BIO topic 27: Gnathostomata(jawed vertebrates)

earliest record of vertebrates

date back to Cambrian period

6 main groups of agnathans are phylogenetically positioned between cyclostomes and gnathostomes

Gnathostomes

form a monophyletic group, known for having jaws

99% of extant vertebrates fall withing Gnathostomata

Shared derived features

1. presence of jaw( opposing mouth elements to open an forceful capture food)

2. presence of mineralized/ossified skeleton( composed of bones and Cartlidge [ ways bones are formed: Endochondral: bones develop from Cartlidge. Dermal: bones develop from connective tissue])

3. presence of paired appendages in the forms of true fins and true limbs (connected to shoulder girdle on the pelvis)

4. enlarged forebrain of the tripartite brain(parried nostrils, enhances small and vision)

5. presence of the lateral line system( sensory system in aquatic vertebrates detecting water vibrations[Neuromast organs with hair to pick up the mechanical signals, embryologically originated from ectodermal placodes])

origin of jaws

at least 8 hypothesis proposed to explain jaws

evidence mostly found in phenotypic features

origin of teeth

-mineralized structures anchored to jaws from environmental interactions, prey capture, processing and defense.

composed of enamel and dentine

phylogenetically the origin of teeth is close to origin of jaws

outside-in hypothesis: tooth originated as a scale and migrated inside the mouth region

paired fins in fossil agnathans

Ostracoderms form a paraphyletic group of agnathans with external body armor and minimal internal ossified skeletons

basal ostracoderm lack paired fins

derived ostracoderms such as osteostracans have unambiguous paired fins.

Diversity of fish

fish: refers to paraphyletic group of gnathostomes, excluding terrestrial tetra pods

main groups: Chondrichthyes (sharks,ect.), Osteichthyes( body fidh)→ Actinopterygii( ray finned fish), Sarcopterygii(lobe-finned fish)

Chondrichthyes

~100 extant mostly marine species

characteristics:

• consisted of unossified Cartlidge, mineralized teeth, and placoid scales

• placoid scales function for protections and swimming

• cartilaginous skeleton is evolved from an ancestor with ossified endoskeleton

Actinopterygii

• include 99% of species of Osteichthyes(bony fish)

  characteristics:

• fins are supported by elongated bony rays connected to the shoulder and pelvis

• tribasic fins with three sets of anatomical parts.

Sarcopterygii

• include fish with lobe-fins and tetrapod’s

  characteristics

• Presence of Lobe-fin

• muscle positioned along the skeletal elements of the fin

• single bone connected to the shoulder girdle and pelvis