klass

Cephalopoda Cuvier, 1797

Peajalgsed

Taksoni iseloomustus

Peajalgsed ehk tsefalopoodid on kõige enam arenenud molluskite klass, kuhu tänapäeva loomadest kuuluvad näiteks kaheksajalad ja kalmaarid. Paleosoikumis ja Mesosoikumis elanud peajalgsetele oli tüüpiline välise koja esinemine.

Peajalgsete kojad (nautiloidid, ammoniidid) ning sisetoese elemendid (belemniidid) on tugevalt mineraliseerunud ning säilivad hästi ka fossiilsetena.


Peajalgse ehitus ja põhiterminid.

Peajalgsete torpeedokujuline või ümmargune kotjas keha koosneb kerest ja peast. Keha katab seljapoolel kokku kasvanud mantel. Suuava ümbritseb iminappadega kombitsate ehk haarmete pärg. Peaaju on neil hästi arenenud, silmad on suured ja sarnanevad ehituselt selgroogsete silmadega. Peajalgsete vanimad kivistised pärinevad Kambriumi ladestust. Praegu kuulub peajalgsete klassi kaks teineteisega kaugelt suguluses olevat haru, nautiloidid (Nautiloidea) ja sisekojalised (Coleoidea). Nautiloidid ehk laevukeselised on primitiivsed, välise mitmekambrilise kojaga peajalgsed. Koda oli kas lamespiraalselt keerdunud või sirge, kambrid olid üksteisest vaheseintega eraldatud ja gaasiga täidetud. Gaasi rõhku sai muuta kambreid ühendava sifooni kaudu. Kõige välimises elukambris elas loom ise. Nautiloidide hulgas eristatakse suureläbimõõdulise sifooniga endotseratiide ja väikseläbimõõdulise sifooniga ortotseratiide. Ammoniidid on väljasurnud peajalgsed, kes valitsesid Keskaegkonna meredes. Eestis nende kivistisi ei leidu.

Enne kalade rolli kiiret kasvu Devonis olid peajalgsed ürgmere suurimateks röövloomadeks. Eestis on peajalgsete kojad suurimateks kivististeks üldse – ortotseratiidide sirged kambrilised kojad võivad olla enam kui meetri pikkused. Osaliste kivististe põhjal on arvatud, et suurimad Maal elanud nautiloidid võisid kasvada aga enam kui 10 m pikkuseks.

Valik taksonit käsitlevast teaduskirjandusest
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Leitud liigid
6. Ancistroceras barrandei Dewitz, 1880 | Aseri lade→Uhaku lade
17. Arionoceras lotskirkense Kröger, 2004 | Aseri lade→Lasnamäe lade
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Klassifikatsioon
organismirühmBiota
riikAnimalia
hõimkondMollusca
alamhõimkondConchifera
klassBivalvia
Cephalopoda
alamklassActinoceratoidea
Ammonoidea
Bactritoidea
Coleoidea
Multiceratia
Nautilia
Orthoceratia
Tarphyceratia
seltsBactritida
Barrandeocerida
Basslerocerida
ülemsugukondTainocerataceae
perekondBotellusoceras
Phylloceras
klassDiplacophora
Gastropoda
Helcionelloida
Hyolitha
Paragastropoda
Polyplacophora
Rostroconchia
Tergomya
sugukondChelodina
perekondEesticonus
Eobucania
Jinonicella
See also