Trilobites are an extinct group of marine arthropods that lived during the Cambrian and Ordovician periods. They evolved rapidly and showed trends in the segments of their body (cephalon, thorax, pygidium) over their short but significant fossil record. Trilobite evolution is studied through these body segments, with trends including a loss of segmentation in the cephalon and reductions in thoracic segments and pygidium size. As stratigraphic index fossils, trilobites provide important information about paleoecology and correlations between rock layers from the Cambrian period. Most trilobites lived in shallow waters but later survivors were restricted to deeper waters.

1. EVOLUTION OF TRILOBITE & ITS
STRATIGRAPHIC SIGNIFICANCE

2. SEMINAR CONTENTS
Introduction
What are Trilobites?
Evolutionary trends in Trilobita
Palaeoecology, Stratigraphic implication
Conclusions
References

3. Introduction
Trilobites are very significant invertebrate group in
Fossil Record. Short and significant fossil record. In
short span of time records as evolved highly shows as a
Time Markers.
Exclusively Aquatic, marine in habit.
Shows evolutionary trends in all Segments like:
Cephalon- Head
Thorax- Body
Pygidium- Tail region
Geological Time Markers
Stratigraphically significant

4. What are Trilobites?
•Trilobites are hard-shelled, segmented creatures that existed
over 300 million years ago.
• Trilobites were among the first of arthropods(a phylum of hard-
shelled creatures with multiple body segments and jointed legs).
•They belong to class Trilobita(9 orders, over 150 families,
thousands of genera, over 15,000 described species.
•The smallest known trilobite is just over a millimeter long, while
the largest include species from 30-70 cm in length.
•Life styles of trilobites include planktonic, swimming, and
crawling forms (predators and scavengers).
•Most trilobites are about an inch long.

6. Morphology:
1) Body divided into 3
lobes: an axial and two
lateral (hence
name)
2) Body also divided into
head (cephalon),
thorax and
abdomen
(pygidium)
3) Segmented body
4) Hard exoskeleton
5) Well developed eyes
6) Biramus limbs
(7) Extinct group of marine
organisms

7. PHACOPS

8. Trilobite Species

9. STUDY OF EVOLUTIONARY CHARECTERS OF TRILOBITES
THE EVOLUTIONARY CHARACTER EXHIBITED IN TRILOBITE CAN BE
BROADLY STUDIED IN THE THREE DIFFERENT BODY SEGMENTS . IN THE
ANTERIOR SIDE THE HEAD OR CEPHALONE REGION SHOWS A PRIMITIVE TYPE OF
COMPLETE SEGMENTED IN THE GLABELLAR REGION FOLLOWED BY
LENGTHENING PROTRUSION OF GLABELLAR ON BOTH SIDES. THE STEPS OF
DISAPPEARENCE OF SEGMENTATION IN THE GLABELLAR REGION CONTINEOUS
FOLLOWED BY CLOSENESS OF GLABELLOR FURROWS.
THE TREND OF GLABELLOR REGION THE SHAPE CHANGING FROM
HIGHLY SEGMENTED TO SEGMENT LESS GLABELLA

10. COMING TO MIDDLE SEGMENT THORAXIC REGION
SHOWS THE TREND OF EVOLUTION AS FOLLOWS
1.PROGRESSIVE DECREASE IN THE NUMBER OF THORAXIC SEGMENT
2.CHANGE FROM SPINOPLEURAL EXTREMITY TO BLENTY ROUNDED
3.PROGRESSIVE ENLARGEMENT OF PYGIDIUM
4.HIGHLY MODIFIED PYGIDIUM WITH SEGMENT DIFFERENT FROM PLEURAL
PYGIDIUM SHOWING DISAPPEARENCE OF ALL SEGMENT AND LASTLY
PYGIDIUM SHOW REDUCTION OF LENGTH IN AXIAL LOBE.

11. EVOLUTIONARY TREND IN
EYE/CEPHALON REGION
THE TREND OF EVOLUTION OF EYE APPEARS TO BE RENIFORM
CLOSE TO GLABELLOR AND MOVES TOWARDS THE
PERIPHERAL MARGIN OF CEPHALON REGION AND BECOME
DISTINCT AND DISAPPEAR AND AGAIN INCREASE IN THE EYE
SIZE HIGHLY EVOLVED INTO COMPOUND EYE OCCUPIES MORE
OR LESS FULL CHEEK REGION AND THE EYE BECOME WITH
STOCKED EYE. THE STOCKED EYE IS THE YOUNGEST TREND
OF EVOLUTION.

12. Role of Trilobites in Paleoecological studies
•Most trilobites lived in fairly shallow water; they
were benthic.
•They walked on the bottom and probably fed on
detritus.
•A few, like agnostids, may have been pelagic,
floating in the water column.
•Cambrian and Ordovician trilobites generally lived
in shallow water.
•After the Ordovician, when many trilobite group
declined or went extinct, the survivors tended to be
restricted to deeper water.

14. Stratigraphy

15. TREMADOC

16. Stratigraphic Implication
•Trilobites, including Agnostids, are useful
for biostratigraphic correlation of Cambrian
rocks (Index fossil)
•Palaeoecology and Environment implication
Most Trilobites lived in shallow water, they
were benthic. After Ordovician, the survivors
tends to be restricted to deeper water.

18. REFERENCES
REFERENCE
RAYMOND C. MOORE (1952) Invertbrate Fossils Pub. Mc
Graw Hill Book Com. NY London, Pp 497-515
Web Address:
http://www.trilobites.info/
en.wikipedia.org/wiki/Trilobite
www.ucmp.berkeley.edu/arthropoda/trilobita/trilob
ita.html
trilobite.electrolux.com
www.georgehart.com/trilobites/trilobite.html