Jurassic

Phylum: Mollusca
Class: Cephalopoda
Subclass: Ammonoidea
Order: Ammonitida

Straight edge
Growth lines
Curved plate

Calcite
Disarticulated from ammonite shell

One could be forgiven for thinking, at first glance, that this was the valve of a bivalve mollusc. However further interrogation of the fossil: the shape, the fact that the growth lines are on one side of the shell but there is no sign of adductor muscle scars, palial sinuses or other characteristic bivalve signs, and the 'bobbled' texture on the plate, leads us to the conclusion that this is not from a bivalve.

Rare examples of ammonite shells with aptychi preserved articulated have shown these plates to be located in the body chamber of ammonites. Upon death most pairs of aptychi disarticulate and hence they are found as single plates, such as this one.

Phylum: Mollusca
Class: Cephalopoda
Order: Belemnitida

Guard of radial calcite crystals
Conical cavity where phragmocone used to be

Aragonitic phragmocone dissolved
Sediment infil of conical cavity where phragmocone used to be (set 1 only)

Phylum: Mollusca
Class: Cephalopoda
Order: Belemnitida

Set 1:
Group: Cylindroteuthis
Set 2:
Group: unknown

Guard with radial calcite crystals
Crushed conical hole where phragmocone used to be
Long, near cylindrical shape

Dissolution of phragmocone
Sediment infill of conical cavity where phragmocone used to be
Crushing of infilled phragmocone area

Jurassic

Phylum: Mollusca
Class: Cephalopoda
Order: Belemnitida
Genus: Hibolites

'Bullet' shape
Guard with radial crystals
Hollow phragmocone area

Mineral replacement of calcitic guard
Dissolution of aragonitic phragmocone
Sediment infil of phragmocone area

Hibolites are not as bullet shaped as most belemnites, and at first glance could be confused with an Echinoid spine. However the radial arrangement of crystals and the presence of a hollow region where the phragmocone would once have been allows us to, after some examination, confirm these specimens as belemnites.

'Bullet' shape
Conical hole where phragmocone used to be
Radial crystals (here in cross section)

Peel of a cross section through a belemnite (manmade)

Have a look at this specimen under a microscope. The peel allows us to see very fine details.

Phylum: Mollusca
Class: Cephalopoda

Straight
Chambers
Septa

Manmade internal cast

Nautiloids originated in the Cambrian and radiated during the Ordovician. Straight nautiloids went extinct in the Permian, but planispirally coiled nautiloids, for example the nautilus, live in the sea today.

Cambrian

Phylum: Arthropoda
Class: Trilobita
Order: Ptychopariida
Genus: Ellipsocephalus

Cephalon
Thorax
Pygidium
No eyes

Cast of a mould (manmade)
External moulds of trilobite hard parts

Ellipsocephalus hoffi was a blind species of trilobite. It likely lived in deep, poorly lit, aphotic environments.

Phylum: Arthropoda
Class: Trilobita
Order: Corynexochida
Genus: Ogygopsis

Glabella
Eye ridges
Cephalon
Thorax
Pygidium

External mould (manmade cast)

Cambrian

Phylum: Arthropoda
Class: Trilobita
Order: Redlichiida
Genus: Paradoxides

Cephalon
Thorax
Pygidium
Glabella

External mould (manmade cast)

Phylum: Arthropoda
Class: Trilobita
Order: Asaphida
Family: Trinucleidae

Cephalon
Thorax
Pygidium
Large fringe at front of cephalon

External mould (manmade)

Trinucleids were fast moving low level epifaunal suspension feeders. The large fringe at the front of the cephalon may have been for food gathering, and their large cephalon size may have been an antipredatory adaptation. Some exceptionally preserved specimens have been found with sediment in the guts of the animals, perhaps supporting the suspension-feeding hypothesis.