Alas, for Darwinism; the fossil record's gonna fossil record.

Fossil Friday: Desmostylia, and the Problem of Horizontal Tooth Displacement 

Günter Bechly 

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. 

References 

Asher RJ & Lehmann T 2008. Dental eruption in afrotherian mammals. BMC Biology 6:14, 1–11. DOI: https://doi.org/10.1186/1741-7007-6-14.

Blainville HMD de 1836. Comptes rendus de l’Académie des sciences, séance du 20 mars 1836, vol. IV, p. 426.

Beatty BL & Cockburn TC. 2015. New insights on the most primitive desmostylian from a partial skeleton of Behemotops (Desmostylia, Mammalia) from Vancouver Island, British Columbia. Journal of Vertebrate Paleontology 35(5):e979939, 1–15. DOI: https://doi.org/10.1080/02724634.2015.979939.

Beatty BL, Vitkovski T, Lambert O & Macrini TE 2012. Osteological Associations With Unique Tooth Development in Manatees (Trichechidae, Sirenia): A Detailed Look at Modern Trichechus and a Review of the Fossil Record. The Anatomical Record 295(9), 1504–1512. DOI: https://doi.org/10.1002/ar.22525.

Cooper LN, Seiffert ER, Clementz M, Madar SI, Bajpai S, Hussain ST & Thewissen JGM 2014. Anthracobunids from the Middle Eocene of India and Pakistan Are Stem Perissodactyls. PLoS ONE 9(10): e109232, 1–15. DOI: https://doi.org/10.1371/journal.pone.0109232.

Domning DP 1982. Evolution of manatees: A speculative history. Journal of Paleontology 56(3), 599–619. JSTOR: https://www.jstor.org/stable/1304394.

Doming DP & Hayek L-AC 1984Horizontal tooth replacement in the Amazonian manatee (Trichechus inunguis). Mammalia 48(1), 105–127. DOI: https://doi.org/10.1515/mamm.1984.48.1.105.

Domning DP, Ray CE & McKenna MC 1986. Two new Oligocene desmostylians and a discussion of Tethytherian systematics. Smithsonian Contributions to Paleobiology 59(59), 1–56. DOI: https://doi.org/10.5479/si.00810266.59.1.

Gheerbrant E, Filippo A & Schmitt A 2016. Convergence of Afrotherian and Laurasiatherian Ungulate-Like Mammals: First Morphological Evidence from the Paleocene of Morocco. PLoS ONE 11(7): e0157556, 1–35. DOI: https://doi.org/10.1371/journal.pone.0157556.

Gomes Rodrigues H & Šumbera R 2015. Dental peculiarities in the silvery mole-rat: an original model for studying the evolutionary and biological origins of continuous dental generation in mammals. PeerJ 3:e1233, 1–19. DOI: https://doi.org/10.7717/peerj.1233.

Gomes Rodrigues H, Marangoni P, Sumbera R, Tafforeau P, Wendelen W & Viriot L 2011. Continuous dental replacement in a hyper-chisel tooth digging rodent. PNAS 108(42), 17355–17359. DOI: https://doi.org/10.1073/pnas.1109615108.

Lanyon JM & Sanson GD 2006. Degenerate dentition of the dugong (Dugong dugon), or why a grazer does not need teeth: morphology, occlusion and wear of mouthparts. Journal of Zoology 268(2), 133–152. DOI: https://doi.org/10.1111/j.1469-7998.2005.00004.x. 

Marsh H 1980. Age determination of the Dugong (Dugong dugon (Müller)) in Northern Australia and its biological implications. Report International Whaling Commission (Special Issue) 3(3), 181–201. https://www.researchgate.net/publication/240628646

Matsui K & Tsuihiji T 2019. The phylogeny of desmostylians revisited: proposal of new clades based on robust phylogenetic hypotheses. PeerJ 7:e7430, 1–17. DOI: https://doi.org/10.7717/peerj.7430.

McKenna MC 1975. Toward a Phylogenetic Classification of the Mammalia. pp. 21–46 in: Luckett WP & Szalay FS (eds.). Phylogeny of the primates: a multidisciplinary approach. Proceedings of WennerGren Symposium no. 61, Burg Wartenstein, Austria, July 6–14, 1974. Plenum Press, New York (NY). DOI: https://doi.org/10.1007/978-1-4684-2166-8_2.

McKenna MC & Bell SK 1997. Classification of Mammals above the Species Level. Columbia University Press, New York (NY), xii+631 pp.

Mitchell J 1973. Determination of relative age in the dugong Dugong dugon (Müller) from a study of skulls and teeth. Zoological Journal of the Linnean Society 53(1), 1–23. DOI: https://doi.org/10.1111/j.1096-3642.1973.tb01409.x. 

Reinhart R 1953. Diagnosis of the New Mammalian Order, Desmostylia. The Journal of Geology 61(2), 187. DOI: https://doi.org/10.1086/626067.

Roth VL & Shoshani J. 1988. Dental identification and age determination in Elephas maximus. Journal of Zoology 214(4), 567–588. DOI: https://doi.org/10.1111/j.1469-7998.1988.tb03760.x

Saether OA 1979. Underlying Synapomorphies and Anagenetic Analysis. Zoologica Scripta 8(1-4), 305–312. DOI: https://doi.org/10.1111/j.1463-6409.1979.tb00644.x

Sanders WJ 2018. Horizontal tooth displacement and premolar occurrence in elephants and other elephantiform proboscideans. Historical Biology 30(1-2), 137–156. DOI: https://doi.org/10.1080/08912963.2017.1297436

Sanson GD 1989. Morphological adaptations of teeth to diet and feeding in the Macropodoidea. pp. 151–168 in: Grigg G, Jarman P & Hume I (eds). Kangaroos, Wallabies and Rat-kangaroos. Vol. 1. Surrey Beatty & Sons, Chipping Norton (NSW)

Santos G-P, Parham JF & Beatty BL 2016. New data on the ontogeny and senescence of Desmostylus (Desmostylia, Mammalia). Journal of Vertebrate Paleontology 36(2):e1078344, 1–7. DOI: https://doi.org/10.1080/02724634.2016.1078344

Self-Sullivan C, Domning DP & Velez-Juarbe J 2014. Evolution of the Sirenia. http://67.59.130.204/sirenianevolution.pd

Thomas O 1904. On a collection of mammals made by Mr. J. T. Tunney in Arnhem Land, Northern Territory of South Australia. Novitates Zoologicae 11, 222–229. https://www.biodiversitylibraryf.....ibrary.org

 

Planet X: a brief history.

 Hypothetical Planet X 

Overview: 

Caltech researchers have found mathematical evidence suggesting there may be a "Planet X" deep in the solar system. This hypothetical Neptune-sized planet orbits our Sun in a highly elongated orbit far beyond Pluto. The object, which the researchers have nicknamed "Planet Nine," could have a mass about 10 times that of Earth and orbit about 20 times farther from the Sun on average than Neptune. It may take between 10,000 and 20,000 Earth years to make one full orbit around the Sun.

The announcement does not mean there is a new planet in our solar system. The existence of this distant world is only theoretical at this point and no direct observation of the object nicknamed "Planet 9" have been made. The mathematical prediction of a planet could explain the unique orbits of some smaller objects in the Kuiper Belt, a distant region of icy debris that extends far beyond the orbit of Neptune. Astronomers are now searching for the predicted planet. 

In Depth 

In January 2015, Caltech astronomers Konstantin Batygin and Mike Brown announced new research that provides evidence of a giant planet tracing an unusual, elongated orbit in the outer solar system. The prediction is based on detailed mathematical modeling and computer simulations, not direct observation.

This large object could explain the unique orbits of at least five smaller objects discovered in the distant Kuiper Belt.

"The possibility of a new planet is certainly an exciting one for me as a planetary scientist and for all of us," said Jim Green, director of NASA's Planetary Science Division. "This is not, however, the detection or discovery of a new planet. It's too early to say with certainty there's a so-called Planet X. What we're seeing is an early prediction based on modeling from limited observations. It's the start of a process that could lead to an exciting result."

The Caltech scientists believe Planet X may have has a mass about 10 times that of Earth and be similar in size to Uranus or Neptune. The predicted orbit is about 20 times farther from our Sun on average than Neptune (which orbits the Sun at an average distance of 2.8 billion miles). It would take this new planet between 10,000 and 20,000 years to make just one full orbit around the Sun (where Neptune completes an orbit roughly every 165 years). 

When was it Discovered? 

Planet X has not yet been discovered, and there is debate in the scientific community about whether it exists. The prediction in the Jan. 20 issue of the Astronomical Journal is based on mathematical modeling. 

What is its Name? 

Batygin and Brown nicknamed their predicted object "Planet Nine," but the actual naming rights of an object go to the person who actually discovers it. The name used during previous hunts for the long suspected giant, undiscovered object beyond Neptune is "Planet X."

If the predicted world is found, the name must be approved by the International Astronomical Union. Planets are traditionally named for mythological Roman gods. 

Why Do They Think It's There? 

Astronomers studying the Kuiper Belt have noticed some of the dwarf planets and other small, icy objects tend to follow orbits that cluster together. By analyzing these orbits, the Caltech team predicted the possibility that a large, previously undiscovered planet may be hiding far beyond Pluto.

They estimate the gravity of this potential planet might explain the unusual orbits of those Kuiper objects. 

What’s Next? 

Astronomers, including Batygin and Brown, will begin using the world's most powerful telescopes to search for the object in its predicted orbit. Any object that far away from the Sun will be very faint and hard to detect, but astronomers calculate that it should be possible to see it using existing telescopes.

"I would love to find it," says Brown. "But I'd also be perfectly happy if someone else found it. That is why we're publishing this paper. We hope that other people are going to get inspired and start searching."

"Anytime we have an interesting idea like this, we always apply Carl Sagan's rules for critical thinking, which include independent confirmation of the facts, looking for alternate explanations, and encouraging scientific debate," said Green. "If Planet X is out there, we'll find it together. Or we'll determine an alternate explanation for the data that we've received so far.

"Now let's go explore." 

Sources 

Fesenmaier, Kimm, "Caltech Researchers Find Evidence of a Real Ninth Planet," press release, last modified January 20, 2015

Konstantin Batygin and Michael E. Brown, "Evidence for a Distant Giant Planet in the Solar System," The Astronomical Journal

Green, James, "A New Planet in our Solar System? NASA Takes a Look," video statement, last modified January 20, 2015