Phylum: Echinodermata
Class: Crinoidea

Stem
Calyx
Crown
Ossicles (fused)

Cast (manmade)

Florigemma

Jurassic

Phylum: Echinodermata
Class: Echinoidea
Order: Cidaroida
Genus: Cidaris

'Club' shaped
Thorns

Mineral replacement (calcite)
Disarticulated from body shell

This is a spine from a regular echinoid. You can tell this by the shape and thickness of the spine.
Irregular echinoids live under the sediment in burrows, using their spines for movement, not for protection, and hence the spines are fine and able to be manipulated.
Regular echinoids live epifaunally, and so are a target for predators. These spines are needed to prevent attack of the shell containing the soft parts of the animal, and the 'club' like shape decreases access opportunities for predators. Other regular echinoid spines are long and thin, pointed, offering a more 'offensive defence' than spines such as this one.

depressus Lestre

Jurassic

Phylum: Echinodermata
Class: Echinoidea
Subclass: Irregularia
Order: Holectypoida
Genus: Holectypus

Bilateral symmetry
Five radial ambulacra
Spine attachment scars (very small)
Peristome and anus out of line

Shell mineral replacement
Sediment infil
Disarticulation of spines

This is a primitive irregular echinoid which would have had a functional jaw apparatus (not preserved) held within the peristome. The anus has migrated and the symmetry is bilateral, so this echinoid is clearly identifiable as irregular.

Praecursor

Cretaceous

Phylum: Echinodermata
Class: Echinoidea
Subclass: Irregularia
Order: Spatangoida
Genus: Micraster

Bilateral symmetry
Five radial ambulacra
Peristome (opening for mouth) and periproct (opening for anus) migrated
Spine attachment scars

Sediment infil
Shell mineral replacement
Disarticulation of spines

Note the streamlined shape for burrowing through the sediment, and how the ambulacra, which bear the tube feet, do not continue onto the underside of the animal where the mouth is.
This is because the tube feet are used for respiration but not for movement, for which small spines are used.

Cretaceous

Phylum: Echinodermata
Class: Echinoidea
Subclass: Irregularia
Order: Echinoneoida
Genus: Conulus

Bilateral symmetry
Five radial ambulacra
Interambulacra
Migrated periproct (opening for anus)
Peristome (opening for mouth)

This fossil was collected in Quaternary river gravel. Originally it was preserved in the Upper Chalk (Cretaceous). This was broken up into clasts by erosive processes, one of which was this irregular echinoid. The echinoid clast was carried by a river downstream and deposited in the river gravel unit during the Quaternary.

Note how the anus has migrated, but the peristome is still in the position you would expect from a regular echinoid (not migrated). Notice also how the ambulacra bearing the tube feet continue onto the underside, a feature not seen in more streamlined and derived irregular echinoids.

Upper Cretaceous

Phylum: Echinodermata
Class: Echinodea
Subclass: Irregularia
Order: Holasteroida

Bilateral symmetry
Mouth and anus out of line
Five radial ambulacra
Interambulacra with plates

Sediment infill
Mineral replacement of shell

Set 1: Anus and mouth appear to have been drilled to emphasise their positions.
Set 2: Fracturing of shell around the anus which obscures it. This would be a weak part of the shell which diagenetic processes have crushed.

Phylum: Echinodermata
Class: Echinoidea
Subclass: Regularia

Pentaradial symmetry
Anus and peristome lie in line
Spine attachments
Five sets of ambulacra

Cast (internal mould of an external mould)

Spine attachments very much larger than for irregular echinoids as they have to hold much larger spines. Spines on regular echinoids are for defence and preventing predation, unlike for irregular echinoids where the spines are for movement in burrows. Watch a video of regular echinoids moving: BBC Wildlife Army of Sea Urchins

Modern

Phylum: Echinodermata
Class: Echinoidea
Subclass: Regularia

Pentaradial symmetry
Anus and peristome in line
Spine attachments
Ambulacra
Attachments for the jaw apparatus visible on the inside of the peristome

Original shell material
Jaw apparatus disarticulated
Soft parts removed

Look at the colour of the shell and compare it to the permineralised specimens. Colour is almost always lost in the fossilisation process, even when the preservation looks to be near-perfect.

Cambrian to Carboniferous

Phylum: Hemichordata
Class: Graptolithina
Order: Dendroidea

Numerous branching stipes
Colonial
Thecae

Flattened

In this specimen we cannot see the size of a potential holdfast. Because of this we cannot say if this is a planktonic or benthic animal.

Dendroid graptolites began as benthic organisms attaching to the seafloor with a holdfast, but some diversified as planktonic animals, with a reduced holdfast.

Ordovician

Phylum: Hemichordata
Class: Graptolithina
Order: Graptoloidea

Thecae
Stipes
Rhabdosome
Nema

Flattened
Carbon film
Red colour from iron in sediment seeping into fossil

The number of stipes decreased over time, from 4 to 3 to 2. This specimen has 3 stipes, placing it between four stiped animals such as Tetragraptus and two stiped animals such as Didymograptus in time.