26-514: Ptychagnostus is a member of the agnostida that lived during the Cambrian period. Ptychagnostidae generally do not exceed one centimetre in length. Their remains are rarely found in empty tubes of the polychaete worm Selkirkia . The genus probably ranged throughout the water column. It has two glabellar lobes, and three pygidial lobes. Agnostus punctuosus Angelin, 1851 from the Pt. punctuosus Zone of
52-576: A group of highly modified trilobites , though some recent research has doubted this placement. Regardless, they appear to be close relatives as part of the Artiopoda . They are present in the Lower Cambrian fossil record along with trilobites from the Redlichiida , Corynexochida , and Ptychopariida orders, and were highly diverse throughout the Cambrian . Agnostidan diversity severely declined during
78-458: A part. Instead, the limbs of agnostids closely resemble those of stem group crustaceans, although they lack the proximal endite , which defines that group. The study suggested that they were likely the sister taxon to the crustacean stem lineage, and, as such, part of the clade, Crustaceomorpha . Other researchers have suggested, based on a cladistic analyses of dorsal exoskeletal features, that Eodiscina and Agnostida are closely united, and
104-404: Is subdivided into five vertical regions. From the top down, these are: The illuminated zone at the surface of the sea with sufficient light for photosynthesis. Nearly all primary production in the ocean occurs here, and marine life is concentrated in this zone, including plankton , floating seaweed , jellyfish , tuna , many sharks and dolphins . The most abundant organisms thriving into
130-490: Is the ecological region at the very bottom, including the sediment surface and some subsurface layers. Marine organisms such as clams and crabs living in this zone are called benthos . Just above the benthic zone is the demersal zone. Demersal fish can be divided into benthic fish , which are denser than water and rest on the bottom, and benthopelagic fish , which swim just above the bottom. Demersal fish are also known as bottom feeders and groundfish . The pelagic zone
156-481: The Greek underworld . This is the deepest part of the ocean at more than 6,000 m (20,000 ft) or 6,500 m (21,300 ft), depending on authority. Such depths are generally located in trenches . The pelagic ecosystem is based on phytoplankton . Phytoplankton manufacture their own food using a process of photosynthesis . Because they need sunlight, they inhabit the upper, sunlit epipelagic zone, which includes
182-411: The hadopelagic . Coastal waters are generally the relatively shallow epipelagic. Altogether, the pelagic zone occupies 1,330 million km (320 million mi ) with a mean depth of 3.68 km (2.29 mi) and maximum depth of 11 km (6.8 mi). Pelagic life decreases as depth increases. The pelagic zone contrasts with the benthic and demersal zones at the bottom of the sea. The benthic zone
208-500: The water column of coastal, ocean, and lake waters, but not on or near the bottom of the sea or the lake. They can be contrasted with demersal fish, which do live on or near the bottom, and coral reef fish . Pelagic fish are often migratory forage fish , which feed on plankton , and the larger predatory fish that follow and feed on the forage fish. Examples of migratory forage fish are herring , anchovies , capelin , and menhaden . Examples of larger pelagic fish which prey on
234-600: The Alum Shale ( Drumian ), Sweden (by original designation). Official ruling on the conservation of accepted usage of A. punctuosus as the type species was given by the International Commission on Zoological Nomenclature, 1993. Ptychagnostus affinis (Brøgger 1878) was once considered a subspecies of Pt. punctuosus . Laurie (2008) grouped punctuosus and affinis within Ptychagnostus , but preferred to place
260-497: The Artiopoda. Scientists have long debated whether the agnostids lived a pelagic or a benthic lifestyle. Their lack of eyes, a morphology not well-suited for swimming, and their fossils found in association with other benthic trilobites suggest a benthic (bottom-dwelling) mode of life. They are likely to have lived on areas of the ocean floor which received little or no light and fed on detritus which descended from upper layers of
286-544: The Cambrian- Ordovician transition, and the last agnostidans went extinct in the Late Ordovician. The Agnostida are divided into two suborders — Agnostina and Eodiscina — which are then subdivided into a number of families . As a group, agnostids are isopygous, meaning their pygidium is similar in size and shape to their cephalon . Most agnostid species were eyeless. The systematic position of
SECTION 10
#1733085287015312-515: The Eodiscina descended from the trilobite order Ptychopariida . A 2019 study of adult specimens with preserved soft tissue from the Burgess Shale found that agnostidans shared morphological similarities to trilobites and other related artiopodans like nektaspids , and their placement as stem-crustaceans was unsupported. The study recovered agnostidans as the sister group to other trilobites within
338-441: The absence of any moulted carapaces suggests that moulting was not their primary reason for seeking shelter. Pelagic The pelagic zone consists of the water column of the open ocean and can be further divided into regions by depth. The word pelagic is derived from Ancient Greek πέλαγος ( pélagos ) 'open sea'. The pelagic zone can be thought of as an imaginary cylinder or water column between
364-657: The closely related atavus within Acidusus . Høyberget & Bruton (2008) concluded that the following genera belong in the Ptychagnostidae: Ptychagnostus, Goniagnostus, Tomagnostus, Lejopyge, Aotagnostus and Onymagnostus . This Agnostida -related article is a stub . You can help Misplaced Pages by expanding it . Agnostida Suborder Agnostina Suborder Eodiscina Isopygia Gürich, 1907 Miomera Jækel, 1909 Agnostida are an order of extinct arthropods which have classically been seen as
390-753: The coastal or neritic zone . Biodiversity diminishes markedly in the deeper zones below the epipelagic zone as dissolved oxygen diminishes, water pressure increases, temperatures become colder, food sources become scarce, and light diminishes and finally disappears. Some examples of pelagic invertebrates include krill , copepods , jellyfish , decapod larvae , hyperiid amphipods , rotifers and cladocerans . Thorson's rule states that benthic marine invertebrates at low latitudes tend to produce large numbers of eggs developing to widely dispersing pelagic larvae, whereas at high latitudes such organisms tend to produce fewer and larger lecithotrophic (yolk-feeding) eggs and larger offspring. Pelagic fish live in
416-512: The first years of their lives in the pelagic zone, moving closer to shore as they reach maturity. Pelagic birds , also called oceanic birds or seabirds , live on open seas and oceans rather than inland or around more restricted waters such as rivers and lakes. Pelagic birds feed on planktonic crustaceans , squid and forage fish . Examples are the Atlantic puffin , macaroni penguins , sooty terns , shearwaters , and Procellariiformes such as
442-477: The forage fish are billfish , tuna , and oceanic sharks . Hydrophis platurus , the yellow-bellied sea snake, is the only one of the 65 species of marine snakes to spend its entire life in the pelagic zone. It bears live young at sea and is helpless on land. The species sometimes forms aggregations of thousands along slicks in surface waters. The yellow-bellied sea snake is the world's most widely distributed snake species. Many species of sea turtles spend
468-553: The mesopelagic zone are heterotrophic bacteria. Animals living in this zone include swordfish , squid , wolffish and some species of cuttlefish . Many organisms living here are bioluminescent . Some mesopelagic creatures rise to the epipelagic zone at night to feed. The name stems from Ancient Greek βαθύς 'deep'. The ocean is pitch black at this depth apart from occasional bioluminescent organisms, such as anglerfish . No plants live here. Most animals survive on detritus known as " marine snow " falling from
494-431: The number of layers depending on the depth of the water. Marine life is affected by bathymetry (underwater topography) such as the seafloor, shoreline, or a submarine seamount , as well as by proximity to the boundary between the ocean and the atmosphere at the ocean surface, which brings light for photosynthesis, predation from above, and wind stirring up waves and setting currents in motion. The pelagic zone refers to
520-408: The open, free waters away from the shore, where marine life can swim freely in any direction unhindered by topographical constraints. The oceanic zone is the deep open ocean beyond the continental shelf , which contrasts with the inshore waters near the coast , such as in estuaries or on the continental shelf. Waters in the oceanic zone plunge to the depths of the abyssopelagic and further to
546-415: The order Agnostida within the class Trilobita remains uncertain, and there has been continuing debate whether they are trilobites or a stem group . The challenge to the status has focused on Agnostina partly due to the juveniles of one genus have been found with legs differing dramatically from those of adult trilobites, suggesting they are not members of the lamellipedian clade , of which trilobites are
SECTION 20
#1733085287015572-524: The sea to the bottom. Their wide geographic dispersion in the fossil record is uncharacteristic of benthic animals, suggesting a pelagic existence. The thoracic segment appears to form a hinge between the head and pygidium allowing for a bivalved ostracodan -type lifestyle. The orientation of the thoracic appendages appears ill-suited for benthic living. Recent work suggests that some agnostids were benthic predators, engaging in cannibalism and possibly pack-hunting behavior. They are sometimes preserved within
598-451: The surface of the sea and the bottom. Conditions in the water column change with depth: pressure increases; temperature and light decrease; salinity, oxygen, micronutrients (such as iron, magnesium and calcium) all change. In a manner analogous to stratification in the Earth's atmosphere , the water column can be divided vertically into up to five different layers (illustrated in the diagram), with
624-438: The voids of other organisms, for instance within empty hyolith conchs, within sponges , worm tubes and under the carapaces of bivalved arthropods , presumably in order to hide from predators or strong storm currents; or maybe whilst scavenging for food. In the case of the tapering worm tubes Selkirkia , trilobites are always found with their heads directed towards the opening of the tube, suggesting that they reversed in;
650-406: The zones above or, like the marine hatchetfish , by preying on other inhabitants of this zone. Other examples of this zone's inhabitants are giant squid , smaller squid and the grimpoteuthis or "dumbo octopus". The giant squid is hunted here by deep-diving sperm whales . The name is derived from Ancient Greek ἄβυσσος 'bottomless' - a holdover from times when the deep ocean
676-410: Was believed to indeed be bottomless. Among the very few creatures living in the cold temperatures, high pressures and complete darkness here are several species of squid; echinoderms including the basket star , swimming cucumber, and the sea pig ; and marine arthropods including the sea spider . Many species at these depths are transparent and eyeless. The name is derived from the realm of Hades ,
#14985