Fossil Friday: Desmostylia, and the Problem of Horizontal Tooth Displacement
This Fossil Friday features the skeleton of the Miocene mammal Neoparadoxia cecilialina, which is on display at the Natural History Museum of Los Angeles County. This species belongs to the extinct order Desmostylia, which is an enigmatic group of Tertiary mammals that had an amphibious lifestyle similar to modern hippos.
Desmostylia have generally been considered to be close relatives of sirenians (manatees) and proboscideans (elephants) (Reinhart 1953) within a common clade called Tethytheria (McKenna 1975, Domning et al. 1986, McKenna & Bell 1997). This was disputed by a phylogenetic study of Cooper et al. (2014, also see Beatty & Cockburn 2015), who suggested that desmostylians are basal odd-toed ungulates (Perissodactyla). Of course, this hypothesis would have to dismiss all the similarities with Tethytheria as convergences, which seems quite implausible. However, other new evidence again supported a tethytherian relationship (Gheerbrant et al. 2016). Therefore, desmostylian affinities are still considered controversial until this day (Matsui & Tsuihiji 2019). Phylogenetics over and over proves to be a rather “soft” science, if it can be called a science at all. We will come back to this question.
Anyway, with Desmostylia at least the genus Desmostylus also has some kind of horizontal tooth replacement (Santos et al. 2016, contra Domning et al. 1986), similar to the possible relatives of Desmostylia, sea-cows and elephants. Indeed, this phenomenon represents a genuine problem for neo-Darwinism.
Here Is Why
If Darwinian evolution is true, we should expect that congruent derived similarities in modern representatives of given lineages are confirmed as homologies in the ground plan of these lineages when early fossil representatives are studied. In other words: similarities in modern relatives should go back to their common ancestor. In many cases the exact opposite is the case, the pattern of similarity found in modern species dissolves into a mess of incongruent homoplasy in their earlier fossil representatives.
A good example is found in the mammal clade Tethytheria, which includes modern elephants and sea-cows (sirenians). Elephants and sea-cows, even though quite different in habitus and way of life, are considered to be most closely related (sister groups) based on numerous shared derived characters like the position of a single pair of teats on the breast, a two-pointed spherical heart, skull structures, etc. They also share a unique type of tooth replacement: They lack permanent premolars, and as these cheek teeth wear down and fall out, they are replaced by new cheek teeth that slowly shift forward from behind along the dentary in a kind of “conveyor belt” manner. This distinct mode has been called horizontal tooth displacement.
The similarities between sirenians and elephants were first recognized in 1836 by the French naturalist Henri de Blainville, who classified them together in a group called “les gravigrades.” Blainville was especially impressed by the shared mode of horizontal tooth replacement. Such a horizontal tooth replacement is only shared by recent elephants and manatees, but absent in almost all other mammals, so that it would naturally be interpreted as a trait inherited by the common ancestor of Tethytheria. So far so good.
Or Not So Much
A first indication that the mode of tooth replacement may be a convergence could be the fact that among living sirenians it is only present in manatees but not in dugongs (Mitchell 1973, Marsh 1980). However, this might as well represent a secondary reduction due to the degenerate cheek teeth of dugongs (Lanyon & Sanson 2006).
Another possible indicator of convergence might be the fact that, contrary to a common misconception in the literature, the mode of horizontal tooth replacement in manatees and elephants is not really identical (Domning & Hayek 1984, Beatty et al. 2012): in elephants the replacement (“mesial drift”) is limited to just three normal molars (Roth & Shoshani 1988, Sanders 2018), while manatees add supernumerary molars as long as they live (Domning 1982, Domning & Hayek 1984). Of course, the latter mode could just be an extension of the former, so that this difference does not exclude homology per se. An even earlier precursor state might be the eruption of permanent teeth well past sexual maturity, which is very uncommon in mammals in general, but found in some afrotherian mammals such as elephants, sirenians, and hyraxes (Asher & Lehmann 2008).
Unfortunately, the fossil record revealed even more incongruence that definitely closed the case in favor of convergence: horizontal tooth replacement is only found in Neogene elephants (Sanders 2018), but not in older and more primitive fossil proboscideans. Among fossil sirenians only the trichechine genera Ribodon and Trichechus had horizontal tooth replacement (Beatty et al. 2012, Self-Sullivan et al. 2014). Finally, in Desmostylia most of the dozen genera apart from Desmostylus show no indication of horizontal tooth replacement. The unique horizontal tooth replacement in Desmostylus, manatees, and elephants obviously originated independently in each of these three lineages and does not support their close relationship.
A Window into the Past
Without the benefit of a good fossil record as window into the past, we would never have suspected this. The apparent homologous pattern of derived similarity (so-called synapomorphy), congruent with other data, evaporated under closer scrutiny, and had to be explained away as convergent or parallel development. How many other similarities might mislead us to infer common descent?
After all, convergence turned out to be much more common than believed in the golden era of neo-Darwinism, prior to the genomic revolution. This is also confirmed by the discovery of horizontal tooth replacement similar to manatees in two very far removed species, the Australian rock wallaby or Nabarlek (Peradorcas concinna) (Thomas 1904: 226, Sanson 1989), and the African rodent Heliophobius argenteocinereus (Gomes Rodrigues et al. 2011, Gomes Rodrigues & Šumbera 2015).
Such striking convergences have often been mentioned as problem for Darwinian evolution, because biological similarities are incongruently distributed among organisms. But in examples like the horizontal tooth replacement the problem seems to go even deeper, because the incongruences are clustered in groups that are thought to be closely related based on other evidence, so that the relationships seem to be reinforced by false homologies. Nature appears to be deceptive. Are Darwinists bothered by such problems? Not at all: Saether (1979) even boldly declared this bug to be a feature and called it “underlying synapomorphies.” So, can phylogenetics be considered a serious scientific discipline? Not according to any reasonable measure.
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