Related Research Articles
Hemichordate is a phylum of marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and include two main classes: Enteropneusta, and Pterobranchia. A third class, Planctosphaeroidea, is known only from the larva of a single species, Planctosphaera pelagica. The extinct class Graptolithina is closely related to the pterobranchs.
Graptolithina is a subclass of the class Pterobranchia, the members of which are known as graptolites. These organisms are colonial animals known chiefly as fossils from the Middle Cambrian through the Lower Carboniferous (Mississippian). A possible early graptolite, Chaunograptus, is known from the Middle Cambrian. One analysis suggests that the pterobranch Rhabdopleura represents extant graptolites. Studies on the tubarium of fossil and living graptolites showed similarities in the basic fusellar construction and it is considered that the group most probably evolved from a Rhabdopleura-like ancestor.
Opabinia regalis is an extinct, stem group arthropod found in the Middle Cambrian Burgess Shale Lagerstätte of British Columbia, Canada. It flourished from 505 million years ago to 487 million years ago during the Cambrian Period of the Paleozoic Era. It measured 2-3 inches in length and is presumed to have been a carnivore. Fewer than twenty good specimens have been described; 3 specimens of Opabinia are known from the Greater Phyllopod bed, where they constitute less than 0.1% of the community. Opabinia was a soft-bodied animal, averaging about 5.7 cm in length, and its segmented body had lobes along the sides and a fan-shaped tail. The head shows unusual features: five eyes, a mouth under the head and facing backwards, and a proboscis that probably passed food to the mouth. Opabinia probably lived on the seafloor, using the proboscis to seek out small, soft food.
Pterobranchia is a clade of small worm-shaped animals. They belong to the Hemichordata, and live in secreted tubes on the ocean floor. Pterobranchia feed by filtering plankton out of the water with the help of cilia attached to tentacles. There are about 30 known living species in the group.
An aptychus is a type of marine fossil. It is a hard anatomical structure, a sort of curved shelly plate, now understood to be part of the body of an ammonite. Paired aptychi have, on rare occasions, been found at or within the aperture of ammonite shells. The aptychus was usually composed of calcite, whereas the ammonite shell was aragonite.
The Coelosclerithophorans are a polyphyletic group of organisms bearing hollow sclerites made of aragonite, and with an allegedly distinctive microstructure.
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 radar.
Deuterostomes comprise a superphylum of animals. It is a sister clade of Protostomia, with which it forms the Nephrozoa clade.
The ectocochleate cephalopods are the oldest known representatives of their class, dating back to the Cambrian period. Their aragonitic shells are not prone to fossilisation. They contain the modern Nautilus and many fossil forms including the ancient Ellesmerocerida and the ammonoids.
Janospira is a microfossil known from Ordovician deposits, whose affinity is uncertain. It resembles a spiral shell mounted on a cylinder, probably calcareous, about 1 mm in length. There are compelling reasons to discount a foramaniferan or molluscan affinity, though, some researchers presume it to be an archaeogastropod.
Isoxys is a genus of extinct, pelagic bivalved arthropod; the various species may have been roam-swimming predators. It had a pair of large spherical eyes, and two large appendages, which have led to speculation that it may be related to the great appendage arthropods.
Girvanella is thought to represent the calcified sheath of a filamentous cyanobacterium known from the Burgess Shale and other Cambrian fossil deposits.
Stenothecidae is an extinct family of fossil univalved Cambrian molluscs which may be either gastropods or monoplacophorans.
Anabarella is a species of bilaterally-flattened monoplacophoran mollusc, with a morphological similarity to the rostroconchs. Its shell preserves evidence of three mineralogical textures on its outer surface: it is polygonal near the crest of the shell, subsequently changing to both spiny and stepwise. Its internal microstructure is calcitic and semi-nacreous. Its name reflects its provenance from Anabar, Siberia. It has been interpreted as ancestral to the rostroconchs, and has been aligned to the Helcionellidae.
Paleoceras is a genus of fossil cephalopods from the Upper Cambrian.
Carnarvonia is a genus of arthropods of uncertain affinities, known from the Middle Cambrian Burgess Shale. Its bivalved carapace bears the imprints of its veins.
Rhabdopleurida is one of three orders in the class Pterobranchia, which are small, worm-shaped animals. Members belong to the hemichordates. Species in this order are sessile, colonial, connected with a stolon, living in clear water and secrete tubes called tubarium. They have a single gonad, the gill slits are absent and the collar has two tentaculated arms. Rhabdopleura is the best studied pterobranch in developmental biology. Some claim Rhabdopleura is an extant graptolite.
Cephalonyx is a genus of filamentous acritarchs known from the Precambrian and early Cambrian.
Proconodontida is an order of conodonts.
Proconodontus is an extinct genus of conodonts in the family Proconodontidae. The specimens are found in Cambrian formations.
References
- ↑ Bengtson, S.; Urbanek, A. (1986), "Rhabdotubus, a Middle Cambrian rhabdopleurid hemichordate", Lethaia, 19 (4): 293–308, doi:10.1111/j.1502-3931.1986.tb00743.x, archived from the original on 2012-12-16, retrieved 2009-06-06
- ↑ Maletz, J.; Steiner, M.; Fatka, O. (2005), "Middle Cambrian pterobranchs and the Question: What is a graptolite?" (PDF), Lethaia, 38 (1): 73–85, doi:10.1080/00241160510013204 , retrieved 2009-06-06
- ↑ Durman, P.N.; Sennikov, N.V. (1993), "A new rhabdopleurid hemichordate from the Middle Cambrian of Siberia" (PDF), Palaeontology, 36: 283–283, retrieved 2009-06-06
- ↑ Urbanek, A. (1994), "When is a pterobranch a graptolite?", Lethaia, 27 (4): 324–324, doi:10.1111/j.1502-3931.1994.tb01582.x
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