Some Cretaceous Echinoidea from Jamaica

Extract

(a) Palæontological.—A new genus of the Holectypoida, Metholectypus, is described. Its only known species, M. trechmanni sp.n., shows remarkable superficial similarity with Lanieria lanieri. It is argued that these two genera may be divergent end-forms of the Coenholectypus stock.

A large species of Heterosalenia (H. occidentalis sp.n.) gives occasion for discussion of the affinities of that genus. Arguments are put forward to show that Heterosalenia and Pseudosalenia may be composite genera, including occasional orthogenetic offshoots from a persistent stock (perhaps Hemicidaris) consisting of individual, or group, cases of an exaggeration of fundamental tendencies that may be accounted for on a Mendelian basis. A similar relation is believed to unite and disintegrate Salenia and Hyposalenia.

A new species of Botriopygus (B. rudistarum sp.n.) shows a very simple stage in the production of petals and phyllodes.

(b) Stratigraphical.—Since all the determinable species and one of the genera are considered to be new, exact comparison of faunas is impossible. This is the only crumb of comfort that I can give to Jamaican stratigraphers. They must make what use of it they can; but to a palæontologist it is almost immaterial. The two alternative published views as to the horizons of the three beds from which the Echinoids come are (i) Trechmann's opinion that the whole series is probably Maëstrichtian, and (ii) Hill's belief that the lower part is Cretaceous (presumably uppermost) and the upper part Eocene. In the light of the results derived from study of the Echinoids, these two views seem very little different, and both untenable.

The solitary specimen from the Barrettia beds (Metholectypus trechmanni) invites comparison with Lanieria from Cuba. The horizon of the latter is not known definitely, but is accepted as Cretaceous. The only other member of the family to which Lanieria belongs is Discholectypus, an Albian genus.

The specimens from the shales overlying the Barrettia beds and beneath the Rudist Limestones are Leiocidaris sp. and Hemiaster sp. The former is closely comparable with L. hemigranosus from an approximately Cenomanian horizon in Texas. The latter, whose preservation prevents satisfactory comparison, suggests affinity with Cenomanian or Turonian species of Hemiaster rather than with any from higher horizons.

The excellently represented species from the Rudist Limestone are Heterosalenia occidentalis and Botriopygus rudistarum spp.n. The latest previous record for Heterosalenia is from the base of the Senonian, and that represents by a long way the latest appearance of the family to which it belongs. Even if the explanation of the “genus” given above (sect, vi) be admitted, the fact remains that no form from which H. occidentalis could be derived is known from above the “Coniacian”. Botriopygus is chiefly developed in the Lower Cretaceous, but seems to range almost throughout the period. The simple structure of the new species points to a lower, rather than a higher, position.

This record of simple deduction obviously excludes all likelihood of a Tertiary date for any part of the sequence considered. It fails, equally, to indicate any specially high Cretaceous zones. Further, it can hardly be a coincidence that the stages suggested by the Echinoids are in conformity with the known sequence of the beds. The literal evidence is, then, that we have in these three members of the Jamaican Cretaceous system a time-range from the base of the Upper Cretaceous to about the base of the Senonian. Some latitude in interpretation may be allowed, but it does not seem that Mäestrichtian could be included, at least for the parts of the Rudist Limestone represented by the Echinoids.

This result came as a great surprise to me; and it will, I fear, give a shock to West Indian stratigraphers. But with the evidence available “I can do no other”. It is possible, of course, that the West Indies was a region to which belated forms retreated to die, so that these types were surviving long after their extinction elsewhere; but there is no existing authority for assuming this—I suggest it out of sympathy with those workers to whom my conclusions may be disconcerting.