New thylacocephalans from the Cretaceous Lagerstätten of Lebanon

Abstract

Thylacocephalans (Euarthropoda, Thylacocephala) are characterized by their “bivalved” carapace and three anterior prehensile appendages. It is still not clear how they used to live, or what their evolutionary history is. This study focuses on new thylacocephalans from the Late Cretaceous Konservat-Lagerstatten of Lebanon, which yielded the youngest representatives of the group. Three new genera and species are described in the Cenomanian sublithographic limestones of Hakel and Hadjoula, and two new genera and one new species are described in the Santonian chalky limestones of Sahel Alma. Among the specimens from Hakel and Hadjoula, Paradollocaris vannieri , Thylacocaris schrami and Globulacaris garassinoi are the first reports of thylacocephalans in the Cenomanian of Lebanon. Paradollocaris and Thylacocaris are assigned to Dollocarididae based upon their large optic notches limited by rostral and antero-ventral processes, their hypertrophied eyes, and their posterior notches with dorsal and ventral spines. Moreover, Thylacocaris presents a very peculiar character: an optic notch with two strong optic spines protecting the eye. Globulocaris is assigned to Protozoeidae based upon its small carapace with a distinct dorsal notch anterior to a strong postero-dorsal spine. Among the specimens from Sahel Alma, Keelicaris deborae is a new form of thylacocephalans in the Santonian of Lebanon. It presents a very unusual keel-shaped carapace with terraces and punctuations, and is assigned to Microcarididae. The new genus Hamaticaris , presenting a very peculiar hooked rostrum, is also erected for Protozoea damesi Roger, 1946 (Roger J. 1946. Invertebres des couches a poissons du Cretace superieur du Liban. Memoires de la Societe geologique de France (Nouvelle serie) 51: 5–92). These two species add to the well-known thylacocephalans from Sahel Alma: Pseuderichtus cretaceus Dames, 1886 (Dames W. 1886. Ueber einige Crustaceen aus den Kreideablagerungen des Libanon. Zeitschrift der Deutschen Geologischen Gesellschaft 38: 551–575), Protozoea hilgendorfi Dames, 1886 and Thylacocephalus cymolopos Lange et al. , 2001 (Lange S, Hof CHJ, Schram FR, Steeman FA. 2001. New genus and species from the Cretaceous of Lebanon links the Thylacocephala to the Crustacea. Palaeontology 44 (5): 905–912). The occurrence of such diverse fauna of thylacocephalans markedly increases the diversity of the group during the Late Cretaceous. The diversity and abundance of the Sahel Alma thylacocephalans pose also the problem of causes of their disappearance from the fossil record after the Santonian.