Important: A new version of the identification key and of the knowledge base is available

The identification key is here http://archaeocyatha.identificationkey.org/mkey.html
The knowledge base is now updated here http://xper3.fr/xper3GeneratedFiles/publish/html/7713921382789369737/

Introduction

Archaeocyaths are exclusively Cambrian fossils. They built the first metazoans bioconstructions. Since the discovery of living aspiculate sponges, their systematic position has been controversial but now a consensus has been found: archaeocyaths represent an extinct class of the phylum Porifera, closed to the Demospongiae. (Debrenne & Vacelet 1984).
The archaeocyathan skeleton is often composed of two inverted porous cones, fitting into each other, interpreted as outer and inner walls delimiting the intervallum. Vertical radial elements (septa, taeniae...) and/or horizontal elements (tabulae) connect the two walls.

Figure 1: Stylized archaeocyathan skeleton (DEBRENNE 1964, modified)

The archaeocyathan cups display various architectural types: one-walled conical, single-chambered subspherical, multi-chambered conical (thalamid), chaetetid, and syringoid.

Figure 2: Different architecture of archaeocyathan cups
(DEBRENNE, ROZANOV & ZHURAVLEV 1990, DEBRENNE & ZHURAVLEV 1992b, modified
© Publications Scientifiques du Muséum National d'Histoire Naturelle, Paris)

They can be solitary or modular (pseudocolonial). Their skeleton is primarily made of globally polyhedral crystallites of high-magnesian calcite, probably the result of an organic matrix mediated process at a very primitive stage.

	SOLITARY
	MODULAR

Figure 3: Examples of solitary and modular archaeocyathan (A.KERNER)

Archaeocyathan systematics is based on skeletal ontogeny determining the order of appearance of skeletal elements, their complexification and the stabilization of adult features (Debrenne et al. 1990, Debrenne & Zhuravlev 1992b and Debrenne et al. 2002)

• Orders are characterized by the architecture of cup
• Suborders by the growth pattern models
• Superfamilies by the outer wall types
• Families by the inner wall types
• Genera by variations in walls and intervallar primary types and distribution of pores in each element
• Species by different numerical coefficients

The Class Archaeocyatha is composed by six orders and twelve Sub-orders. The previous subdivision into Regulares and Irregulares is often still used in biostratigraphy or paleoecology when archaeocyathan are concerned. They roughly correspond to Ajacicyathida and Coscinocyathida (ex-Regulares) and Archaeocyathida and Kazachstanicyathida (ex-Irregulares).