Chapter 16: Deuterostomes: Echinoderms and Hemichordates

What are echinoderms characterised by?

1. Watervacular system including tube feet

2. Mesodermal skeleton of calcitic plates with a stereom structure

3. Pentameral symmetry

Clade Ambulacraria

echinoderms and hemichordates

Water vascular system in echinoderms

system in which water is forced around the plumbing by muscular action while tube feet, extending from the system are modified for food processing, locomotion, respiration

What animals are included in the phylum echinodermata

• Sea lilies,

• Sea urchins

• Sand dollars

• Star fish

• Sea cucumbers

What is the phylum echinodermata generally split into?

1. non-stalked eleutherozoans

2. fixed, stalked pelmatozoans

Low-level epifaunal echinoderms

Edrioasteroids, Helicoplacoid, Holothurian

High level epifaunal echinoderms

Blastoids, crinoids, calcichordate, rhombiferan

Mobile infaunal and epifaunal echinoderms

Asteroid, Echinoid

Mobile epifaunal echinoderms

Echinoid, Ophiocistioid

What is echinoderm classification based on?

1. Main body enclosed by plates (theca or test)

2. Areas bearing the tube feet (ambulacra) with perforations/ holes (brachioles)

3. possession of cup (calyx) and arms (brachia)→ only in pelmatozoans

Classes within the Subphylum Pelmatozoa

1. Class EOCRINOIDEA

2. PARACRINOIDEA

3. Class BLASTOIDEA

4. Class DIPLOPORITA

5. Class RHOMBIFERA

6. Class CRINOIDEA

Class CRINOIDEA

• Calyx with lower cup and upper tegmen

• Sea lilies and feather stars.

• Ordovician (Tremadocian) to Recent

Classes within the subphylum Eleutherozoa

1. Class EDRIOASTEROIDEA

2. Class ASTEROIDEA

3. Class OPHIUROIDEA

4. Class ECHINOIDEA

5. Class HOLOTHUROIDEA

Class EDRIOASTEROIDEA

• Disk‐shaped thecae with straight or curved ambulacra with the mouth situated centrally and the anus sited on the interambulacra.

• Precambrian (Ediacaran)?, Cambrian (Lower) to Carboniferous (Gzelian)

Class ASTEROIDEA

• Between 5 and 25 arms with large tube feet extend from a central disk.

• Starfishes or sea stars.

• Ordovician (Floian) to Recent

Class OPHIUROIDEA

• Five long, thin, flexible arms, consisting of vertebrae (in advanced forms) and with small tube feet, extend from large, circular central disk

• Brittle stars or basket stars.

• Ordovician (Floian) to Recent

Class ECHINOIDEA

• Test is usually globular with plates differentiated into ambulacral and interambulacral areas

• Mouth on underside, anus on upper-side or sited posteriorly

• Sea urchins, heart urchins, and sand dollars.

• Ordovician (Katian) to Recent

Class HOLOTHUROIDEA

• Body is cucumber‐shaped with leathery skin with muscular mesoderm and spicules. A ring of modified tube feet surrounding the mouth.

• Sea cucumbers.

• Ordovician (Floian) to Recent

What is Helicoplacus?

• Potentially a part of the stem group for the phylum echinodermata

• Did not survive the Cambrian substrate revolution

• Helicoplacoids had 3 ambulacral areas with tube feet wrapped around spindle-shaped bodies

• Group lacked appendages

• Helicoplacoids have many planes + stereom structure + ambulacra and a mouth sited laterally together with an apical anus

Crinoidea

• sea-lilies

• sessile, pentameral symmetry

• 2 main life strategies: stalked + mobile

• Some non-stalked genera free to crawl and swim - flexible arms

• rooted to sea bed by a stalk→ some forms are free-living

• modern forms live in dense clusters from tropics → polar latitudes

• feather stars→ continental shelf

• fixed sea lilies→ deep water environments of continental slope

• calyx from 2 rings of calcitic plates

Crinoidea Evolutionary history

• True crinoids appear in Tremadocian - lower Ordovician

• expansion in Early Ordovician tropics - adaptive radiation

• Paleozoic crinoids mostly stalked

• Articula - all post Paleozoic crinoids (exception of some Triassic cladid survivors)

• Crinoid diversity peaked in Mississippian - Age of Crinoids

^ How/ Why?

1. five major groups were in various states of recovery after the Frasnian‐Famennian extinction event, particularly the advanced cladids

2. Disappearance of shelf edge coral-stromatoporoid buildups at end Devonian → improved water circulation → stenohaline conditions→ growth of communities → new ecospace + lack of predation pressure

• Cooler water conditions associated with Late carboniferous glaciation → decreased diversity

• Mesozoic crinoids very recovered Paleozoic diversity

• In Jurassic → mobile, stalkless comatulids → adaptation to increasing predation pressures from echinoids

Blastozoans

• Informal grouping including extinct groups: cystoids, blastoids, eocrinoids

• Pelmatozoans

• short stem, lacked brachia

• high level filter feeders

Cystoids

• mid-Paleozoic

• Diploporita and Rhombifera

Blastoids

• Small petamerally symmetric animals

• Short stems, hydrospires for respiration

• Silurian → Permian

• Mouth surrounded with spiracles

Eocrinoids

• Earliest of the brachiole-bearing echinoderms

• Huge range of thecal shapes with primitive holdfasts

• Sutural pores rather than thecal pores

• Lack of respiratory structures

• Biserial brachial appendages

• In rocks from early Cambrian → Late Silurian

Echinoidea

• sea urchins, sand dollars

• Robust, rigid exoskeletons or tests composed of plates of calcite coated by an outer skin covered by spines

• shallow water marine environments, part of the nektobenthos

• first radiation in Ordovician

Echinoidea Basic Morphology

• Exoskeleton or test of most regular echinoids = hemispherical

• test is built from network of interlocking calcite plates

• ambulacral areas carry tube feet and alternated with interambulacral areas

• genital plate perforated by a hole to allow the release of gametes

• peristome contains mouth parts

• organs suspended in the test and supported by fluid

• digestive system lacks a stomach → esophagus + large, small intestine → waste material from periproct

• NS: nerve ring and 5 radial nerves

Regular vs. Irregular Transition of Echinoids

• regular echinoids e.g sea urchins are the most basal

• irregulars - specialised for forward movement

• irregular morphotype evolved rapidly + changes for burrowing

Some changes include

1. asymmetric test

2. short numerous spines

3. large adapical pores + posteriorly placed periproct together with keeled teeth

4. secondary bilateral symmetry superimposed on pentameral

5. ambulacral area modified→ food groove

6. tube feet extendable with flattened ends- respiration

Echinoid life modes

From epifaunal mobile behaviour to infaunal burrowing strategies

Lower Jurassic → Lower Tertiary Transition from the sea urchins through the heart urchins to sand dollars

1. Flattening of Test

2. Elongation of Test

3. Decrease in size of tubercles and spines

4. Increase in number of tubercles and spines

5. Shifting of anus posteriorly

6. Shifting of peristome anteriorly

7. Decrease in size of peristome

Evolutionary history of echinoderms

• First appeared Mid Ordovician but abundant in lower Carboniferous

• early record→ fragile skeleton or not common in Paleozoic benthos

• Paleozoic: increase in number and size of ambulacral areas and sophistication of Aristotle’s lantern - most genera relatively small

• Decline in diversity in Late Carboniferous

• By Permian only 2 groups: detritus feeders and opportunists

• Regular echinoids diversified in Late Triassic, Early Jurassic

• Irregulars appeared E. Jurassic

• Diversity reduced at K-Pg - both recovered rapidly in early Cenozoic

Starfish

asteroidea

Brittle stars

Ophiuroidea

Where is the mouth/ anus of asteroidea + ophiuroidea?

• mouth = underside, oral/ dorsal surface

• anus - ventrally on adoral surface

Asteroids life mode

• mobile lifestyle within benthos

• benthic deposit feeders that ingest prey/ filter feed

• Some asteroids - prey on shellfish but others are slow-moving shore and shallow marine animals

• light detecting cells at ends of arms- no true eyes

Asteroids preservation

• skeletons disintegrate rapidly after death due to feeble cohesion between the skeletal plates

• several starfish Lagerstatten deposits

What are the 3 classes of asterozans

1. basal Somasteroidea

2. Asteroidea or starfishes

3. Ophiuroidea or brittle stars

Somasteroidea

• earliest starfish-like animals

• disappeared in Mid Ordovician

When was the first true starfish?

• end Ordovician

• Immobile, infaunal sediment shovelers

• Relatively uncommon in Paleozoic rocks - important in Mesozoic and Cenozoic

• Now represented by Neoasteroidea

Ophiuroids

• first appeared in E. Ordovician

• Group may be paraphyletic

• Classification based on arm structure and disk plating

• prefer deep-water environments

Ophiuroid body plan

• subcircular central disk and five long, thin, flexible arms

• mouth is situated centrally on the lower surface of the disk

• most of the disk is filled by the stomach and, in the absence of an anus, waste products are regurgitated through the mouth

• arms consist of highly specialized ossicles or vertebrae.

Asteroid basic body plan

• five arms radiating from the disk, which is coated by loosely fitting plates permitting considerable flexibility of movement

• Additional respiratory structures, called papulae, project from the celom through the plates of the upper surface → backup system aids the high metabolic rates of these active starfish

Carpoidea

• Marine animals from Mid Cambrian to Late Carboniferous

• Calcite, echinoderm-type skeleton lacking radial symmetry

What are the 2 types of Carpoidea?

1. Cornutes (boot-shaped, series of gill slits on LS of roof of head)

2. Mitrates (derived from cornutes, more bilaterally symmetrical with covered gill slits on both sides)

Calcichordate hypothesis:

• carpoids and chordates share many characters

• ^ based on anatomy and anatomy of embryos of modern echinoderms and chordates

• But they are echinoderms

• Molecular phylogenetic analyses show that Hemichordata is the sister group of Echinodermata, forming together the Ambulacraria, and that Ambulacraria is the sister group of Chordata

What is the phylum hemichordata characterised by?

• rod-like structure stomochord

• soft-bodied animals, bilateral symmetry, no segmentation

What are the 2 classes of hemichordates?

1. tiny, mainly colonial, pterobranchs that lived in the sessile benthos

2. the larger infaunal acorn or tongue worms, the enteropneusts that lived in burrows mainly in subtidal environments

What features do hemichordates share with chordates?

• do not have a notochord

• share with chordates possession of gill slits + giant nerve cells in the collar region → potentially equivalent to similar nerve cells in basal chordates

• larval mode if not in adult mode

How do pterobranchs superficially resemble bryozoans?

• both colonial

• zooids feed with tentaculate ciliated arms

Which hemichordate genera are used as analogs for graptolites?

• Cephalodiscus and Rhabdopleura

• Genera + graptolites have a housing construction with fusellar tissue and a stolon system or petocaulus, an organic connection between the individual zooids

What are the 2 main orders of graptolites?

1. Dendroidea

2. Graptoloidea

Dendroidea first and last appearance

Cambrian (middle) – Carboniferous (middle)

Graptoloidea first and last appearance

Ordovician (Tremadocian) to Devonian (Pragian).

Morphology of a graptolite colony

• Collagenous housing structure

• tubarium -characterized by a growth pattern of half rings of fuselli interfaced by zigzag sutures

• Each colony/ tubarium grew from a small cone, the sicula, as one or a series of branches or stipes

• Stipes isolated or linked by lateral struts

• thecae house zooids

• aggregates of tubaria = synrhabdosomes → structures either asexual budding or common attachment (more likely) to a single float or patch of substrate

Dendroidea

• dendroid tubarium was multibranched, like a bush, with its many stipes connected laterally by struts or dissepiments

• two differently sized thecae: autotheca and bitheca grew along stipes

• earlier genera benthic → late Cambrian some genera detached themselves → planktonic lifestyle

Graptoloidea

• graptoloid tubarium is superficially simpler and consists of an initial sicula, divided into a prosicula and a metasicula, with at its apex, distally, a long thin, spine, the nema

• metasicula composed of fusellar tissue → bundles of short, branching fibrils

• Graptoloid taxa: The architecture of the graptoloid tubarium depended on three sets of structures: the number of stipes or branches, their mutual attitudes and the shape of the thecae.

Retiolitids

• group of biserial graptoloids

What were graptolites made of?

• most assemblages in black shales diagenetically altered, metamorphosed within/ around orogenic belts

• graptolite fusellum, when preserved, consists mainly of an aliphatic polymer, immune to base hydrolysis

• lacks protein even though both structural and chemical analyses of the fusellum of living Rhabdopleura suggest that it was originally composed of collagen

• suggested that the collagen had been replaced by macromolecular material from the surrounding sediment

• new analyses suggest that the aliphatic composition of graptolite fusellum reflects direct incorporation of lipids from the organism itself by in situ polymerization

• A similar process may account for the preservation of many other groups of organic fossils

Two types of skeletal tissue of graptolites

• fusella tissue + cortical tissue

• cortical tissue potentially secreted by mobile zooids

Graptolite Ecology

• Marine

• debated whether attached to seabed/ seaweed or free floaters in the plankton

• early dendroids a part of the sessile benthos

• detachment at the beginning of Ordovician with genera entering the plankton

• idea of attachment rejected→ no attachment structures; risky in turbulent surface waters

• accepted that colonies could move up and down the water column

• potential support from gas bubbles and fat in their tissues/ vane- like extensions to the nema

• during feeding → upward movement of the colony in the water column→ nutrient-rich photic zone → sweep-net feeders

• food particles → food basket between stipes→ aided by beat of ciliated zooids

4 main stages of morphological development of graptolites

1. The transition from sessile to planktonic strategies in the dendroids during the late Cambrian and Early Ordovician

2. At the end of the Tremadocian (early Early Ordovician), the appearance of the single type thecae of the graptoloids

3. The development of the biserial tubarium in the Dapingian (Middle Ordovician)

4. Finally, the origin of the uniserial monograptids

Why was a trend towards reduction in stipes more advantageous?

simpler stipe configuration hydrodynamically more stable/ better adapted to turbulence/ aided motion of graptolites through the water column/ prevented interference between thecae on adjacent stipes → more efficient colony structure

3 phases of graptolite expansion

1. Dichograptid radiations

2. Axonophoran radiations

3. Monograptid/Retiolitid radiations

→ losses of diversity in the Tremadocian and Hirnantian crises, and final disappearance in the Early Devonian

Four sequential graptolite faunas from Early Ordovician → Early Devonian

1. Anisograptid → Tremadocian

2. Dichograptid → Floian

3. Diplograptid → Darriwilian, Sandbian, Katian, Hirnatian

4. Monograptid → Pragian