New adherent foraminiferans from the Lower Cretaceous (Aptian) of south-central England

Three new species and one new genus of adherent foraminiferans have been found within bivalve borings in cobbles from the Faringdon Sponge-gravel (Upper Aptian) of south-central England. The new genus and species, Lapillincola faringdonensis is a uniserial textulariine with an initial planispiral coil and a multiple aperture. Lapillincola gen. nov. is a remarkable homeomorph of Arenonina Barnard, which was originally described as an agglutinated form. Arenonina is shown here to actually be calcareous and perforate, and thus a junior synonym of Rectocibicides Cushman & Ponton. Acruliammina parvispira sp. nov. is another uniserial textulariine with an initial coil. Bullopora ramosa sp. nov. is an adherent polymorphinid with a branching, irregular series of adherent chambers. These foraminiferans may have used the bivalve borings as spatial refuges from the abrasive, high energy environment of the Sponge-gravel.


INTRODUCTION
An unusual adherent foraminiferan fauna was found inside bivalve borings in calcareous claystone cobbles from the top 3m of the Lower Cretaceous Faringdon Sponge-gravel, Faringdon, Oxfordshire, England. One new genus and three new species are described from this collection. The genus Arenonina Barnard (1958) is morphologically similar to the new genus Lapillincola, but re-examination of the type specimen of Arenonina has shown it to be calcareous perforate and not agglutinated as originally described. Arenonina is redescribed here as a junior synonym of Rectocibicides Cushman & Ponton (1932) and a remarkable homeomorph of Lapillincola gen. nov.

STRATIGRAPHY AND LOCATION
The adherent foraminiferans were collected from the "Red gravel" (Meyer, 1864) of the Faringdon Spongegravel, 5.5-8.5m above the disconformable contact with Oxfordian limestones of the Corallian Group, exposed in the Wicklesham gravel pit at the southeast edge of Faringdon, Oxfordshire (nat. grid ref. SU 293943; lat. 51" 41' 2 0 N, long. 1" 34' 45" W; Fig. 1). This gravel has been placed within the Parahoplites nutfieldensis Zone of the Upper Aptian (see Casey, 1961 for the detailed stratigraphy).

MODE OF OCCURRENCE
The foraminiferans are found encrusting the interiors of flask-shaped borings of the ichnogenus Gastrochaenolites Leymarie, 1842. These borings, ranging in size from 2-20mm at widest diameter and 4-40mm at greatest depth, are abundant in rounded calcareous claystone cobbles. One cobble contained the ammonite Prorasenia bowerbanki (kindly identified by H. G. Owen) within its matrix, indicating that some of the cobbles, at least, were originally derived from the lower Kimmeridge Clay or the Upper Oxfordian (Upper Jurassic). These cobbles were rolled periodically, as demonstrated by their rounding and the distribution of borings and encrusters on all outside surfaces.
Approximately 40 additional encrusting and nestling species are also found in the Faringdon Gastrochaenolites borings. They include bryozoans, serpulids, brachiopods, bivalves and sponges. This fauna is described separately (Wilson, 1986).

SYSTEMATIC DESCRIPTIONS
Description. See Barnard (1958, p. 118) for a full description of the species, noting that the wall is now know to be calcareous and perforate and not arenaceous (see below). The drawing of Arenonina cretuceu accompanying the original description (Barnard, 1958, text- fig. 1) is also inaccurate. The broken portion of the last-formed chamber is drawn as a large apertural pore, and where the drawing shows 10 undamaged apertural pores there are actually 17. Barnard's (1958, text- fig. 1) drawing also shows deep sutures on the initial coil, which are not visible on the type specimen. A photograph of the A. cretacea paratype (Barnard, 1958, P1. 22, fig. 2) shows what may be deeper sutures on the initial coil. This specimen was apparently never deposited in a museum collection, so it was unavailable for study. The photograph of the paratype is poor; because it shows none of the characteristic features of the genus, such as spreading chambers or a multiple aperture, it may represent a separate form. The drawing of A. cretacea produced by Barnard (1958, text- fig. 1) was reproduced in the Treatise (Loeblich & Tappan, 1964, fig. 160) as the illustration of Arenonina. Remarks. Barnard (1958, p. 118) erected the genus Arenonina to separate an undescribed Upper Cretaceous foraminiferan from Bdelloidina, to which it had a "superficial external resemblance". He did not place Arenonina in a family, simply listing it as one of a group of "forms with agglutinated tests" (Barnard, 1958, p. 116). Re-examination of the holotype of A. cretacea with the aid of a scanning electron microscope showed that the test is calcareous, coarsely perforate and not "arenaceous" . Because it also possesses a trochospiral initial coil and a multiple aperture on the terminal face, Arenonina is a junior synonym of Rectocibicides Cushman & Ponton (1932), previously described from the Miocene of Florida. R. cretacea differs from R. miocenicus Cushman & Ponton (1932) by the very shallow sutures on the initial coil, the more parallel septa in the uniserial portion of the test, and the reduced apertural lips.
Rectocibicides cretacea is not found in the Faringdon Sponge-gravels, but it is an impressive calcareous homeomorph of the agglutinated Lapillincola faringdonensis gen. et sp. nov. described below (Pl. 1, fig. 4).
Both tests possess an initial coil with very shallow sutures followed by a uniserial portion with spreading chambers and a multiple aperture consisting of a row of pores.  Fig. 1). Description. Test entirely adherent and agglutinated, consisting of coarse, angular grains in a calcareous matrix. Interior walls are simple and not labyrinthic. The initial planispiral coil is one or two whorls, with five to ten chambers in each whorl; sutures are very shallow to indistinct within each whorl. Occasionally an elliptical scar is present near the centre of the coil, possibly indicating the presence of a bulbous proloculus. The uniserial portion of the test contains 10-15 chambers, each delimited by distinct, usually arcuate, sutures. The uniserial chambers increased in height slowly with growth, but chamber width increased much faster, producing a flabellate or spatulate test. The aperture is known only from the uniserial chambers; it is a single row of pores extending across the face of the last-formed chamber parallel to the attachment surface. These pores, numbering in a single aperture from five to eight, are simple openings without lips or associated crenulations. The apertural pore row is usually midway between the attachment surface and the centre of the apertural face. Dimensions. Holotype length = 5.09mm.; diameter of initial coil = 0.81mm.; width of last-formed uniserial chamber = 1.05mm. Derivation of name. From the Latin, parvus (small) and spira (coil).

Diagnosis.
Acruliarnmina with a small initial planispiral coil (diameter less than half the width of the mature apertural face).
Holotype. British Museum (Natural History) no. P 5 1737. Material. 7 specimens. Locality and horizon. As above. Description. Test adherent for all or most of its length, agglutinated, walls consisting of fine to coarse grains in a calcareous matrix. Interior walls are simple. The initial planispiral coil is one or two whorls with 7-10 chambers per whorl; sutures are moderately deep and distinct. A bulbous proloculus is present in some specimens. The initial coil is followed by an irregularly winding series of chambers with distinct sutures; chamber length and width increase in approximately equal proportion. The distal portion of the test may have grown free from its attachment after 10 to 20 adherent uniserial chambers were formed. The aperture consists of two pores on the distal face of the early uniserial chambers. In more mature forms the apertural face has three or four pores arranged in a semi-circle or complete circlet. Dimensions. Holotype length = 4.21mm; width of initial coil = 0.33mm; width of apertural face = 0.87mm.

Remarks.
A . parvispira has a much smaller initial coil than A . longa (Tappan, 1940) or A. serpula (Tappan, 1943) of the North American midwestern Cretaceous.
A. longa has a coil almost twice as wide as the mature apertural face, whereas the diameter of the A. parvispira coil is usually less than half the width of the mature apertural 'face.  (Sollas, 1877) develop irregular tubular chambers (as described by Adams, 1962), and B. tuberculata (Sollas, 1877) has a similar tuberculated exterior.

DISCUSSION
These Faringdon foraminiferans are found in an unusual environment: within the vacated borings of bivalves on loose calcareous claystone cobbles. Most adherent foraminiferans are described from encrusted shells, but a few taxa have been previously noted on hardgrounds (Voigt, 1970;Kaever, 1974;Palmer, 1982). Fossil foraminiferans have been described from within borings, most notably from the Cretaceous of the Netherlands (Hofker, 1965) and the British Jurassic (Hitchings, 1980). Indeterminate calcareous foraminiferans were noted on a loose (or "mobile") hardground of Upper Carboniferous intraclasts by West & Palmer (1982), and foraminiferans have long been described encrusting Jurassic and Cretaceous phosphatic nodules in Europe (Chapman, 1892;Kennedy & Garrison, 1975). This paper, however, records the first occurrence of foraminiferans within borings in a "mobile hardground" (Wilson, 1985) of loose cobbles.
The Faringdon Sponge-gravel was deposited in narrow tidal channels during an Early Cretaceous transgression (Bridges, 1982). The gravel, derived from a variety of pre-Cretaceous units, was continually reworked in its upper layers (Krantz, 1972). Adherent  foraminiferans, with their relatively fragile shells, would have had difficulty surviving on the external surfaces of shells or cobbles under these abrasive conditions. The borings, though, provided a spatial refuge , protecting them from the physical dangers in the loose gravel and from most, if not all, epifaunal predators. Many of the borings intersected other cavities, so there was usually the possibility of flowthrough water currents, even within the centres of the cobbles.