A New Species of Polyconites from the Lower Aptian of Iberia and the Early Evolution of Polyconitid Rudists

Abstract

The main diagnostic character of polyconitid rudists is a distinctive ectomyophoral cavity inserted behind a reflexed posterior myophoral plate in the left valve. The only pre-Aptian Old World polyconitid taxon recognized in the current literature is Horiopleura dumortieri (Matheron): this species clearly shows the prominent posterior myophoral shelf in the right valve that is diagnostic of the genus, which continues into the Albian. Polyconites, by contrast, has a more depressed (operculiform) left valve and its posterior adductor was inserted on an inward-sloping swelling on the right valve inner wall, with no projecting shelf. Hitherto, the earliest known species of Polyconites was P. verneuili (Coquand), ranging from the Middle Aptian (Gargasian). However, smaller specimens (of similar size to H. dumortieri) from the uppermost Lower Aptian (Dufrenoyia furcata zone) of the Maestrat Basin of eastern Spain, together with similar though slightly older specimens from the southern Lusitanian Basin of Portugal show the relatively depressed left valve and myophoral configuration of Polyconites, to which genus we refer them as a new species, P. hadriani. Its similarity to P. verneuili suggests direct chronospecific descent of the latter, with phyletic size increase, as seen in many other rudist lineages. Recognition of the inception of this Polyconites lineage from the mid-Lower Aptian resolves the status of certain uppermost Lower Aptian polyconitids previously assigned to H. baylei but recognized as problematical. Moreover, we suggest that H. baylei (Coquand) and P. verneuili may be synonymous. The progressive depression of the left (free) valve and extension of the right (fixed) valve ventral margin during development in P. hadriani allowed upward growth-projection of the compressed ventral valve margins. This new mode of growth, relative to the antecedent Horiopleura, permitted imbricate close-packing of individuals, as in living flat oysters and epibyssate pteriaceans such as Isognomon, as well as the mid-Cretaceous Chondrodonta.