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The Teeth of the Jurassic Mammals

Authors

  • P. M. Butler B.A., B.Sc.


Summary

  • 1On the supposition that similar elements of the tooth change in a similar way when traced along the series, it is possible to determine the homologies of the parts of the teeth of Jurassic mammals.
  • 2The Pantotheria fall into two groups: in the Docodontoidea the upper molar pattern is like that of Ictops, and in the Dryolestoidea it is of a Zalamb-dodont type. The paracone of the molar teeth of Melanodon and related forms is the lingual cusp. The metacone is distinct from the paracone in Jurassic mammals, and no amphicone exists.
  • 3The Symmetrodonta and Triconodonta have similar upper and lower molar teeth, and the same cusps occur in both orders, the main difference being that the main cusp is displaced in the Symmetrodonts. The paracone is the second of the three large cusps of Tricoriodonts, and the lingual cusp of Symmetrodonts. The lower molars have no metaconid, but possess instead a “posterior accessory cusp” on the posterior ridge of the protoconid, hitherto mistaken for the metaconid in Spalacotherium.
  • 4The premolars in Jurassic mammals probably replaced milk molars, and were less molariform than the milk molars they replaced, as in the modern Didelphis.
  • 5The Jurassic mammals, with the exception of the Paurodontidae, resemble the modern Didelphidae with regard to the relation between the length of the molar series, the total length of the lower jaw, and the length of the jaw exclusive of the antemolar region. Many Placentals also show the same relationships.
  • 6The Paurodontidae atnd the Zalambdodont Insectivores (except Solenodon) differ from all the other mammals measured in the relative shortness of the molar series. It is suggested that this may indicate affinity and that the Zalambdodonts are not closely related to the other Insectivores, but retain many more primitive features.
  • 7The total length of the molar series appears not to depend on the number of molars present.
  • 8A suggested scheme of homologies between upper and lower molar cusps is presented, bearing out in principle that put forward in previous papers.
  • 9Three grades of mammalian dental evolution are postulated: (1) similarity of upper and lower patterns (Triconodonta and Symmetrodonta); (2) presence of three-rooted upper molars and two-rooted lower molars; presence of a metaconid on the lower molars; formation of a protocone and buccal cusps (Pantotheria, Zalambdodonts); (3) development of the talonid and enlargement of the metacone (typical Marsupials and Placentals).
  • 10Support is given to the view that anterior teeth represent an earlier stage of evolution than more posterior teeth in the same series.
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Ancillary