Family HORNERIDAE Smitt, 1867

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Family HORNERIDAE Smitt, 1867

Compiler and date details

July 2001 - Dr Philip Bock

Introduction

Smitt (1867) established this family for the European genus Hornera Lamouroux. It was overlooked by Busk (1875) but adopted by Gregory (1899) and subsequent authors. Borg (1926) examined the morphology of two European species and H. antarctica Waters (1904) in detail, and expanded his concept of the family in describing the Antarctic species (Borg 1944). An extensive European Cretaceous fauna was reviewed by Brood (1972). Taylor & Jones (1933) examined the skeletal ultrastructure in two New Zealand species.

Hornerids develop massive, arborescent growths attached to hard substrata by an encrusting base. The branches divide repeatedly and irregularly, giving a complex three-dimensional structure. Autozooids originate from a basal budding zone, curving to open on a defined frontal surface; orifices are initially in regular quincuncial series, but the addition of zooids budded in later ontogeny disturbs this regularity. The brood chamber is a modified zooid, a gonozooid, with an inflated, porous distal portion situated on the basal (abfrontal) surface of the colony. Calcification is entirely cryptocystal, with a laminated structure, traversed by numerous pores, and thickens continuously through ontogeny. The type genus and type species are Mediterranean, but hornerids occur in shelf seas throughout the world. Harmer (1915) described and illustrated two species known from the Indo-Malayan region. One was Hornera spinigera Kirkpatrick (1888), originally reported from Mauritius; the other was H. caespitosa Busk (1875), originally reported from Cape Capricorn, Queensland, as well as from Tierra del Fuego. Borg (1944) noted that Busk's species belonged to two distinct taxa, and reserved the name H. caespitosa for the species from Queensland, which was listed first. MacGillivray described three species from Victoria, and illustrated two of them, H. foliacea and H. robusta, in detail in McCoy's Prodromus. Bock (1982) also figured H. foliacea, which has very large, fenestrate colonies, the branches being regularly joined by crossbars. H. robusta has smaller colonies, up to 50 mm in height, with no crossbars. H. ramosa MacGillivray (1887) is similar, but with narrower and less regular branches. Thus there are at least four species endemic to Australia; there may be a small number of additional undescribed species. Unpublished data shows that most records are from less than 100 metres depth, but rare records are from deeper water. MacGillivray (1895) also described a substantial Tertiary fauna from Victoria. Waters (1904) described H. antarctica from three stations in the Bellingshausen Sea from a depth range of 480 to 569 metres, and from temperatures of 0.8 - 0.9°C.

Fossil hornerids are quite frequent in Tertiary sediments of southern Australia; the earliest recorded hornerid is from the Early Cretaceous of France (Taylor 1993).

Diagnosis

Colony heavily calcified, sometimes massive, erect, arising from an encrusting base, strengthened by kenozooids. Branches three-dimensional, arborescent, sometimes anastomosing. All calcified walls are interior walls, with a coelomic space on the outer side. Zooids open on the frontal face, the gonozooid has a large, inflated portion on the basal (abfrontal) side of a branch. The ooeciostome is often placed laterally, and has a flaring orifice.