Beechers Trilobite Bed - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 54

claws on the end of the most distal
podomere. The outer, gill, branch
(exopodite) consists of many short
podomeres, each supporting a filament, so
that the whole structure resembles a gill
raker. Almost certainly the inner branch
was used for walking, the outer for
respiration, but there is some discussion
concerning whether the gill branch could
also have been used for filter feeding, at
least in some trilobites. The appendages get
smaller and smaller down the thorax until,
beneath the pygidium, they are tiny and
must have been useless in locomotion.

of trinucleids is the fringe around the
front of the relatively large cephalon
which bears grooves and pits. It is
generally considered that trinucleids were
bottom-dwelling (benthic) animals which
used their long spines as an aid to
supporting themselves on the sediment
surface. The pits in the fringe are likely to
have been sensory: useful in a world where
light was probably very dim.
Like all arthropods, trilobites need to
molt their cuticles when they grow from
juvenile to adult. So, for any adult trilobite,
there might be up to a dozen molts left
behind as potential fossils. Beecher (1895b)
studied the juvenile stages of trilobites,
known as protaspids, and found many of
these in Beecher's Bed ( 59 ). He concluded
that they must belong to the commonest
adult, Triarthrus . Whittington (1957)
questioned this assumption but Cisne
(1973b) agreed with Beecher that the
protaspids belonged to Triarthrus and
produced an interesting analysis of the life
stages of the trilobite. Illustrating his study

Other trilobites
Other trilobites are rare in the trilobite
bed, but specimens of Cryptolithus
bellulus, Stenoblepharum beecheri and a
few odontopleurids are known (Whiteley,
1998). Cryptolithus ( 58 ) is a trinucleid;
unlike Triarthrus these trilobites have no
eyes. They have long genal spines, few
thoracic segments, and a triangular
pygidium. The most extraordinary feature

58

59

59 Scanning electron microscope photograph
of a protaspid early stage trilobite of
Stenoblepharum, showing cephalon, pygidium,
and about three thoracic segments YPM.
Animal is about 1.5 mm 0.06 in long.

58 A pyritized Cryptolithus, ventral view
showing thoracic appendages YPM. Animal
is about 2 cm 0.8 in long.