Trace fossils. Trace fossils, also called ichnofossils, meaning a trace or track, are structures preserved in sedimentary rocks that record biological activity. While we are most familiar with relatively spectacular, fossilized hard-part remains such as shells and bones known as body fossils, trace fossils are often less dramatic, but nonetheless very important. Strictly defined, trace fossils must reflect both the anatomy of their maker in some way, and be the result of behaviour. Sedimentary structures made by empty shells rolling along the sea floor are thus excluded as death marks, as are structures such as stromatolites that, although the result of behaviour, do not reflect the anatomy of their maker. Spun coccoons and spiders webs are considered to be trace fossils, as they are manipulated by their makers after secretion; egg cases, on the other hand, are not. Trace fossils include burrows such as Chondrites, borings, ichnites or footprints and track marks, Zoophycus feeding marks, trails such as Cruziana scratched by trilobites, coprolites, fossilized droppings and other gut-derived objects, and rhizoliths or rhizocretions, the fossil remains of roots. The study of trace remains is called ichnology, which is divided into paleoichnology, or the study of trace fossils, and neoichnology, the study of modern trace remains. The science of ichnology is quite challenging, as most trace remains cannot be positively assigned to a specific organism or even to a specific class of organisms. Furthermore, trace remains such as burrows can make the work for paleontologists and paleobiologists more difficult as they rework sediments, causing older strata to be mixed with younger ones. This can cause some confusion in interpretation, unless viewed in geologic context.
Microfossils. Microfossil is a descriptive term applied to fossilized plants and animals whose size is just at or below the level at which the fossil can be analyzed by the naked eye. A commonly applied cut-off point between micro and macro fossils is 1 mm, although this is only an approximate guide. Microfossils may either be complete or nearly complete organisms in themselves such as the marine plankters foraminifera and coccolithophores or component parts such as small teeth or spores of larger animals or plants. Microfossils are of critical importance as a reservoir of paleoclimate information, and are also commonly used by biostratigraphers to assist in the correlation of rock units.
Resin fossils. Fossil resin, colloquially called amber is a natural polymer found in many types of strata throughout the world, even the Arctic. The oldest fossil resin dates to the Triassic, though most dates to the Tertiary. The excretion of the resin by certain plants is thought to be an evolutionary adaptation for protection from insects and to seal wounds caused by damage elements. Fossil resin often contains other fossils called inclusions that were captured by the sticky resin. These include bacteria, fungi, other plants, and animals. Animal inclusions are usually small invertebrates, predominantly arthropods such as insects and spiders, and only extremely rarely a vertebrate such as a small lizard. Preservation of inclusions can be exquisite, including small fragments of DNA.
Pseudofossils. Pseudofossils are visual patterns in rocks that are produced by naturally occurring geologic processes rather than biologic processes. They can easily be mistaken for real fossils. Some pseudofossils, such as dendrites, are formed by naturally occurring fissures in the rock that get filled up by percolating minerals. Other types of pseudofossils are kidney ore, like the round shapes in iron ore and moss agates, which look like moss or plant leaves. Concretions, round or oval-shaped nodules found in some sedimentary strata, were once thought to be dinosaur eggs, and are often mistaken for fossils as well.
Living Fossil. Living fossil is a term used for any living species which closely resembles a species known from fossils, as if the fossil had come to life. This can be a species known only from fossils until living representatives were discovered, such as the coelacanth and the ginkgo tree, or a single living species with no close relatives, or a small group of closely related species with no other close relatives, such as the horseshoe crabs or the nautilus, that are the sole survivors of a once large and widespread group in the fossil record. There is a subtle difference between a living fossil and a Lazarus taxon. A Lazarus taxon is a taxon, either one species or a group of species that suddenly reappears, either in the fossil record or in nature, as if the fossil had come to life again, while a living fossil is a species that seemingly hasn’t changed during its very long lifetime, as if the fossil has always lived. The mean species turnover time, or the time a species lasts before it is replaced varies widely among the phyla, but averages about 2-3 million years. So, a living species that was thought to be extinct, like the coelacanth, Latimeria chalumnae is not a living fossil simply due to that definition though it may still be one because it hasn’t changed much, it is a Lazarus species. Coelacanths disappeared from the fossil record some 80 million years ago. If, however, other Cenozoic Latimeria fossil species were to be found, the coelacanth would be considered a true living fossil, as that would fill in the gap where the species is dead. Of course, species do not just appear out of thin air, so all living Lazarus species excluding disappearing and reappearing red list species are nonetheless considered living fossils, if it can be shown they are not Elvis taxa. Some living fossils are species that were known from fossils before living representatives were discovered. The most famous examples of this are the coelacanthiform fishes Latimeria chalumnae and Latimeria menadoensis and the dawn redwood, Metasequoia, discovered in a remote Chinese valley. Others include glypheoid lobsters, mymarommatid wasps, and jurodid beetles, all of which were first described from fossils, but later found alive with 2 species, 10 species, and one species respectively. Others are a single living species with no close living relatives, but which is the survivor of a large and widespread group in the fossil record, perhaps the best-known example of which is Ginkgo biloba, the ginkgo, though there are others, such as the Syntexis libocedrii, the cedar wood wasp. Note that just because a living fossil is a surviving representative of an archaic lineage does not necessarily require that it retains all of the primitive features or plesiomorphies of the lineage it is descended from that is, they may possess one to many derived features, that have evolved since the time of their lineage’s divergence. All that is required is that they can be unambiguously assigned to an otherwise extinct lineage rarely are they identical to the fossil forms.
Some of these are informally known as living fossils.
-Plants
Ginkgo tree (Ginkgoaceae)
Horsetails Equisetum (Equisetaceae)
Metasequoia Dawn Redwood (Cupressaceae; a borderline example, related to Sequoia and Sequoiadendron)
Sciadopitys tree (Sciadopityaceae)
Whisk ferns Psilotum (Psilotaceae)
Welwitschia (Welwitschiaceae)
Wollemia tree (Araucariaceae; a borderline example, related to Agathis and Araucaria)
-Animals
• Vertebrates
Mammals
Cypriot mouse (Mus cypriacus)
Laotian Rock Rat (Laonastes aenigmamus)
Monotremes (the Platypus and echidnas)
Mountain Beaver (Aplodontia rufa)
Birds
Hoatzin (Ophisthocomus hoazin)
Reptiles
Crocodilia (Crocodiles, Gavials and Alligators)
Tuatara (Sphenodon punctatus and Sphenodon guntheri)
Fish
Coelacanth (Latimeria menadoensis and Latimeria chalumnae)
Queensland lungfish (Neoceratodus fosteri)
Sharks
Frilled shark (Chlamydoselachus anguineus)
Amphibians
Purple frog (Nasikabatrachus sahyadrensis)
• Invertebrates
Insects
Cedar wood wasp (Syntexis libocedrii)
Notiothauma reedi, a scorpionfly relative
Sikhotealinia zhiltzovae, a Jurodid beetle
Mymarommatid wasps (10 known species in genus Palaeomymar)
Parasitic wood wasps (about 70 species in 16 genera)
Peloridiid bugs (fewer than 30 species in 13 genera)
Crustaceans
Neoglyphea inopinata, N. neocaledonica, and Laurentaeglyphea neocaledonica ; three glypheoid lobsters
Triops cancriformis, a notostracid crustacean
Other invertebrates
Crinoids
Horseshoe crab (Limulus polyphemus)
Lingula anatina, an inarticulate brachiopod
Neopilina galateae, a monoplacophorid mollusc
Nut clam (Ennucula superba)
Onychophorans, for instance Peripatus
-Fungi
• Neolecta
