Exam C

• Deuterostomes


  Monophyletic but morphologically disparate clade
• Deuterostomes


  • Radial cleavage
  • Indeterminate development
  • Blastopore becomes anus
  • Coelom via mesoderm budding from wall of archenteron
• • Cells from each division are directly above/below each other
  • Each cell totipotent for several cleavages
  • Blastopore becomes anus (mouth secondary)
  • Mesoderm buds from wall of archenteron to form coelom
• Deuterostomes


  • Chordates + hemichordates + echinoderms
  • Relationship between the 3 clades is highly contentious

  
• Chordates


  Vertebrates, urochordates (tunicates) and cephalochordates (e.g. amphioxus)

  
• Hemichordates


  Enteropneusts (acorn "worms") and pterobranchs (colonial tube-dwelling)

  
• Ambulacraria


  Echinoderms + hemichordates

  
• The inter-relationships of the three deuterostome clades are highly contentious
• Phylum Hemichordata


  • Enteropneusts (acorn worms)
  • Pterobranchs (graptolites)

  
• Phylum Chordata


  • Cephalochordates
  • Urochordates
  • Vertebrates

  
• Xenacoelomorpha


  Lack several features common to most bilaterians, e.g. anus, circulatory system
• Phylogenetic position of Xenacoelomorpha

  • (A) Sister group to all remaining Bilateria (Xenacoelomorpha + Nephrozoa (protostomes + deuterostomes))
  • (B) Clade inside Deuterostomia
  • (C) Taxa split between sister group + inside deuterostomes

  
• Echinoderms


  • Symmetry: bilateral symmetry in larvae, pentaradial in adults
  • Water Vascular System network of water-filled canals that functions in gas exchange, locomotion, and feeding
  • Stereom – unique mesodermal skeleton (endoskeleton) covered by thin epidermis: porous, Calcium carbonate: give shape and rigidity without adding extra mass
  • Mutable Collagenous Tissue: ligaments of collagen under control of nervous system – can change rapidly from highly flexible to rigid
  • Spines + pedicellariae
  • Simple nervous system – nerve ring around mouth with radial nerves branching into nerve net
• Echinoderms from Precambrian - Recent
• Sponges, archaeocyathans and stromatoporoids have calcitic spicules but the remainder are unmineralized, resulting in a poor fossil record
• Crown-group Echinoderms


  • Pelmatozoa
  • Eleutherozoa
• Pelmatozoa


  e.g. Crinoidea: 'cup + stem', attached, mouth upwards, free feeding appendages
• Eleutherozoa


  e.g. echinoids, asteroids, ophiuroids, holothurians – free-living, motile, mouth downwards

  
• Echinoid anatomy


  • Water vascular system: stone canal runs from madreporite (one of 5 genital plates) to circum-oral-ring
  • 5 radial water vessels with tube feet – each runs from 1 of 5 ocular plates to circum-oral ring
  • Tube feet = respiration and locomotion
• Apical system of echinoids

  • Ocular plates
  • Genital plates (with gonads) – inc madreporite
  • Periproct
  • Small plates set in flexible (organic) membrane
  • Includes anus
• Coronal system of echinoids

  • Ambulacra
  • Ambulacral plates
  • Pore pairs = openings for tube feet
  • Interambulacra
  • Interambulacral plates
  • Tubercles for spines

  
• Peristomal system of echinoids

  • Peristome membrane
  • Mouth
  • Gills: internal
  • Aristotle's lantern – 5 strong jaws each with 1 calcitic tooth (rasping)
• The 5 teeth of the Aristotle's lantern would make a pentagonal trace fossil when scratching a surface
• Irregular echinoids


  Adaptations to an infaunal lifestyle
• Changes in irregular echinoids

  • Position of the anus
  • Shape of ambulacra
  • Oral surface
  • Fascioles
• Regular echinoids are epifaunal and mobile, acting as omnivores, carnivores and herbivores, grazing on hard surfaces and scavenging
• The thin excavated channels on oyster shells formed by echinoid grazing are called Gnathichnus trace fossils

  
• The angle between the two scratch marks made by the 5 teeth of the Aristotle's lantern would be 72 degrees
• Irregular echinoids

  Heart-shaped test, deep burrowing, specialization of tube-feet for tunneling and waste disposal
• Irregular echinoids


  Flattened test, shallow self burial, high energy tidal environments, lunules to prevent being lifted out, food harvesting more efficient
• The Ediacaran fossil Arkarua seems to exhibit pentaradial symmetry and may resemble edrioasteroid echinoderms

  
• The Ediacaran fossil Tribrachidium seems to exhibit triradial symmetry

  
• Helicoplacoids


  Enigmatic extinct early Cambrian group, body bulb-shaped + spirally plated, three twisted ambulacra, anus on upper pole, mouth in lateral position, suspension feeding

  
• Homalozoa


  Lack key echinoderm synapomorphies like water vascular system and pentaradial symmetry, have bilateral symmetry or asymmetry, but share stereom microfabrics

  
• Crown group echinoderms


  • Pelmatozoa
  • Eleutherozoa
• Homalozoa is an out of date term that accommodated a wide range of groups
• Helicocystis moroccoensis exhibits key echinoderm features like a stem, cup, spiral regions, pentaradial symmetry, and a mouth on the dorsal side

  
• The Deuterostomes


  • mesoglea
  • blind gut (enteron)
  • through gut
  • mesoderm
  • radial cleavage
  • indirect development
  • moulting
  • spiral cleavage
  • diploblastic
  • sponges
  • archaeocyathans
  • stromatoporoids
  • triploblastic
  • corals
  • chordates + vertebrates
  • hemichordates
  • echinoids
  • crinoids
  • trilobites
  • brachiopods
  • gastropods
  • bivalves
  • cephalopods
• Eumetazoa ("true animals")


  • Porifera
  • Cnidarians
  • Deuterostomes
  • Protostomes
  • Ecdysozoa
  • Spiralia
  • Bilateria
  • Radiata

  
• Echinoderms and hemichordates together are known as ambulacrarians