Phylum Arthropoda-

Trilobita

Prepared by Welfredo Yu

• "Three Lobes" • Earliest known group of Arthropods• Crab like animals, not bugs! • They are a very diverse Class of animal.

WHAT IS TRILOBITES?

• First evolved in the Lower Cambrian and became extinct by the end of the Permian.

• They are most common during the Cambrian, Ordovician and Silurian.

• Therefore they have no modern equivalents and an understanding of their soft parts has to be based on modern day arthropods that show some similarity i.e. crustaceans.

• They are marine animals.

• Some were filter feeders,particle feeding some were scavangers, others were predators. They probably ate anything smaller than themselves, including other trilobites.

• Trilobites had a thick exoskeleton for protection.

• Some have beautiful spines, some have no eyes, some have long tails.

• Some Could Enroll:Some trilobites could enroll them selves into a ball for protection, similar to what little pill bugs do.

Protection

ENROLLED TRILOBITES

• Trilobites, a common fossil, were known to people throughout history. The first "scientifically" described trilobite comes from Wan Shizen of China. In 1689 he described trilobite pygidia (tails) as batstones.

• The first scientific drawing of a trilobite comes from Rev. Edward Lhwyd. He has a sketch of a trilobite in the 1698 volume of "The Philosophical Transactions of the Royal Society". The trilobite, which we know as Ogygiocarella debuchii, is called a "flatfish".

DISCOVERY

• TRILOBITE pygidia (tails) as batstones

• Ogygiocarella debuchii, "flatfish".

• They are segmented animals and have a chitinous exoskeleton.

• They have a bilateral symmetry i.e. either side is symmetrical.

• They have jointed limbs with an identical pair on the either side of the body.

• The body can be divided into segments:Laterally:

• A central or axial segment.• Bounded by two lateral segments.

Transversely into three regions:• Cephalon - “head” area.• Thorax - “body” with hinged segments.• Pygidium - “tail” with fused segments.

DORSAL VENTRAL

TOP SIDE OF CEPHALON BOTTOM OF CEPHALON

DORSAL VENTRAL

• Body Size: 1 mm to 72 cm in size!The smallest trilobite is currently Acanthopleurella stipulae at around 1 mm, that's the size of a fleck of pepper.

• The largest trilobite is currently Isotelus rex which has been found to reach 72 cm. That's over 2 feet!

BODY SIZE

• Trilobites tend to be fairly small being 5 - 8 cm long on average although extremes do occur from 5 mm to 70 cm.

• Because they had a rigid exoskeleton growth caused problems.

• How did they grow?• They malted i.e. they shed their exoskeletons for a larger by

a process called “ecdysis”.

•ECDYCIS

• the facial sutures (red) split, opening the cephalon.• This provides an exit for the molting trilobite (purple) from

its old exoskeleton (orange).

• Trilobites Molted:Like crabs and lobsters, trilobites had to molt when growing. Just after molting, or shedding their exoskeleton, they were soft and vunerable. Most trilbite fossils are actually molted exoskeletons.

• This is the head shield, which consists of a central region GLABELLA, or axial region.

• The glabella is usually convex but does vary in size and shape in different species.

• The FACIAL SUTURE is the line along which the skeleton is cracked to allow ecdysis, it divides the cheeks into 2 areas:

• FIXED CHEEK: That part which stays attached to the glabella.

• FREE CHEEK: That part which becomes separated during ecdysis.

CEPHALON

Here are some examples of the three suture types:

Eodiscina (proparian)

Phacopina (proparian)

Calymenina (gonatoparian)

Ptychopariina (opisthoparian)

Asaphida (opisthoparian)

Glabella end

ANTERIOR

Pygidium end

POSTERIOR

• Trilobites can be divided into three from side to side.

• Trilobites can be divided into three from side to side.

• It has a central axis separated from the 2 lateral regions by the AXIAL FURROW.

• Each segment of the lateral area has PLEURA.

• The thorax contains segments, which are jointed and able to move independently.

• This flexibility allows some trilobites to roll up to give protection to the softer under part.

• The number of these segments varies e.g. 2 - 40.• However, in any Genera the number is constant e.g. 13 in

Calymene.• These are in pairs one on each side of the body.

THORAX

PYGIDIUM• This is a semicircular or triangular shield.• These have a number of fused segments, which varies

from 2 - 30 but cannot move independently.• the pygidium (tail piece) can range from extremely small

(much smaller than the cephalon) to larger than the cephalon.  There are four general categories of pygidium relative size, shown below:

micropygous pygidium much smaller

than cephalon

subisopygous pygidium subequal

to cephalon

isopygous pygidium equal

to cephalon

macropygous pygidium larger than cephalon

eye ridges: These are consistently present in primitive trilobites, connecting the front of the  palpebral lobe with the axial furrow (a feature  lost in many post-Cambrian trilobites) pygidium: The posterior tagma of greater  than one segment is a conspicuous feature of all trilobites (but not restricted to Trilobita). Pygidia are typically very small in primitive  forms (e.g., Olenellina)

Together with the organization of the body into three anterior-posterior divisions (cephalon, thorax, and pygidium), and the three longitudinal lobes (axial lobe and two flanking pleural lobes), the body features on this page serve to readily distinguish trilobites from all other known arthropod groups.

calcitic compound eyes: While other compound eyes are found in Cambrian arthropods, only those of trilobites have corneal surfaces composed of prismatic calcite lenses (with the crystallographic axis normal to the lens surface).circumocular sutures: In Cambrian holochroal trilobite eyes, a suture around the edge of the shared corneal surface assisted in molting of holaspid trilobites. In post-Cambrian trilobites this feature is secondarily lost, leaving the corneal surface attached to the librigena.

rostral plate: a ventral anterior plate separated from the rest of the cephalic doublure by sutures is very well developed in primitive trilobites (e.g., Redlichiida), narrower in other trilobite orders, and secondarily lost in some advanced forms (e.g., Asaphida and Phacopida)

•

•

Compound eye

3 STAGES OF LIFE CYCLE

• TRILOBITES GROW AND MOLTED 1.PROTASPIS-planktonic and consisted of little more than a small cephalon. 2.MERASPIS-the animal is seen to have both cephalon and pyrigium but without thorax. 3.HOLASPIS-three body sections but it still has considerable growing to do before it becomes an adult.

3 STAGES OF LIFE CYCLE

Agnostida

Redlichiida

Corynexochi

da

Odontopleuri

daLichid

aPhacopida

Proetida

Asaphida

Harpetida

Ptychopariida

10 orders in the classAGNOSTIDA - Among the early trilobites, with a basic, clamshell-like appearance. Suborders Agnostina and Eodiscina. Representative species pictured here: Ptychagnostus akanthodes (Agnostina)

REDLICHIIDA - Including the most primitive trilobites from the lower Cambrian. Suborders Olenellina and Redlichiina. Representative species pictured here: Redlichia sp. (Redlichiina)

• CORYNEXOCHIDA - An often spiny group united by a shared hypostomal attachment. Suborders Corynexochina, Illaenina, and Leiostegiina. Representative species pictured here: Kootenia sp. (Corynexochina)

10 orders in the class

10 orders in the class

ODONTOPLEURIDA - Very spiny trilobites, a sister group to the Lichida.Suborder Odontopleurina; superfamilies Dameselloidea and Odontopleuroidea.Representative species pictured here: Selenopeltis buchii.

10 orders in the class

LICHIDA - Some of the most ornately sculptured species fall into this group. Suborder Lichina; families Lichidae and Lichakephalidae.Representative species pictured here: Arctinurus boltoni (Lichioidea)

10 orders in the class

PHACOPIDA- The well-known Phacops, with its beautiful compound eyes belongs here. Suborders Calymenina, Phacopina, and Cheirurina. Representative species pictured here: Phacops sp.(Phacopina)

10 orders in the class

PROETIDA - Includes some of the last trilobite species before the Permian Extinction. Suborder Proetina, with three Superfamilies. Representative species pictured here: Proetus granulosus (Proetoidea)

10 orders in the class

ASAPHIDA - All share a ventral median suture, and most a similar development. Suborder Asaphina, with six Superfamilies comprising ~20% of alltrilobites. Representative species pictured here: Homotelus sp. (Asaphoidea)

10 orders in the class

PTYCHOPARIIDA - Bearing the "generic trilobite" body plan, but many weird variations! Suborders Ptychopariina and Olenina (Harpina has been elevated to order Harpetida; see below) Representative species pictured here: Modocia sp. (Ptychopariina)

10 orders in the class

HARPETIDA - Bearing the distinctive, broad, often intricately pitted, cephalic fringe. In 2002, split out of the Ptychopariida and elevated from suborder to full order. Representative species pictured here: Eoharpes sp.

NEKTASPIDA - The so called "soft-shelled trilobites" such as Naraoia have been classified as an order of trilobites by some. Click on the image or link to learn more about them, and to see how they are handled in the 1997 Treatise.

10 orders in the class