Origin and Early History of the Vertebrates

The Protochordates as Ancestors

• Some living chordate groups may resemble the primitive vertebrate ancestors (protochordates)

Hemichordates• Burrows in marine

sediments (acorn worms)• (+) pharyngeal gill slits;

tornaria larvae; • Considered as direct

ancestor because it has primitive notochord but appears as an extension of the digestive tract to the proboscis region.

Haltabo 09-10

Urochordates:Tunicates

• Small marine invertebrate chordates quite common today; hold the key to vertebrate origin.

• Also known as tunicates• Appears thick-walled, squirt

sac with 2 openings. Inside the central cavity of the animal are numerous gill slits.

• (+) notochord-supported tail for locomotion

• Nervous system: dorsal hollow nerve cord, several ganglia, nerves

• Adult form: Sessile; (-) notochord; (+) gills – for filter feeding

Tunicate larva

• Characteristics: Tadpole shape and possesses the three chordate characteristics.

• It has been suggested that such larva may have become sexually mature without developing the other adult tunicate characteristics.

• It evolves into a fishlike vertebrate similar to the lancelet, which is a chordate that shows the three chordate characteristics in its adult form, as well.

Haltabo 09-10

Cephalochordates• Small, eel-like, sand-burrowing

animals; entirely marine dwellers (free-living; sessile; solitary; colonial)

• Filter-feeders; larva is similar to jawless fishes (hagfish and lampreys)

• Microscopic suspension feeders• Gender: Dioecious (separate

male & female species); • Fertilization: external• Earliest known predecessor:

Pikaia, and Yunnanozoon

Characteristics of a Hypothetical Primitive Vertebrate

• The prechordate ancestor was probably a primitive, sessile echinoderm which used tentacles for filter feeding.– Loss of tentacles and formation of pharynx

with gill slits resulted in the first Urochordates, which soon replaced the earlier ciliated larval form with a tailed larva containing a notochord and dorsal nerve cord

– This new larval form is found in modern Urochordates (tunicates) which retain the sessile filter-feeding stage as adults

– Some of the ancient tailed larvae probably gained the ability to reproduce without the adult stage and thus gave rise to both modern Cephalochordates and the first vertebrates.

From comparison between modern and fossilvertebrates and cephalochordates, we canreconstruct a hypothetical primitive vertebrates It would be small and look like fish It would have myotomes (segments of

muscles) the muscles would run longitudinally between

the septae, which would be anchored to the notochord

Haltabo 09-10

Branchiostoma lanceolata, Amphioxus

FEATURES:(-) developed head ; (+) pharyngeal gill slits; (+) tail with anus; body (myotomes); (+) notochord; Closed circulation; (-) colorless blood; (+) thyroid gland

Similar to Amphioxus but there would be several structures absent in Cephalochordates

Fossil Records of the Earliest Vertebrates

• Cathaymyrus-earliest confirmed chordate fossil from the Lower

Cambrian Chengjiang Fauna in China

• Soft-bodied fossil assemblage much like that of younger Burgess Shale

The first vertebrate fossils are tiny bits of bone (Calcium phosphate) found in the Late Cambrian Deadwood Formation of Wyoming• This is a marine

sandstone belonging to an ostracoderm of the Class Agnatha

• Class Agnatha is characterized by:– No jaws, no paired

fins– Simple vertebrae

and primitive brain

Characteristics :

1. Presence of an armored shield

2. Absence of joint between the head and body

3. Body is dorsoventrally flattened

4. Eyes on the dorsal part of the head

5. Possess a cartilaginous internal skeleton

Ostracoderm

• extinct during Devonian period due to incapability to compete with jawed fishes

Placoderms

-origin: agnathan fish- Include marine and freshwater forms; integument appears like plaques around their bodies-presence of jaw and mobile head

*jaws are derived from the third gill arch

-partial attainment of bony internal skeleton

-most were effective predators

Haltabo 09-10

Haltabo 09-10

Bothriolepis - most primitive placoderm

Class Chondricthyes (Sharks)• Gr. chondros, cartlilage + ichthys,

fish– Streamlined fish-like body with

heterocercal tail; jawed, paired appendages and cartilaginous skeleton, 5-7 gills with separate openings; no operculum; no swim bladder

– Evolved from Placoderms

Class Chondricthyes

Acanthodians or spiny fishes (Class Acanthodii, Silurian-Permian) probably descended from the Agnathans

Features:•(+) jaws•Spines along the leading edges of heir fins•Paired fins for mobility

Class Osteichthyes(Teleostomi)

• Gr. Osteon, bone + ichthys, fish• Primitively fusiform body but

variously modified; mostly ossified skeleton single gill opening on each side covered with operculum; usually with swim bladder.

• May also be classified into separate classes:– Class Sarcopterygii (lobed-finned fishes)– Class Actinopterygii (ray-finned fishes)

Osteichthyes

Class Sarcopterygii (lobed-finned fishes)

Class Actinopterygii (ray-finned fishes)