The first non-marine ostracod fauna from the Lower Barremian dysodiles of Lebanon

Abstract

Dysodiles are finely laminated sedimentary rocks rich in organic matter and in exceptionally well-preserved fossils, hence their petroleum and palaeontological interest. However, few studies focused on the dysodiles of Lebanon, leaving their palaeontological content and their depositional environment largely unknown. Our previous investigations show a variety of well-preserved lacustrine fossils including ichthyofauna, chelonians, insects, plant debris, gastropods, palynomorphs and ostracods. The present study focuses on the ostracods of five sampling localities from the Lower Barremian of Lebanon. Being the most abundant fossils, ostracods raise some taphonomic and palaeoecological questions that may help us understand the depositional environment and enrich the story of these Early Barremian lakes. Faunal differences between the sampled sections provide clues to the specific characteristics of each lake. The abundance of 'butterfly' preservation of carapaces, together with the association of adults and juveniles, indicates calm depositional environments without post-mortem transport. The assemblages comprise ostracods with known swimming ability (Cyprois and Zonocypris) or that are thought to have been swimmers (Cypridea), while non-swimmers are absent, suggesting that nektobenthonic ostracods must have lived only in the shallow margins of the lakes or among floating vegetation because hypolimnion conditions were hostile to benthos. This study provides different scenarios on how this fauna ended up preserved in the laminated lake sediments. □ Black shales, Cretaceous, lake, micropaleontology, palaeoenvironment, taphonomy.

Dysodiles from the lower Barremian of Lebanon: Insights on the fossil assemblages and the depositional environment reconstruction

“…Where ostracods are so abundant as to cover the entire surface of a layer, we consider this to be possible evidence of mass mortality (El Hajj et al, 2020), but no other fossils are visible in these layers.…”

“…Among these, we could identify three genera: Cypridea Bosquet, 1852 represented by four putative species, Cyprois Zenker, 1854 and Zonocypris Müller, 1898. These ostracods and their paleoenvironmental implications are detailed in a separate publication (El Hajj et al, 2020).…”

“…Historically, Lebanon is paleontologically famous for its Late Cretaceous (Cenomanian) marine carbonate beds yielding abundant and exquisitely preserved fishes (Forey et al, 2003;Gayet et al, 2003;Kachacha et al, 2017), and its numerous Early Cretaceous amber outcrops with biological inclusions (Azar, 2007;Azar et al, 2010). The recent re-/discovery of unusual Lower Cretaceous continental sediments (dysodiles) brings important and exceptional paleontological assets (El Hajj et al, 2019;Azar et al, 2019;El Hajj et al, 2020). Dysodiles are sedimentary rocks characterized by finely laminated (micrometric) layers, high organic matter content (Cordier, 1808) and well-preserved fossils (Fraas, 1878).…”

“…Then, since the 1930s, dysodiles were forgotten until their recent rediscovery through our extensive geological fieldwork which resulted in the finding of seven localities, five of them in the lower Barremian and two in the Albian. Recently, in addition to their petroleum interest and source rock potential (El Hajj et al, 2019), the ostracod fauna found within the dysodile layers was identified and its contribution to understanding the paleoenvironment was discussed (El Hajj et al, 2020). However, these dysodiles certainly deserve multidisciplinary approaches in order to reveal and understand their depositional conditions, their relationship with environmental regional context and their continental paleobiodiversity.…”

“…However, these dysodiles certainly deserve multidisciplinary approaches in order to reveal and understand their depositional conditions, their relationship with environmental regional context and their continental paleobiodiversity. Besides the ostracods identified from five dysodile sampling localities (El Hajj et al, 2020), only fossiliferous amber [with biological inclusions (mainly insects and other terrestrial arthropods) (Azar et al, 2010;Azar, 2012;Maksoud & Azar, 2020)] and lignite have provided insights on the terrestrial flora and fauna during the early Barremian of Lebanon (Azar et al, 2010;Azar, 2012;Maksoud & Azar, 2020). Thus, the study of the fossil assemblages found in the dysodiles will significantly increase our knowledge of these Early Cretaceous lacustrine environments and their biodiversity.…”