Ichnofossils – Trace fossils Fossilized Bahavior
There is no branch of detective science so important and so much neglected as the art of tracing footsteps. Sherlock Holmes Sir Conan Doyle, A Study in Scarlet, 1801
Biogenic sedimentary structures or ichnofossils (Greek ichnos, trace)
Ichnofossils are signs of animal activities preserved in rocks. • Ichnology: study of ichnofossils, involves identifying the activity and making deduction about the animal resposible for it. • Trace fossils made skeletal and non- skeletal organisms.
Trace fossils: represent behavioral response of organisms, and are controlled by the environments, substrate consistency, rates of sedimentation, temperature and salinity. • PALEOENVIRONMENTAL RECONSTRUCTION: some ichnofossils are restriced to particular environment
What are ichnofossils? • Locomotion (footprints and crawling trails); Laetoli (Tanzania) some 3.6 to 3.75 my ago
Feeding process (tooth marks, grazing furrows, mining burrows, stomach contents, gastroliths);
Classification • Adopted system of giving each trace fossil a Linne’s taxonomic name complete with genus and species. Based upon morphology of the respective biogenetic structure. • There are no ichnofamilies and higher category.
Because one organism can produce several different types of traces, and several different organisms may leave the same trace and the same activity can produce different traces within different substrate this name gives a misleading connotation
One organisms leaves different traces • Same activity done on different substrates produce different traces (Stratinomic classification)
The same behavior of the different organisms can leave the indentical traces. • Comensalisms of two or more organisms leabves only one trace.
Ichnofossil’s categories • On the sediment surface (tracks and trails) • In the sediment (signs of dwelling, feeding and locomotion)
Ichnofossils and etholological classification • Seilacher (1953) : relation between traces morphologies and animal’s behavior • Behavioral categories are:
Cubichnia – resting traces, impressions caused when the animal interrupted its locomotion for rest and refuge.
Domichnia – dwelling traces such as deep elongate burrows or excavations that served as long term residence of the animal. • Repichnia – moving traces (crawling motion). They are continous, elongated trails with delicate marks from the leg motions.
Agrichnia – farming traces. Regularly patterned burrow system reflecting permanent dwelling & feeding behavior. • Fodichnia – deposit feeding traces formed when orgainsm made 3D burrows, eating the sediment to digest out all of the food within it.
Ichnofossils categories: behaviorlogy • Fugichnia – signs of forced escape (9) • Pasichnia – grazing traces (3)¸ horizontal pattern showing that organism was systematically combing the surface
Marine trace fossils • Certain types of trace fossils are related to certain types of sedimentary environments and depth condition (four regions).
Marine ichnofacies: Intertidal zone • High energy zone with a variety of substrates ranging from rocky foreshore and sheets of well sorted shifting sands to more sheltered bays where muddy snads and silts occur.
Skolithos (Sk) Ichnofacies • infaunal filter-feeders living in burrows, vertical or U shaped. • Skolithos: unknown organisms that lived in rapidly moving water and shifting sands made vertical tubes resamble to organ pipe (Cm – K) • Ophiomorpha: P- rec. (crabs) vertical burrows with bumpy outer surface, pellets that lined the burrow are used to reinforce the walls • Diplocraterion
Subtidal zone • Region of moderate to low energy, that passes to a region unaffecetd by storms and merges into the bathyal zone.
Cruziana (Cr) • Cruziana: Trilobites crawling trails (two parallel grooves separated by median ridge, common Cm-P) • Thalassinoides: vertical and oblique feeding burrows made by crustaceans • Asteriacites: resting traces • Rhizocorallium: U shaped burrow with horizontal attitude (feeding burrow)
Cruziana ichnofacies: the most diverse ichnofossil communities, Repinchia (Cruziana, Aulichnites), Cubichnia (Asteriacites), Fodichnia and Domichia (vertical burrows Thalassoinoides)
Asteriacites • Cubichnia of sea-stars
Thalassinoides (Fodichnia and domichnia) • Complex 3D network of cylindrical burrows that form an irregular web of crisscrossing tubes.
Rhizocorallum • Animal moved horizontally through the sediment in a systematic feeding pattern.
Bathyal: Zoophycos (Z) • Region (continental slope) of low energy (below storm wave base) of muddy fine sands, silts and muds rich in organic matter
In place the oxygene level may be low. Depost-feeders graze the surface or mine along shallow tunnels parallel to the substrate
Zoophycos ichnofacies indicates lowered oxygen levels and abundant organic material in the sediment in quiet water settings. • Zoophycos is not depth indicators (occasionally occurs in the Cruziana and Nereites ichno-region)
Abyssal: Nereites (N) • Deposits are very fine (pelagic) muds associated with turbidites
Condition are though at times disturbed by turbidity currents. • It is dark, pressure is high, uniformly cold. Scavengers and deposit fedders live there. • Traces include crawling, grazing and shllow feeding-cum-dwelling traces
Nereites ichnofacies: horizontal burrows in the muddy bottom • Nereites burrows are elaborate and complex in design. • They indicate a systematic searching for food. • Meandering (Nereites, Neonereites, Helminthoides), spiral (Spirorphaphe), honeycomb (Paleodictyon) structures, and dendritic systems are present.
Bioturbation and bioerosion • Bioturbation: biogenic activities in sediments depend on rate of sedimentation, rate of erosion, population density of organisms, degree of physical energy, supply of nutrients… • Epiliths vs. Endoliths • Golubić (1981): active and passive bioeroders and, those that inhabit natural cavities • Endoliths had a great success during the Mesozoic