Brood chambers constructed from spines in fossil and Recent cheilostome bryozoans

Most cheilostome bryozoans brood their larvae in skeletal structures called ovicells which, in evolutionary terms, were derived from spines. Ovicells in the great majority of fossil and Recent cheilostomes, however, have lost all or most  traces  of  their  spinose  origin.  Here  we  review  the  occurrence  of  spinose  (including  costate)  brood  chambers in cheilostomes, investigating in detail 32 species belonging to ten genera among five families (Calloporidae, Monoporellidae, Macroporidae, Cribrilinidae and Tendridae). Spinose ovicells are moderately common in the Upper Cretaceous, where they are recorded in 28 species, and also occur in one Palaeocene, seven Eocene-Miocene and 11 Recent species. The most primitive cheilostome ovicells occur in mid-Cretaceous calloporids in which a group of mural spines belonging to the distal zooid were apparently bent towards the maternal zooid to form a cage-like structure for reception of the embryo. The bases of these spines were initially aligned in a distally concave row that later became straight, distally convex and finally horseshoe-shaped, affording progressively better protection for the developing embryo. We suggest that primitive monoporellids inherited from calloporid ancestors a distally concave arrangement of ovicell spine bases, while cribrilinids inherited a horseshoe-shaped arrangement. Important trends that can be recognized in early ovicell evolution include: (1) loss of basal spine articulation; (2) spine flattening; (3) closure of the gaps between spines; (4) reduction in spine number (through loss or fusion), and (5) development of a concave ovicell floor. The conventional ‘unipartite’ ovicells found in the majority of cheilostomes may have originated either by spine fusion, as seems likely in some cribrilinids, or through a progressive loss of spines via an intermediate stage, seen in some calloporids and in two monoporellids, where the ovicell comprises a large pair of flattened spines. The spinose ovicells of some monoporellids and macroporids subsequently evolved investments of hypostegal coelom that allowed secretion of a surface layer of cryptocystal calcification. Acanthostegous brood chambers characteristic of Tendridae apparently provide an example of independent evolution of spinose brooding structures. © 2005 The Natural History Museum, London, Zoological Journal of the Linnean Society, 2005, 144, 317−361.