A quantitative study of variation within the mesozoic echinoid Nucleolites

Abstract

Bivariate (reduced major axes) and multivariate (discriminant functions, canonical variates, principal components, cluster analysis) computer techniques are used to analyse quantitative measurements on fifteen test dimensions of the Mesozoic cassiduloid echinoid Nucleolites and its probable living descendant Apatopygus recens. A modification to standard techniques is described for analysis of reduced major axes and the cluster analysis.

Twenty samples are analysed. They include nine species of Nucleolites from the Jurassic (N. amplus, N. burgundiae, N. elongatus, N. latiporus, N. micraulus, N. scutatus, N. woodwardii) of England and France and Cretaceous (N. rotundus, N. subquadratus) of France and North Africa.

The genotype, Nucleolites scutatus, is represented by seven samples. Significant infraspecific variation occurs primarily in size, shape and position of the periproct and in the length of the anal sulcus. A large periproct correlates with large sediment grain size.

No such differences are proved between three samples of N. latiporus ( = N. clunicularis auct.). Statistical differences between N. latiporus and N. scutatus are of the same order as between infraspecific samples of N. scutatus.

Samples of the seven remaining fossil species are compared to the genotype. Quantitatively, differences can be expressed in terms of two test variables, always including either the length of the periproct and/or its distance from the apical disc. The size of the periproct is facies dependent. Within the Jurassic species, the length of the anal sulcus is consistently shorter in the stratigraphically younger species. This trend is not continued in the Cretaceous species. Overall, the distance of the apical disc from the anterior of the test becomes consistently shorter in the stratigraphically younger of the nine species studied.

N. scutatus shows a similar range of variation to infra specific samples of Apatopygus recens from New Zealand. A. recens burrows completely in a coarse substrate and injests large substrate particles. N. scutatus probably did likewise. These habits are therefore of ancient origin in the Cassiduloida.