Phylum Hemichordata

1

• Phylum Hemichordata
• Includes acorn worms and pterobranches
  (colonial).
• Have gill slits, dorsal nerve chord, but lack
  notochord (skeletal rod protecting nerve chord)
• It used to be thought that some of these
  organisms had a notochord, but it turned out to
  be part of the digestive tract. Oops!
• Class Pterobranchia
• Ordovician to Recent
• Most similar extant group to graptolites.
• All are sessile, benthic, colonial, filter
  feeders.
• Have an exoskeleton of scleroprotien, fusellar
  half rings, with zig-zag suture. Secreted from
  the external side.
• Body of each zooid contains a cephalic shield,
  trunk, and collar.
• Cephalic shield is a cilliated discoidal
  structure which aids in feeding. All secretes
  skelton (rhabdosome).
• Collar surrounds mouth. Distal portion of collar
  has arms.
• Arms have tentacles and are an extension of the
  coelom. (A lot like a lophophore!) Used in
  feeding.
• Trunk contains stomach, u-shaped gut, gonads,
  etc.
• Reproduce sexually. Sexually-produced
  individuals start a colony and then colony grows
  by budding.
• Buds form from internal fleshy tube called a
  stolon. Older portions of the stolon are dark
  (black stolon)

2

• Class Graptolithina
• Important index fossils for Ordovician and
  Silurian. Range from Cambrian to Pennsylvanian.
• All colonial. There are bushy forms, sheet like
  forms, and rod-like forms.
• Skeleton composed of tough scleroprotien. Made
  of two layers
• Fusellar half rings with zig-zag suture
• Cortical layer (like a wrapped sheet).
• Fusellar layer (inner) secreted first, then
  cortical layer. Must have had external
  secretion.
• Organic, so preserve best in anoxic substrate.
  Black shale graptolite biofacies.
• Best preserved specimens are from limestone.
  Collected by dissolving limestones.
• Graptolite colony rhabdosome
• Initial individual that starts colony sicula
• Aperture typically points opposite direction of
  other zooids.
• Sexually produced? (ancestrula-like?)
• Underwent metamorphism different skeletal
  structure in pro- and metasicula
• Close to sicula proximal. Away from sicula
  distal
• Individuals in colony thecae. Theca size and
  shape change through astogeny.
• Arrangement of theca, thecal shape, and aperture
  shape are important in classification.
• Each linear branch of thecae is called a stipe.
• A thin tube (nema) extends from the apex of the
  sicula. Called a virgula if it contacts theca.

3

• Orders of Graptolites
• Order Dendroidea
• Middle Cambrian to Early Pennsylvanian
• Bushy, benthic, sessile (Dictyonmena
  epiplanktonic?)
• Formed golf ball size to grapefruit size colonies
• Stipes often connected by dissepiments
• Two distinct types of theca autotheca and
  bitheca.
• Colony attached to seafloor by stem-like
  extension of sicular aperture.
• Tolerant of low O2? Preservational bias?
• Most are bradytelic at generic level, but some
  species are important for biostratigraphy.
• Order Camaroidea
• Encrusters
• Densly packed auto- and bitheca. Autotheca look
  like headless geese (to me, at least)
• Late Cambrian to Early Ordovician (glacial
  erratics from Poland, mostly)
• Order Crostoidea
• Encrusters.
• Sparselly packed auto- and bitheca. Authothecae
  are vase-like with constricted apertural neck.

4

• Order Graptoloidea
• Early Ordovician to Early Devonian
• Super index fossil
• Few stipes
• Shapes
• Pendant (theca face in, aperture of theca and
  sicula face same direction)
• Horizontal
• Reclined (theca face out, apertures of theca and
  sicula face opposite directions)
• Scandent
• Uniserial or biserial
• Planktonic
• Some with floats
• Spines and lightweight skeleton may have aided in
  floatation
• Coiling forms may have spiraled up or down in
  water column
• Mass blooms and die-offs.
• Graptolids through time
• Early Ordovician Multiple stipes, dichotomously
  branching, leafy phyllograptids (4 stipes)
• Early to Middle Ordovician Pendant graptolites
• Middle Ordovician tuning fork graptolites
  (biserial to uniserial stipes