mammal), Ptolemaiida and Afredentata (the infamous Eurotamandua). Something to note: African golden moles, under this premise, are not closely related to other moles and are similar due to convergent evolution. Additionally, the placement of Eurotamandua within Afrotheria is still highly controversial, and I have not found much information on this. Previously classified as an anteater, then pangolin, the relationships of Eurotamandua are still subject to change even though we have a complete skeleton in great preservation state. The inclusion of Bibymalagasia, Ptolemaiida and Afredentata is still tentative and subject to further evidence. As an interesting side note, affinities between Afrotherians and South American hoofed mammals known as Meridungulates have been considered (Agnolin et al.) based on vertebral count, knee anatomy and tooth replacement patterns. Now this makes sense (remember how Africa and S.America fit together perfectly?) but I'd like to see more research on this topic. God, how I'd love to see more research on this topic.
Paengulata group together orders Hyracoidea(Hyraxes), Proboscidae (Elephants and allies), Sirenia (Sea Cows) and the extinct Desmostylia and Embrithopoda (see fig. 2). Suffice it to say right now that Desmostylians are another highly intriguing group of large hoofed mammals that had amphibious anatomy, protruding incisors, and so many other neat anatomical feature. To put a modern analogy, think something like a beach hippo, but totally different. Bizarrely, Cooper et al have even proposed that Desmostylians are tentative perissodactyls. Embrithopods are another fascinating group that resemble a rhinoceros on steroids, although both lineages are not closely related.
Back to Ocepeia. Within Afrotheria Ocepeia is reconstructed as more closely related to insectivore-like Afroinsectiphilians (i.e., aardvarks, sengis, tenrecs, and golden moles) than to Paenungulates. One of the highly interesting features of the Ocepeia skull is the “character mosaic” nature of it; it represents the first known “transitional fossil between insectivore-like and ungulate-like Afrotherians”. The family Ocepeiidae supports the idea Afrotheria evolved in the either the earliest Tertiary or latest Cretaceous, making it one of the oldest lineages of mammals. This simultaneously knocks down hypotheses rooting the individual afrotherian lineages in Condylarth lineages of Paleogene Laurasia.
As previously mentioned, research into Afrotherian systematics is dismembering Condylarthra, and another group libreated in this taxonomic carnage is the family Hyopsodontidae, consisting of a few not-very specialized small taxa. The idea of Hyopsodontidaeas stem Afrotherians is rather new. Randolphe et al have proposed Eocene paenungulates and elephant-shrews are to be related to some Early Tertiary Euramerican ‘hyopsodontid condylarths. Like Ocepeia, these creatures evolved right after the KT boundry, when there was a wealthof niches available for mammals to exploit. The relationship between Ocepeia and hypsodontids awaits further fossil elucidation, but it does appear they are both close to the ancestor of all Afrotherians. The study also reports of cranial and dental evidence of Eocene stem hyrax and macroscelid from North Africa supporting Afrotheria. The publication was a seminal milestone in understanding the relationships of this amazing assortment of animals. Another interesting tidbit thrown out by the paper is that of possible Ptolemaiida affinities with Tubulidentata.
Bibymalagsia is the order that has only one member currently, the The ‘Malagasy aardvark’ (Plesiorycteropus). Found in Madagascar, this mysterious creature (the other more mysterious taxon right off the top of my head is Necrolestes from South America) was written off as “just another aardvark (Orycteropus), but bigger” until Buckley released a comprehensive study on the taxon. A complete skull has not been recovered, with only cranial fragments providing the only information on that region but with sufficient post-cranial material to work with. Buckley has challenged Plesiorycteropus's affinities with Tubulidentates on the grounds that similarities with Orycteropus are based on convergence, not shared ancestry: both genera show adaptations for digging lifestyles. Plesiorycteropus’s skeletal anatomy shows diagnostic traits of several different eutherian mammalorders as well and thus cannot be placed in any existing taxon. The Buckley paper also includes the first ever molecular data for the genus, “obtained from the bone protein collagen (I), which places the ‘Malagasy aardvark’ as more closely related to tenrecs than aardvarks. More specifically, Plesiorycteropus was recovered within the order Tenrecoidea (golden moles and tenrecs) within Afrotheria, suggesting that the taxonomic order ‘Bibymalagasia’ is obsolete.” This indeed means Plesiorycteropus is not just “another big aardvark” buta very large semi-mole insectivorous lipotyphyan. Holy shit! For the time being, though, the existence of Bibymalagasia as a taxon remains as we await new findings.
Agnolin, F. L., & Chimento, N. R. (2011). Afrotherian affinities for endemic South American “ungulates”. Mammalian Biology-Zeitschrift für Säugetierkunde, 76(2), 101-108.
Buckley, M. 2013: A molecular phylogeny of Plesiorycteropus reassigns the extinct mammalian order ‘Bibymalagasia’. PLoS ONE, 8(3): e59614. doi: 10.1371/journal.pone.0059614
Cooper, L. N.; Seiffert, E. R.; Clementz, M.; Madar, S. I.; Bajpai, S.; Hussain, S. T.; Thewissen, J. G. M. (2014-10-08). "Anthracobunids from the Middle Eocene of India and Pakistan Are Stem Perissodactyls". PLoS ONE 9 (10): e109232. doi:10.1371/journal.pone.0109232
Gheerbrant, Emmanuel; Amaghzaz, Mbarek; Bouya, Baadi; Goussard, Florent; Letenneur, Charlène; (2014). "Ocepeia (Middle Paleocene of Morocco): The Oldest Skull of an Afrotherian Mammal". PLoS ONE 9 (2): e89739. doi:10.1371/journal.pone.0089739
Horovitz, Inés, Gerhard Storch, and Thomas Martin (2005). "Ankle structure in Eocene pholidotan mammal Eomanis krebsi and its taxonomic implications". Acta Palaeontol. Pol. 50 (3): 545–548
Nishihara, H.; Maruyama, S.; Okada, N. (2009). "Retroposon analysis and recent geological data suggest near-simultaneous divergence of the three superorders of mammals". Proceedings of the National Academy of Sciences 106 (13): 5235. doi:10.1073/pnas.0809297106
Rodolphe Tabuce et al. (2007) Early Tertiary mammals from North Africa reinforce the molecular Afrotheria clade.
Seiffert, Erik R.; (2007). "A new estimate of afrotherian phylogeny based on simultaneous analysis of genomic, morphological, and fossil evidence".BMC Evolutionary Biology, 7:224doi:10.1186/1471-2148-7-224
Stanhope, M. J.; Waddell, V. G.; Madsen, O.; de Jong, W.; Hedges, S. B.; Cleven, G. C.; Kao, D.; Springer, M. S. (1998). "Molecular evidence for multiple origins of Insectivora and for a new order of endemic African insectivore mammals". Proceedings of the National Academy of Sciences 95 (17): 9967–9972. doi:10.1073/pnas.95.17.9967. PMC 21445. PMID 9707584