Echinoderms Flashcards
Deuterostomes
▪ Bilateral metazoans (Bilateria)
▪ Joined based on nature of larval growth
▪ Unity supported by molecular genetics
vertebrates, cephalocordada, echonidermenta, hemichordada
Echinoderms
▪ Includes starfish, sand dollars, sea urchins, brittle
stars, sea cucumbers, crinoids
▪ About 7,000 living species and 13,000 described
fossil species
▪ All marine
“spiny skins”
Features that join echinoderms into a clade
epidermal skeleton made up of ossicles, five fold radial symmetry, water vascular system,
individual calcite crystals with a porous (”Swiss cheese”)
structure or stereom.
ossicles
– Can be loosely held together by liagments and muscles or fused
Water vascular system
hydrostatic system of internal water-filled canals, which in many echinoderms form suckered “tube feet”, which are
sucker-like appendages that the echinoderm can use to move, grip the substrate, or manipulate objects
Class Echinoidea
Sea urchins and sand dollars.
▪ Skeleton (test) made of closely interlocking plates, sometimes fused together -good fossil record!
▪ Test shape ranges from globose (urchins) to flat (sand dollars)
▪ Show near perfect five-fold symmetry (regular echinoids), presumed
primitive condition, or can be bilateral (irregular echinoids), presumed derived condition
▪ Jaw apparatus made of five hard teeth arranged in a circle -Aristotle’s lantern
▪ Regular echinoids mostly epifaunal grazers; some predators
▪ Irregular mostly infaunal deposit feeders
▪ Ordovician -Recent
Class Holothuroidea
Sea Cucumbers
▪ Usually grouped with echinoids (no arms, stem, or tail)
▪ Generally soft-bodied; skeleton reduced to isolate
calcareous plates = ossicles
▪ calcareous ring encircles the pharynx or throat.
▪ Suspension feeders, deposit feeders
▪ Limited fossil record; Silurian? - Recent
Class Asteroidea
True starfish ▪ Five arms (or multiples of five) ▪ Internal body parts, including water vascular system, extend into arms ▪ Tube feet line ambulacral grooves ▪ Highly mobile, use tube feet to move ▪ Predominantly predators Ordovician-Recent
Class Ophiuroidea
Brittle starfish and basket stars
▪ Well-defined central disk and separate arms
▪ Arms are flexible - move fairly rapidly by wriggling their
arms (rather than tube feet)
▪ Most are scavengers and deposit feeders, although they
also prey on small live animals such as small crustaceans
and worms. Some, in particular the basket stars, filter-feed
on plankton with their arms.
▪ Ordovician-Recent
▪ Presumed closely related Asteroidea
Class Crinoidea
Crinoids ▪ Cup-like body carrying arms ▪ Arms have ambulacra and tube-feet ▪ May or may not have a stalk attaching to the substrate ▪ One living subclass ▪ Ordovician-Recent
Small side branches on crinoids
pinnules
Crinoids with stalk
Sea lillies
Crinoids without stalk
feather stars
Crinoids fossil record
▪ 3 subclasses of crinoids are known from the
Paleozoic
▪ Generally stalked
▪ Most abundant echinoderms from the
Ordovician -Permian
▪ Often major constituent Paleozoic limestones
▪ Nearly become extinct end Permian -only one
early Triassic genus!
Blastozoans
Blastoids and “cystoids”
▪ Body covered by theca
Blastozoans
Blastoids and “cystoids”
▪ Body covered by theca
▪ Brachioles extend out along ambulacra (not arms)
▪ Stalked filter feeders
How do classes of blastoids differ?
From the arrangement of plates on theca
theca
Covers the body of blastoids, made of interlocking plates with ambulacra
Class Blastoidea
▸ Highly standardized arrangement of plates
▸ Complex internal folds of calcite below
ambulacra (hydrospires)
▸ Ordovician - Permian; very diverse in
Mississippian
Class Edrioasteroidea
▪ Grouped with holuthurians and echinoids in subphylum Echinozoa
▪ Sessile suspension feeders
▪ Often found growing on brachiopod shells
▪ ambulacra grew in a curved, often spiral or nearly spiral pattern.
Subphylum Homolozoa
▪ Flattened, bilateral to irregular - not pentaradial!
▪ Elongate extension of the body -tails??
▪ Questioned by some whether they are really
echinoderms -generally accepted that they are.
▪ Rare!
▪ Cambrian-Devonian
First definite echinoderms appear in…
The early Cambrian. All are low diversity; suspension feeders
primitive blastozoans (paracrinoids), ancestral crinoids, and homolozoa appear….
In the late cambrain. All are low diversity; suspension feeders.
Echinoderm fossil record during the paleozoic
▸ Crinoids became dominant group - high tiering filter feeders
▸ Gradual loss of other classes
Dominant group of modern echinoderms
echinoids
Major expansion in the Echinoderm fossil record during…
Ordovician - 17 classes in Middle Ordovician! -nearly all
ways of life - many classes low diversity
Paleontological problems with Echinoderms
▪ Origin of group and relations to some odd later Precambrian (Ediacaran) forms.
▪ Why such high diversity at the class level early on?
▪ Relationships of the classes to each other
▪ Relationships with chordates
Symmetry in larval echinoderms
bilateral
