Lecture notes on Sub-phylum Trilobita

In Greek tria means  three, lobose means  lobe.Trilobita includes the extinct three-lobed anthropods. Trilobites are the oldest of all known arthropods, They are entirely extinct and exclusively marine forms, abundant and widely distributed in the Palacozoic seas. Most trilobites were bottom-dwelling crawling forms. Their creeping adaptations include flat body, leg-like endopodite and dorsal eyes. Some forms were burrowing, some were free-swimming, and still others were planktonic. Burrowing forms had darting, wedged and plough-like carapace. Swimming forms had flat, oar-like endopodites, lean body and lateral eyes. Planktonic forms had floating devices, such as long and radiating marginal spines.They flourished during the Cambrian and Ordovician periods, and extinct  towards the close of the Palacozic, nearly 240-250 million years ago.

The diagnostic features of trilobites are the following:

(i) Body is 3 to 10 cm long, segmented, dorsoventrally flattened and covered by an exoskeleton of calcite .

(ii) Body has three divisions, namely head or cephalon, thorax or trunk and tail or pygidium

(iii) Body is marked out into three longitudinal lobes by two longitudinal grooves. The

three lobes include a median or axial lobe and two lateral or pleural lobes. The name

"trilobita" refers to this trilobed division of the body.

(iv) Head is formed of five fused segments and an acron , the anterior non-segmented part. It is covered by a broad and five-segmented carapace or head-shield.Carapace has an elevated median lobe,called glabella, and two depressed lateral lobes,called genae or cheecks.

(v) Head bears dorsálly a pair of compound eyes dorsally on the genae

.Ventrally it bears the mouth and five pairs of cephalic appendages. Mouth is guarded by the labrum or hypostome and the labium or metastome.

(vi)Cephalic appendages include a pair of antennae and four pairs of maxillipeds. Antennae are preoral, many-jointed, sensory and uniramous. Maxillipeds are jointed and biramous, with exopodite and endopodite (telopodite). Exopodite is respiratory, and endopodite walking in function. Coxa of each maxilliped bears a jaw-like lobe for passing the food into the mouth.

(vii)Thorax is formed of several articulated segments, covered dorsally by hinged exoskeletal plates. The movable articulation of the exosketal plates enables some trilobites to

roll to a ball, when alarmed or attacked (as some wood lice do)

(viii) Thoracic appendages are typically biramous, with leg-like endopodite (telopodite),

respiratory exopodite, and jaw-like coxal lobe.

(ix) Pygidium is formed of a few fused segments, cach with a pair of biramous caudol

appendages. Its exoskeletal plates fuse to form a dorsal tail shield and a terminal telson.

(x) A mid-ventral food groove for the passage of food from behind forwards to the mouth.

(xi) J-shaped alimentary canal, with its crop in the head region,

(xii) Haemocoel is in the axial lobe of the body.

(xiii) Sexes are separate. No sexual dimorphism. Development involves three larval stages

namely protapsis, merapsis and holapsis.

(xiv)Development includes Protapsis ,Merapsis and  holapsis larval stages. Holapsis has head,

thorax and pygidium. It grows to the adult

Evolutionary  significance     

Trilobites are biologically interesting in that they stand in the ancestral line of arthropod

evolution. They are believed to have descended from a precambrian ancestral stock.Trilobites and the extant arthropods have had a common ancestor in the pre-cambrian. It is also believed that trilobites represent the ancestral stock from which the present day arthropods have evolved. The nearest arthropod relatives of trilobites are crustaceans. The crustacean form Apus is regarded as a connecting link between trilobites and crustaceans. However, the current view is that there are probably four lines of arthropod evolution from different annelidan and near annelidan ancestors, and also the evolution of chitinous exoskeleton and jointed appendages ,occurred independently four times in evolution.