marine animals hard shells upper lower surfaces unlike left right arrangement rear end opened feeding closed protection major groups recognized articulate articulate toothed simple opening closing muscles complex system muscles halves aligned typical stalk like projects opening near known valve keeping animal anchored clear opening word formed ancient greek words arm foot known lamp shells curved shells class look like pottery oil lamps range thirty years sperm float main exit mantle cavity larvae miniature adults enable larvae feed swim months animals heavy settle larvae articulate species resemble adults look like yolk remain days leaving water column addition traditional classification articulate approaches appeared 1990s approach groups articulate use material mineral layers shell approach makes group outer organic layer different believe premature suggest higher levels classification order recommend approach identifies genera groups intermediate groups traditionally regarded members sister group includes type analysis evolutionary relationships placed type split placing suggested evolved ancestor similar like animal chain mail shell rear end thought ancestral converted shells pair folding rear body new fossils found showed chain mail formed tube animal resembled close relatives bore symmetrical plates early form fossil extant taxa appeared early periods respectively arisen extinct severe mass peak era abundant filter reef builders occupied ecological including swimming jet propulsion style fossils useful climate changes extinction event recovered diversity study concluded especially vulnerable extinction built hard parts calcium low rates weak respiratory systems thought went decline extinction competed study found species increased modern times increasing faster extinction time diverse live sea species avoid locations strong currents waves larvae articulate species settle quickly form dense populations defined areas larvae species swim month wide ranges live mainly cold water low light fish crustaceans find flesh seldom attack commercially small scale species measure environmental conditions oil terminal built russia shore description shells mechanisms modern range millimetres long species millimetres largest known reaching centimetres width occurred upper shell sections cover dorsal ventral surface animal unlike shells cover lateral surfaces termed valve bears inner surface arms gets support feeding valve inner surface attachment stalk like attach substrate called dorsal upper ventral lower regard terms dorsal ventral irrelevant believe ventral valve formed folding upper surface body valve typically larger articulate species convex surfaces bearing growth lines ornamentation similar size shape articulate tooth arrangement locking lateral displacement matching teeth held muscles muscles set inside valve close pulling valve ahead muscles quick fibers close catch fibers slower closed long periods articulate open means muscles known lie rear pull valve use different opening mechanism muscles reduce length main body cavity pushing apart classes open angle complex set muscles employed operate mechanism use valve consists layers outer organic compounds layers articulated proteins primary layer form calcium mixture proteins shells similar sequence layers composition different articulated varies classes matrix long materials embedded containing calcium primary layer complex mixture layer containing proteins cement directly hard surfaces layers shells grow new material added margin new material added posterior region shell anterior direction growing shell mantle mantle lines shell internal organs body occupies internal space inside shell nearest rest space lined mantle extensions water filled space sits extends lobe network canals carry nutrients edges mantle relatively new cells groove edges mantle material extends cells gradually displaced underside mantle recent cells groove switch material shell words edge valve extended reinforced extension layers species edge mantle bears called help defend animals act sensors groups help channel flow water mantle cavity hollow extensions mantle penetrate layers function uncertain suggested storage chambers chemicals deter organisms stick shell help experiments oxygen consumption drops petroleum shell like crown fine hairs create water current enables filter food particles water ring mounted single retracted stalk basic form shaped forming arms gets non retractable occupy thirds internal space area opened provide capacity restricted space larger folded moderately complex shapes loops common species resemble hand fingers species supported skeleton words pressure internal fluid fluid extends articulate support attached inside valve bear fine mobile hairs edges center beating outer drives water current tips bases exits food particles trapped middle drive mixture base groove runs round bases pass food groove mouth method known upstream collecting food particles captured enter field creates feeding current method related use similar looking crown solid flow runs bases tips forming downstream collecting system catches food particles exit attachment substrate modern species attach hard surfaces means cylindrical stalk extension body wall cuticle non cellular skin opening genera articulate cement rear valve surface slightly inclined away surface articulate genera adults grow finally lie loosely surface genera shells shaped opening kept free sediment species extensions main houses internal organs layer longitudinal muscles lines members order long use soft raise shell opening feed shell disturbed moves body thirds remaining occupied end builds concrete anchor order short attach hard surfaces articulate constructed different body core composed tissue muscles rear body bend rotate far end generally extensions short attach hard surfaces articulate genus use branched anchor sediment emerges valve notch species valve longer hole valve doubles touch valve species stand end upwards lie horizontal valve functional opening feeding water flow enters sides open exits animal entrance exit channels formed groups function funnels entry exit channels organized shape captures food particles especially tiny organisms deliver mouth grooves bases mouth base food passes mouth muscular throat lined cells digestive stomach wall branched food mainly cells nutrients transported including mantle produced broken eliminated mantle dissolved sign perform phase process appear sperm majority food consumed little solid waste produced change direction isolated particles matter animal encounters larger matter lining entry channels pause contact apart form large gaps slowly use dump lining mantle wash opening snap sharply creates digestive tract shaped ends body wall articulate curved gut ends animals bundle solid waste periodically sharp gut muscles circulation mantle surfaces absorb oxygen eliminate carbon dioxide oxygen distributed fluid circulated mantle driven lining beating species oxygen partly carried respiratory pigment transported cells maximum oxygen consumption low minimum requirement blood circulated muscular heart lying dorsal body stomach blood passes vessels extend body branch organs including gut muscles rear blood circulation completely closed fluid blood mix degree main function blood deliver nutrients nervous system senses brain adult consists adult lower join nerves run mantle muscles operate edge mantle probably greatest concentration sensors directly connected sensory mantle probably send signals mantle close shadows appear cells responsible unknown detect changes animals position reproduction life cycle range years adults species sex lives masses developing sperm species valve lie channels mantle lie near gut float main exit mantle cavity open mouth species release sperm water females species embryos brood chambers larvae hatch cell division embryo radial cells form rings directly cells separate adjoining type tissue cell develops controlled interactions adjacent cells cell animals develop mouth deepening surface early embryo closes mouth develop new openings larvae swim months like miniature adults mantle mantle cavity small feeding swimming shell heavier juvenile adult larvae articulate species live yolk remain days type lobe body rear lobe mantle like skirt rear adult surface lobe develops organs mantle rolls lobe starts shell cold seas growth seasonal animals lose weight winter variations growth form growth lines shells members genera survived year food taxonomy history fossils great diversity morphology shells modern genera diversity provide soft bodied characteristics fossils extant species limitations difficult produce comprehensive classification based morphology experienced significant evolution recent group lost characteristic seen intermediate group characteristic seen older group believe premature define higher levels classification order recommend instead approach identifies genera groups intermediate groups believe patterns characteristics sufficiently stable higher level classifications worthwhile different views higher level classifications traditional classification defined approaches established 1990s traditional classification toothed held muscles classification devised 1990s based materials shells based united articulate shells combined shells calcium scheme 1990s places separate group grouped living species recognized grouped genera great majority modern excluding modern classification genetic analysis performed extended understanding relationship different organisms clear belong earlier included worms called adult morphology different sequence indicates worms closest relatives articulate weight evidence inconclusive exact relations consequently suggested included worms class named addition contains class subdivided orders ecology distribution habitat live sea species avoid locations strong currents waves typical sites include rocky caves steep slopes continental shelves deep ocean floors articulate species attach exceptionally sheltered sites zones smallest living millimetre long lives gravel excluding larvae consume settle develop quickly specific areas form dense populations reach thousands meter young adults attach shells mature ones hand swim month settling wide ranges members genus cosmopolitan distribution interactions organisms rates tenth abundant warm shallow seas cretaceous period live mainly cold low light conditions shells occasionally evidence damage predators subsequent repair fish crustaceans find flesh fossil record shows drilling predators like attacked times suggesting predators attacked mistake prey scarce waters food scarce steals food tube worms commercially small scale seldom settle artificial surfaces probably vulnerable pollution population useful measure environmental conditions oil terminal built russia shore evolutionary history fossil record fossil species recognized grouped genera largest modern millimetres long fossils measure millimetres wide earliest confirmed found early forms appearing followed soon articulate forms species found apparently represent distinct groups evolved ancestors called living fossil similar genera found way hand articulate produced major suffered severe mass articulate diverse present day groups appeared start respectively proposed hypothesis development adapted colleagues hypothesis earliest evolution fold hypothesis suggests evolved ancestor similar like animal chain mail shell rear end hypothesis proposes converted shells pair folding rear body fossils onwards supported new interpretation early new hypothesis evolved armor mail known assembled form generally assumed like animals similar armor compounds fossils new showed assembled mail coat formed tube indicate animal like tube resembled animals feed regarded close relatives sub group assembled fossil found described symmetrical plates like fully animal body peak abundant filter reef builders occupied ecological including swimming jet propulsion style extinction event informally known great dying recovered diversity thought actually decline diversity way competed produced statistical analysis concluded increased way modern times increased faster extinction moderately severe devastating time diverse diversity increased low base evidence competed short term increases decreases groups appeared times concluded groups vulnerable extinction hard parts calcium low rates weak respiratory systems fossils useful climate changes era global temperatures low large difference temperatures equator poles created different collections fossils different latitudes hand warmer periods created smaller difference temperatures seas low middle latitudes species evolutionary family tree 1940s 1990s family trees based morphological features placed sister group super includes closer examination found difficulties grounds affiliated radial earliest divisions egg appears original condition ancestral earliest possibly earliest major sub group radial imply affiliated traditional view form different process called respectively research early 1990s found significant exceptions types construction appear imply terms originally defined distinct ways forming mouth depression appears early stage embryo forming mouth process like typical forming mouth like process imply affiliated nielsen views closely related affiliated driven cell regards multiple cell probably closely related evidence latest common ancestor latest common ancestor fed means onwards analyses based molecular phylogeny compares features similarities dna placed super includes super members include conclusion unanimous molecular phylogeny studies use wide selection genes genes mitochondrial protein genes single nuclear protein genes sets nuclear protein genes combined studies molecular morphological data support concluded relationship feed surfaces build layered tubes possibly mixed particles material traditionally regarded separate increasingly detailed molecular phylogeny studies concluded sub group analysis concluded sub group molecular phylogeny studies half combined studies conclude identify closest relatives sub group analyses found closest relatives authors found surprising spiral early stages cell division form radial shows sign having evolved study concluded closely related casting doubt idea clade feeding animals gallery
Brachiopod