New insights into endolithic palaeocommunity development in mobile hard substrate using CT imaging of bioeroded clasts from the Pliocene (Almería, SE Spain).

Bioeroded carbonate clasts from a Pliocene shallow-marine succession of Almería (SE Spain, Betic Cordillera) were analysed with computed tomography (CT). This revealed the detailed 3D architecture of bioerosion structures hidden within and allowed for their ichnotaxonomic identification (14 ichnospecies of 5 ichnogenera) and quantification. Borings are produced by worms, mostly polychaetes and sipunculids dominated, followed by bivalves and lastly by sponges. The crosscutting relationship between the borings and their preservation characteristics points to a complex colonization history of the clasts with repeated bioerosive episodes interrupted by physical disturbances, including overturning and abrasion of the clasts followed by their recolonization. Our findings facilitated paleoenvironmental interpretation and can be compared to analogous modern-day ecological succession. The sharp dominance of worm borings - early successional species - may be related to frequent, periodic, physical disturbance that possibly prevented the cobble-dwelling macroboring community from being overtaken by sponges - late successional taxa. CT, hand sample and petrographic observations detected, aside from borings, other irregularly shaped pores which are interpreted to be generated by diagenetic processes including dolomitization, silicification and dissolution, representing an intraparticle moldic and moldic enlarged porosity. Boring porosity crosscutting the diagenetically altered grains suggests the later occurrence of bioerosion processes. Irregular shapes ranging from roughly spherical, elongate sub-polyhedral to amoeboid resemble morphologies produced by modern sponges. Moldic pores possibly acted as primary domiciles for boring sponges, which infested, altered and enlarged pre-existing pores as they grew (as happens in the modern), providing an example of how biological and non-biological processes interacted and together influenced endolithic palaeocommunity development.

Intermittent and temporally variable bioturbation by some terrestrial invertebrates: implications for ichnology.

Bedding planes and vertical sections of many sedimentary rock formations reveal bioturbation structures, including burrows, produced by diverse animal taxa at different rates and durations. These variables are not directly measurable in the fossil record, but neoichnological observations and experiments provide informative analogues. Comparable to marine invertebrates from many phyla, a captive beetle larva burrowing over 2 weeks showed high rates of sediment disturbance within the first 100 h but slower rates afterwards. Tunnelling by earthworms and adult dung beetles is also inconstant-displacement of lithic material alternates with organic matter displacement, often driven by food availability with more locomotion when hungry. High rates of bioturbation, as with locomotion generally, result from internal and external drives, slowing down or stopping when needs are filled. Like other processes affecting sediment deposition and erosion, rates can drastically differ based on measured timescale, with short bursts of activity followed by hiatuses, concentrated in various seasons and ontogenetic stages for particular species. Assumptions of constant velocities within movement paths, left as traces afterward, may not apply in many cases. Arguments about energetic efficiency or optimal foraging based on ichnofossils have often overlooked these and related issues. Single bioturbation rates from short-term experiments in captivity may not be comparable to rates measured at an ecosystem level over a year or generalized across multiple time scales where conditions differ even for the same species. Neoichnological work, with an understanding of lifetime variabilities in bioturbation and their drivers, helps connect ichnology with behavioural biology and movement ecology.