Tylopoda

Tylopoda (meaning "calloused foot")[1] is a suborder of terrestrial herbivorous even-toed ungulates belonging to the order Artiodactyla. They are found in the wild in their native ranges of South America and Asia, while Australian feral camels are introduced. The group has a long fossil history in North America and Eurasia. Tylopoda appeared during the Eocene around 50 million years ago.

Tylopoda
Temporal range: Early Eocene-Holocene
A dromedary camel
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
Suborder: Tylopoda
Illiger, 1811
Families

Camelidae
and numerous prehistoric families (see text)

Tylopoda has only one extant family, Camelidae, which includes camels, llamas, guanacos, alpacas and vicuñas. This group was much more diverse in the past, containing a number of extinct families in addition to the ancestors of living camelids (see below).

Tylopods are not ruminants.[2]

Taxonomy and systematics

Tylopoda was named by Illiger (1811) and considered monophyletic by Matthew (1908). It was treated as an unranked clade by Matthew (1908) and as a suborder by Carroll (1988), Ursing et al. (2000) and Whistler and Webb (2005). It was assigned to Ruminantia by Matthew (1908); to Artiodactyla by Flower (1883) and Carroll (1988); to Neoselenodontia by Whistler and Webb (2005); and to Cetartiodactyla by Ursing et al. (2000) and by Agnarsson and May-Collado (2008).[3][4][5]

The main problem with circumscription of Tylopoda is that the extensive fossil record of camel-like mammals has not yet been thoroughly examined from a cladistic standpoint. Tylopoda is a highly distinctive lineage among the artiodactyls, but its exact relationships are somewhat elusive because the six living species are all closely related and can be considered "living fossils", the sole surviving lineage of a prehistorically wildly successful radiation. More recent studies suggest that tylopods are not as closely related to ruminants as traditionally believed, expressed in cladogram form as:[6][7][8][9][10][11]

Artiodactyla 

Tylopoda (camels)

 Artiofabula 

  Suina (pigs)

 Cetruminantia 

 Ruminantia (ruminants)  

 Cetancodonta/Whippomorpha 

 Hippopotamidae (hippopotamuses)

 Cetacea (whales)

Tylopoda are extremely conservative in their lifestyle and (like ruminants) seem to have occupied the same ecological niche since their origin over 40 million years ago. Thus, it seems that the previous assumption of a close relationship between Tylopoda and ruminants is simply because all other close relatives (whales, pigs etc.) are so divergent in their adaptations as to have obscured most indications of relationship, or at least those visible to phenetic analyses. However, the rather basal position that Tylopoda appears to have among the even-toed ungulates and relatives means that the oldest members of this lineage are still morphologically very primitive and hard to distinguish from the ancestors of related lineages. The first major modern and comprehensive analysis of the problem (in 2009) supported this; while some taxa traditionally considered Tylopoda could be confirmed to belong to this suborder (and a few refuted), the delimitation of this group is still very much disputed despite (or because of) an extensive fossil record.[6]

Life restoration of Agriochoerus antiquus

The taxa currently assigned (with some reliability) to Tylopoda are:[6]

Basal and incertae sedis

Superfamily †Anoplotherioidea

Superfamily Cameloidea

Superfamily †Merycoidodontoidea (=Oreodontoidea)

Superfamily †Xiphodontoidea

Life restoration of the primitive artiodactyl Diacodexis pakistanensis (foreground) being stalked by Pakicetus

Disputed Tylopoda

Several additional prehistoric (cet)artiodactyl taxa are sometimes assigned to the Tylopoda, but other authors consider them incertae sedis or basal lineages among the (Cet)artiodactyla:

Some studies have considered Protoceratidae closely related to Tylopoda, but others have considered them more closely related to the ruminants.[12]

References
  1. Donnegan, James (1834). "A New Greek and English Lexicon"
  2. Fowler, M.E. (2010). "Medicine and Surgery of Camelids", Ames, Iowa: Wiley-Blackwell. Chapter 1, General Biology and Evolution, addresses the fact that camelids (including llamas and camels) are not ruminants, pseudoruminants, or modified ruminants.
  3. Matthew, W. D. (1908). "Osteology of Blastomeryx; and phylogeny of the American Cervidae". Bulletin of the American Museum of Natural History. 24 (27): 535–562. hdl:2246/1442.
  4. R. L. Carroll. 1988. Vertebrate Paleontology and Evolution. W. H. Freeman and Company, New York 1-698
  5. Ursing, B. M.; Slack, K. E.; Arnason, U. (April 2000). "Subordinal artiodactyl relationships in the light of phylogenetic analysis of 12 mitochondrial protein-coding genes". Zoologica Scripta. 29 (2): 83–88. doi:10.1046/j.1463-6409.2000.00037.x. S2CID 84619585.
  6. Spaulding, M., O'Leary, M.A. & Gatesy, J. (2009): Relationships of Cetacea (Artiodactyla) Among Mammals: Increased Taxon Sampling Alters Interpretations of Key Fossils and Character Evolution. PLoS ONE no 4(9): e7062. doi:10.1371/journal.pone.0007062 article
  7. Beck, N.R. (2006). "A higher-level MRP supertree of placental mammals". BMC Evol Biol. 6: 93. doi:10.1186/1471-2148-6-93. PMC 1654192. PMID 17101039.
  8. O'Leary, M.A.; Bloch, J.I.; Flynn, J.J.; Gaudin, T.J.; Giallombardo, A.; Giannini, N.P.; Goldberg, S.L.; Kraatz, B.P.; Luo, Z.-X.; Meng, J.; Ni, X.; Novacek, M.J.; Perini, F.A.; Randall, Z.S.; Rougier, G.W.; Sargis, E.J.; Silcox, M.T.; Simmons, N.B.; Spaulding, M.; Velazco, P.M.; Weksler, M.; Wible, J.R.; Cirranello, A.L. (2013). "The Placental Mammal Ancestor and the Post-K-Pg Radiation of Placentals". Science. 339 (6120): 662–667. Bibcode:2013Sci...339..662O. doi:10.1126/science.1229237. hdl:11336/7302. PMID 23393258. S2CID 206544776.
  9. Song, S.; Liu, L.; Edwards, S.V.; Wu, S. (2012). "Resolving conflict in eutherian mammal phylogeny using phylogenomics and the multispecies coalescent model". Proceedings of the National Academy of Sciences. 109 (37): 14942–14947. Bibcode:2012PNAS..10914942S. doi:10.1073/pnas.1211733109. PMC 3443116. PMID 22930817.
  10. dos Reis, M.; Inoue, J.; Hasegawa, M.; Asher, R.J.; Donoghue, P.C.J.; Yang, Z. (2012). "Phylogenomic datasets provide both precision and accuracy in estimating the timescale of placental mammal phylogeny". Proceedings of the Royal Society B: Biological Sciences. 279 (1742): 3491–3500. doi:10.1098/rspb.2012.0683. PMC 3396900. PMID 22628470.
  11. Upham, N.S.; Esselstyn, J.A.; Jetz, W. (2019). "Inferring the mammal tree: Species-level sets of phylogenies for questions in ecology, evolution, and conservation". PLOS Biology. 17 (12): e3000494. doi:10.1371/journal.pbio.3000494. PMC 6892540. PMID 31800571.(see e.g. Fig S10)
  12. Robson, Selina Viktor; Seale, Brendon; Theodor, Jessica M. (2021-07-29). Louys, Julien (ed.). "The petrosal and basicranial morphology of Protoceras celer". PLOS ONE. 16 (7): e0251832. Bibcode:2021PLoSO..1651832R. doi:10.1371/journal.pone.0251832. ISSN 1932-6203. PMC 8321106. PMID 34324518.
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