EVOLUTION OF TRILOBITE & ITS STRATIGRAPHIC SIGNIFICANCE

SEMINAR CONTENTSIntroductionWhat are Trilobites?Evolutionary trends in TrilobitaPalaeoecology, Stratigraphic implicationConclusionsReferences

IntroductionTrilobites 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- HeadThorax- BodyPygidium- Tail region

Geological Time MarkersStratigraphically significant

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.

Morphology:1) Body divided into 3lobes: an axial and twolateral (hencename)2) Body also divided intohead (cephalon),thorax andabdomen(pygidium)3) Segmented body4) Hard exoskeleton5) Well developed eyes6) Biramus limbs (7) Extinct group of marine organisms

PHACOPS

Trilobite Species

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

COMING TO MIDDLE SEGMENT THORAXIC REGION SHOWS THE TREND OF EVOLUTION AS FOLLOWS1.PROGRESSIVE DECREASE IN THE NUMBER OF THORAXIC SEGMENT2.CHANGE FROM SPINOPLEURAL EXTREMITY TO BLENTY ROUNDED 3.PROGRESSIVE ENLARGEMENT OF PYGIDIUM4.HIGHLY MODIFIED PYGIDIUM WITH SEGMENT DIFFERENT FROM PLEURAL PYGIDIUM SHOWING DISAPPEARENCE OF ALL SEGMENT AND LASTLY PYGIDIUM SHOW REDUCTION OF LENGTH IN AXIAL LOBE.

EVOLUTIONARY TREND IN EYE/CEPHALON REGIONTHE 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.

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.

Stratigraphy

TREMADOC

Stratigraphic Implication

•Trilobites, including Agnostids, are useful for biostratigraphic correlation of Cambrian rocks (Index fossil)

•Palaeoecology and Environment implicationMost Trilobites lived in shallow water, they were benthic. After Ordovician, the survivors tends to be restricted to deeper water.

REFERENCESREFERENCERAYMOND 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/Trilobitewww.ucmp.berkeley.edu/arthropoda/trilobita/trilobita.htmltrilobite.electrolux.comwww.georgehart.com/trilobites/trilobite.html