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An introduction to the palaeontology of rudist bivalves


Rudist are a group of bivalves which evolved during the Late Jurassic to Cretaceous and lived in warm, shallow oceans of low latitudes. They became extinct at the Cretaceous/Tertiary boundary. Most rudists have not much in common with 'normal' bivalves and developed bizarre, occasionally large shells.


Various morphologies of rudist shells (from Schumann & Steuber 1997)


Different to other bivalves, one or both valves are uncoiled which allowed for accretion of the shell along the complete mantle margin and the construction of tubular shells.


Progressive uncoiling in the evolution of rudists (from Skelton 1979)


According to their life habit, rudist morphotypes are classified as 'elevators', 'clingers' or 'recumbents', each morphotype being adapted to specific environmental conditions (type of sediment, sedimentation rate, current regime).


Rudist morphotypes (from Skelton 1991)

Life habits of different rudists which developed different morphotypes (from Skelton 1978)


Although rudists are sometimes considered as characteristic 'reef-builders' of the Cretaceous, several important differences exist between typical coral-algal-hydrozoan reefs and rudist formations. Rudist formations are typically of low relief and form more or less tabular bodies. Bound, wave-resistant fabrics were uncommon, and elevator morphotypes were supported by sediment which accumulated during their vertical shell growth. Consequently, growth fabrics of rudist associations were constratal, in contrast to superstratal fabrics of modern coral reefs.


Superstratal (left) versus constratal (right) growth fabric (from Gili et al. 1995).


Comprehensive information about rudist formations are provided by, e.g., Gili et al. (1995), Höfling (1997), Kauffman & Sohl (1973), Philip (1984), Ross & Skelton (1993), Sanders & Pons (1999).


The evolution of rudist bivalves


The Upper Jurassic - lower Cretaceous Diceratidae are considered as the root stock of rudist bivalves. Uncoiling was only incipient, and some genera were attached with the left valve while other were attached with the right valve. The Requieniidae were particularly abundant in Barremian-Lower Aptian Urgonian-type carbonate platforms. They were attached with the left valve. The Caprotinidae and Polyconitidae, and all other, derived families were attached with the right valve. The Caprinidae lived mainly as recumbents and have a characteristic pattern of canals in one or both valves. Radiolitidae evolved since the Late Aptian to become the most species-rich family among the group. Ichthyosarcolitidae were particularly widespread during the Cenomanian, and have been rarely recorded from older sediments. The Cenomanian/Turonian transition was critical in the evolution of rudist. The Caprinidae and Ichthyosarcolitidae became extinct, and the Hippuritidae which dominated many Late Cretaceous rudist formations occurred for the first time. Plagioptychidae and Antillocaprinidae are superficially similar to Caprinidae but differ in the arrangement of myophores, supporting the adductor muscles. The Antillocaprinidae are restricted to the Americas and the Caribbean, and the Dictyoptychidae are only known from the latest Cretaceous of the northeastern Afro-Arabian plate. The extinction of rudist bivalves is hypothesised to have been stepwise during the Maastrichtian, but the exact timing and pattern of extinction during the latest Cretaceous is still enigmatic.


A simple phylogeny of rudist families (from Skelton 1978)


Cumulative plot of rudist species records in the central-eastern Mediterranean and Middle East as reported in the literature. This plot shows, to some extent, the relative abundance of various rudist families. Black and red lines delineate numbers of species originations and extinctions per stage (see Steuber & Löser 2000 for a detailed discussion of these patterns).

Cumulative plot of rudist species records in the Americas as reported in the literature.


Most important for the distinction of rudist families is the myocardinal arrangement and the structure of inner and outer shell layers. Shells structures and ornaments are important for the subdivision of families, genera and species.


Arrangement of adductor attachment in some rudist genera; mp = posterior myophore, ma = anterior myophore (from Steuber 1999)


Tentative phylogeny of rudist bivales (Steuber 1999), a more detailed and rigorous cladistic analysis is provided by Skelton & Smith (2000). Derived characters: (1) left valve (LV) = attached valve; (2) thickened outer shell layer; (3) right valve (RV) = attached valve; (4) two cardinal teeth in LV, and one tooth, which is reduced in some taxa, in RV; (5) ligament invaginated; (6) inclined to erect lamina in RV, running from central tooth to posteroventral shell wall to demarcate omp’; (7) LV posterior myophore (mp) insertion surface facing outwards onto mp´ of RV, the latter on posterior shell wall; (8) LV mp insertion surface facing inwards onto outward facing mp´ of RV, the latter on erect myocardinal lamina; (9) thick outer shell layer in RV; (10) mp´ of RV on thickening of posterior shell (Polyconites), that may be raised to extend parallel to the commissure or is even slightly tilted backwards (Horiopleura); (11) canals in inner shell layer; (12) mp´ of RV tilted backwards, in tapering socket that receives elongated mp of LV; (13) pores and canals in LV outer shell layer; (14) cellular structure of outer shell layer in one or both valves. Phylogeny of the Caprinidae d´Orbigny after Skelton and Masse (1998); the Coalcomaninae Coogan include Retha Cox, and the Caprininae d´Orbigny include Pachytraga Paquier.


Suggested reading on the evolution of rudists: Kauffman & Johnson (1988), Masse & Philip (1986), Skelton (1978), Skelton & Smith (2000).

A wealth of information on all aspects of rudist palaeontology can be found in the proceedings volumes of the 'International Conferences on Rudist Bivalves':

·        Geologica Romana, 28: 372 pp.; Roma (1992).

·        Alencáster, G. & Buitrón-Sánchez, B.E.: (eds.), Number devoted to the Third international Conference on Rudists. -- Revista mexicana de Ciencias geológicas, 12/2 (for 1995): 316 pp.; Mexico City (1996).

·        Masse, J.-P. & Skelton, P.W.: (eds.), Quatrième Congrès international sur les Rudistes. -- Geobios, Mémoire spéciale, 22: 427 pp.; Lyon (1998).


A Bibliography is also available:

·        Steuber, T. & Löser, H. (1996): Jurassic-Cretaceous rudists (Mollusca, Hippuritacea) - Bibliography and data base 1758-1994.- Neue Paläontologische Abhandlungen, 1: 123 pp., Diskette; Dresden.