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-Lagerstätten 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. Invertébrés des couches à poissons du Crétacé supérieur du Liban. Mémoires de la Société géologique de France (Nouvelle série) 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.

New thylacocephalans from the Early Triassic Paris Biota (Bear Lake County, Idaho, USA)

“…UV fluorescence is obtained by illuminating the fossil by a "darklight" (blue to UV light). Under this kind of light, numerous fossil euarthropods, especially from limestone display a strong yellow autofluorescence (for instance see: Haug et al, 2009;Charbonnier et al, 2017). Pictures are digitally white-balanced to ease the visualisation of fluorescence.…”

“…The ventral notch is found in only two thylacocephalan genera (Parisicaris, Mayrocaris) and we do not known if this unusual structure is really a distinctive antero-ventral embayment or a well-developed mid ventral flap. Such ventral flap, but with an extreme development in that case, is observed in the very peculiar keel-shaped carapace of Keelicaris Teruzzi and Charbonnier in Charbonnier et al, 2017 from the Cretaceous of Lebanon (seeCharbonnier et al, 2017).…”

“…Thylacocephalans are among the most intriguing euarthropods of the Palaeozoic and Mesozoic. They have a stratigraphic range extension from the Silurian (Haug et al, 2014) and possibly earlier (Vannier et al, 2006: Cambrian) to the Late Cretaceous (Dames, 1886;Schram et al, 1999;Charbonnier et al, 2017), when they probably became extinct. They also show a high palaeobiogeographic distribution and are known from all continents except Antarctica and South America (Hegna et al, 2014).…”

First African thylacocephalans from the Famennian of Morocco and their role in Late Devonian food webs

“…Their first undisputed appearance in the fossil record dates back to the Silurian 1 , although some fossils of Early Cambrian age 2 may also belong to this group. Thylacocephalans lived until the Late Cretaceous period; the youngest fossils were found in the Cenomanian of Lebanon [3][4][5] .…”

“…Additionally, Haug et al points out that the presence of several enditic structures on the legs supports a position in the Eucrustacea 1 . Thylacocephalans from the Devonian are known to occur in a few places in Europe 5,[7][8][9] , Australia 10 and North America (e.g.) 11 .…”