The invasion of deserts: an ichnological perspective

Abstract

The transition from water to land and the consequent invasion of terrestrial ecosystems encompass some of the most significant events in the history of life. Among all terrestrial settings, deserts are one of the most extreme environments on Earth to be conquered, being hot and dry with greatly fluctuating surface temperatures. Consequently, deserts were one of the most difficult environments to be conquered by multicellular life, and possibly the last one to be colonized. Surprisingly, the invasion of deserts is one of the only events of the terrestrialization process that has not been studied systematically. By the elaboration of a comprehensive database that summarizes all life known in eolian deposits from the Cambrian to the Recent, we propose a summary of colonization trends and evolutionary turnovers in deserts. The first phase in the colonization of deserts coincides with the initial phases of terrestrialization during the early Paleozoic (Middle Cambrian-Silurian), and involves incursions into adjacent coastal dunefields by nearshore marine animals (phase 1). It is documented by large trackways produced by amphibious arthropods, although it is unlikely that these animals would have remained for long periods of time in coastal deserts. The colonization of inland deserts began as early as the Middle Devonian, involving fluvial-habitat animals that temporarily or permanently entered inland desert dunefields (phase 2). Among the earliest true inland desert inhabitants were stem-group arachnids or arachnids, together with myriapods. Currently, there is no evidence of plant colonization of deserts in Devonian times. Winds may have brought small detrital particles that served as a food resource for these early desert inhabitants. The first steps in the invasion of deserts by plants dates to the Middle Pennsylvanian when plants colonized stabilized deflation surfaces within coastal deserts, having been apparently incapable of colonizing and stabilizing mobile eolian dunes. Tetrapods are generally considered to have colonized most land habitats and diversified into many new modes of life further away from the water during the Carboniferous. Presumably, by the Early Permian tetrapods were present in desert on a global scale (phase 3). The ichnogenus Chelichnus is recorded in all desert ichnoassemblages with tetrapod footprints, and it is essentially the only ichnotaxon present. During the Mesozoic inland deserts experienced a major exploitation of the infaunal ecospace as reflected by the appearance of more varied behavioral patterns of sub-superficial structures (phase 4). Finally, during the Neogene modern desert communities were established (phase 5).