The Devonian is a geologic period and system of the Paleozoic era during the Phanerozoic eon, spanning 60.3 million years from the end of the preceding Silurian period at 419.2 million years ago (Ma), to the beginning of the succeeding Carboniferous period at 358.9 Ma. It is named after Devon, South West England, where rocks from this period were first studied.
A trace fossil, also known as an ichnofossil, is a fossil record of biological activity by lifeforms but not the preserved remains of the organism itself. Trace fossils contrast with body fossils, which are the fossilized remains of parts of organisms' bodies, usually altered by later chemical activity or mineralization. The study of such trace fossils is ichnology and is the work of ichnologists.
Bioerosion describes the breakdown of hard ocean substrates – and less often terrestrial substrates – by living organisms. Marine bioerosion can be caused by mollusks, polychaete worms, phoronids, sponges, crustaceans, echinoids, and fish; it can occur on coastlines, on coral reefs, and on ships; its mechanisms include biotic boring, drilling, rasping, and scraping. On dry land, bioerosion is typically performed by pioneer plants or plant-like organisms such as lichen, and mostly chemical or mechanical in nature.
The halkieriids are a group of fossil organisms from the Lower to Middle Cambrian. Their eponymous genus is Halkieria, which has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the small shelly fossil assemblages. The best known species is Halkieria evangelista, from the North Greenland Sirius Passet Lagerstätte, in which complete specimens were collected on an expedition in 1989. The fossils were described by Simon Conway Morris and John Peel in a short paper in 1990 in the journal Nature. Later a more thorough description was undertaken in 1995 in the journal Philosophical Transactions of the Royal Society of London and wider evolutionary implications were posed.
Stromatoporoidea is an extinct clade of sea sponges common in the fossil record from the Middle Ordovician to the Late Devonian. They can be characterized by their densely layered calcite skeletons lacking spicules. Stromatoporoids were among the most abundant and important reef-builders of their time, living close together in flat biostromes or elevated bioherms on soft tropical carbonate platforms.
Hexasterophora are a subclass of glass sponges in the class Hexactinellida. Most living hexasterophorans can be divided into three orders: Lyssacinosida, Lychniscosida, and Sceptrulophora. Like other glass sponges, hexasterophorans have skeletons composed of overlapping six-rayed spicules. In addition, they can be characterized by the presence of hexasters, a type of microsclere with six rays unfurling into multi-branched structures.
The Hunsrück Slate is a Lower Devonian lithostratigraphic unit, a type of rock strata, in the German regions of the Hunsrück and Taunus. It is a lagerstätte famous for exceptional preservation of a highly diverse fossil fauna assemblage.
Trace fossils are classified in various ways for different purposes. Traces can be classified taxonomically, ethologically, and toponomically, that is, according to their relationship to the surrounding sedimentary layers. Except in the rare cases where the original maker of a trace fossil can be identified with confidence, phylogenetic classification of trace fossils is an unreasonable proposition.
Cliona celata, occasionally called the boring sponge, is a species of demosponge belonging the family Clionaidae. It is found worldwide. This sponge creates round holes up to 5 millimetres (0.20 in) in diameter in limestone or the shells of molluscs, especially oysters. The sponge itself is often visible as a rather featureless yellow or orange lump at the bottom of the hole.
Carbonate hardgrounds are surfaces of synsedimentarily cemented carbonate layers that have been exposed on the seafloor. A hardground is essentially, then, a lithified seafloor. Ancient hardgrounds are found in limestone sequences and distinguished from later-lithified sediments by evidence of exposure to normal marine waters. This evidence can consist of encrusting marine organisms, borings of organisms produced through bioerosion, early marine calcite cements, or extensive surfaces mineralized by iron oxides or calcium phosphates. Modern hardgrounds are usually detected by sounding in shallow water or through remote sensing techniques like side-scan sonar.
Skolithos is a common trace fossil ichnogenus that is, or was originally, an approximately vertical cylindrical burrow with a distinct lining. It was produced globally by a variety of organisms, mostly in shallow marine environments, and appears as linear features in sedimentary rocks.
An ichnofacies is an assemblage of trace fossils that provides an indication of the conditions that their formative organisms inhabited.
The "Cambrian substrate revolution" or "Agronomic revolution", evidenced in trace fossils, is a sudden diversification of animal burrowing during the early Cambrian period.
Diplocraterion is an ichnogenus describing vertical U-shaped burrows having a spreite between the two limbs of the U. The spreite of an individual Diplocraterion trace can be either protrusive or retrusive. Some ichnospecies have both types. The presence/absence of funnel-shaped openings should not be used as an ichnotaxobase due to the high probability that the upper portions of the trace may have been eroded away. Observation of the orientation of Diplocraterion in the field is frequently used to determine the way up of rock strata at outcrop.
Trypanites is a narrow, cylindrical, unbranched boring which is one of the most common trace fossils in hard substrates such as rocks, carbonate hardgrounds and shells. It appears first in the Lower Cambrian, was very prominent in the Ordovician Bioerosion Revolution, and is still commonly formed today. Trypanites is almost always found in calcareous substrates, most likely because the excavating organism used an acid or other chemical agent to dissolve the calcium carbonate. Trypanites is common in the Ordovician and Silurian hardgrounds of Baltica.
Mildred Adams Fenton trained in paleontology and geology at the University of Iowa. She coauthored dozens of general science books with her husband, Carroll Lane Fenton, including Records of Evolution (1924), Land We Live On (1944), and Worlds in the Sky (1963).
Gastrochaenolites is a trace fossil formed as a clavate (club-shaped) boring in a hard substrate such as a shell, rock or carbonate hardground. The aperture of the boring is narrower than the main chamber and may be circular, oval, or dumb-bell shaped. Gastrochaenolites is most commonly attributed to bioeroding bivalves such as Lithophaga and Gastrochaena. The fossil ranges from the Ordovician to the Recent. The first Lower Jurassic Gastrochaenolites ichnospecies is Gastrochaenolites messisbugi Bassi, Posenato, Nebelsick, 2017. This is the first record of boreholes and their producers in one of the larger bivalves of the globally occurring Lithiotis fauna which is a unique facies in the Lower Jurassic Tethys and Panthalassa.
Petroxestes is a shallow, elongate boring originally found excavated in carbonate skeletons and hardgrounds of the Upper Ordovician of North America. These Ordovician borings were likely made by the mytilacean bivalve Corallidomus as it ground a shallow groove in the substrate to maintain its feeding position. They are thus the earliest known bivalve borings. Petroxestes was later described from the Lower Silurian of Anticosti Island (Canada). and the Miocene of the Caribbean.
Rogerella is a small pouch-shaped boring with a slit-like aperture currently produced by acrothoracican barnacles. These crustaceans extrude their legs upwards through the opening for filter-feeding. They are known in the fossil record as borings in carbonate substrates from the Devonian to the Recent.
Sclerobionts are collectively known as organisms living in or on any kind of hard substrate. A few examples of sclerobionts include Entobia borings, Gastrochaenolites borings, Talpina borings, serpulids, encrusting oysters, encrusting foraminiferans, Stomatopora bryozoans, and “Berenicea” bryozoans.
