Microbial mats in dinosaur ichnocoenoses

ABSTRACT Until now, the significance of microbial mats in preservation of dinosaur tracks and in reconstructing the palaeoenvironment in which dinosaurs roamed was rarely studied. Dinosaur tracks are commonly found close to ancient aquatic bodies where moist sediment had once allowed footstep registration. At such sites, epibenthic or endobenthic microbial mats may have developed as well and may have led to exquisite track preservation. Microbial mats in coastal, lacustrine or flood plain environments cause ‘microbially induced sedimentary structures—MISS’, which are micobialites found in sandy or evaporitic settings. The microbial structures record environmental parameters including meteorological conditions that existed at the time of burial. The aim of this contribution is to give an overview on track morphologies and track distribution coinciding with ancient mat type and palaeoenvironmental situation. In concert, this information assembles a set of palaeoenvironmental and taphonomic parameters characteristic for lagerstätten of well‐preserved tracks. Twelve Jurassic and Cretaceous track sites were investigated, all located in Utah and Colorado, United States. A great variety of MISS including ‘multidirectional ripple marks’, ‘mat chips’, ‘erosional remnants and pockets’, ‘petees’ and others document the occurrence of endobenthic and epibenthic microbial mats once developed in coastal settings of semi‐arid tropical and humid tropical palaeoclimates. Tattered mat surfaces document degradation of microbial mats at the end of the ancient growth season. Epibenthic mat types include planar, spongy and petee mats. These epibenthic mats as well as the infield view less pronounced endobenthic mats were either healthy or desiccated by the time of burial. The type of microbial mat and its condition during footprint registration affected the morphologies of the tracks. Track morphologies include deep footprints with only few anatomical details preserved in ancient, epibenthic microbial mats of planar and of spongy types. Moist mats offered a better medium for registration than dry, brittle mats. Petee microbial mats, originally formed in sabkha settings, were thin and did not allow a well‐contoured footprint to form. Surprisingly, fossil endobenthic microbial mats display relatively well‐preserved tracks. Degrading of the ancient microbial mats was accompanied by the release of mat chips, disintegrating a good part of track information. Pedestal tracks are the result of erosion around tracks that were protected by a microbial mat filling in the footprints. A variety of undertracks were found that allow conclusions on the water content of the buried fossil microbial mats. The track morphologies offer a wider spectrum of information on environment and track producers than tracks preserved in pure sand substrates would. A short exposure time of the ancient surface limited postdepositional disturbance of tracks by weathering, continuous mat growth, baffling and trapping or subsequent dinoturbation. Minimum exposure times ranged from a few hours or days (duration of development of endobenthic mats) to a few months (duration of development of epibenthic mats). These palaeoenvironmental and lithological characteristics of the studied dinosaur track sites may be representative of many exquisite track sites and may assist the future detection of tracks in palaeontological field survey.