Vertebrate Life - Sarcopterygians Overview

Three Extant Clades of Sarcopterygians:
- More than 99% of the species fall under Tetrapoda (literally meaning “four feet”).
- Molecular Phylogenetic Studies indicate:
- Coelacanths: Most basal extant sarcopterygians.
- Lungfishes: Extant sister group to tetrapodomorphs.
- Tetrapoda Definition: Encompasses all sarcopterygians with 4 limbs.
- Crown Tetrapoda: Refers to all extant forms of Tetrapoda.

Monobasic Paired Fins:
- Notable characteristics include fleshy paired pelvic and pectoral fins which are scaled and have a bony central axis.
Cosmine:
- Composed of dermal bones and scales, an important feature in sarcopterygians.
Intracranial Joint:
- This joint is found between anterior and posterior portions of the cranium, distinguishing these species from others.

Two species of coelacanths (Subclass Actinistia).
Six species of lungfishes (Subclass Dipnoi).
All tetrapods (four-legged vertebrates) are descended from extinct sarcopterygian fishes, specifically an extinct group known as Rhipidistians.
Special attention to Dipnoi (lungfish).

Fossil Record: Prolific between Devonian period and Cretaceous, then believed extinct for ~50 million years until rediscovery in 1938.
Discovery:
- 1938 Catch: A specimen captured by a South African fishing boat, recognized by Marjorie Courtney-Latimer and identified by icthyologist J.L.B. Smith.
- Subsequent Specimens: A second specimen was caught in 1952 near the Comoro Islands, leading to further research and study, with “hundreds of specimens” caught but challenges in maintaining them alive post-capture.
- Second Species: Identified through DNA sequencing in 1998; both species are listed as CITES Appendix 1 indicating they are threatened with extinction.

Size: Large fish—up to 5 feet long.
Coloration: Blue-grey with white/pinkish spots, possessing reflective golden eyes (due to the tapetum lucidum).
Physical Traits:
- Muscular lobes at the base of fins, unique rostral organ for electroreception, and the distinguished intracranial joint which enhances feeding capabilities.
Habitat: Lives in deep cold waters (70-400m) and primarily feeds on lanternfish, squid, and octopus.

OvoViviparous: Indicates live-bearing with presumed internal fertilization, though males lack a delivery organ.
Potential for Monogamous behavior?

Fins: Unique features include:
- Caudal fin with a small median lobe.
- Highly mobile paired fins with long fleshy basal lobes.
- Anterior dorsal fin reduced and equipped with long protective hollow spines (the name coelacanth translates to “hollow spine”).
Swimming Movement: Fins move in a pattern reminiscent of tetrapod locomotion (i.e., walking).

Fat-filled Swim Bladder: Unique adaptation.
Rostral Organ: Contains gelatinous material, with surface pores for sensory reception.
Physiological Traits: Includes high blood urea levels, a liquid-filled notochord, a specialized rectal gland for salt secretion, and lobed fins.
Debate on Phylogenetic Affiliations: Generally consensus places coelacanths as sister to Rhipidistia; provides context for their relationship to lungfish and tetrapods.

Total of six extant species: comprises one from South America, one from Australia, and four from Africa.
Lungfish Adaptation: As representative sarcopterygians, they possess alveolar lungs and exhibit ability to breathe air.

Australian Lungfish: Capable of gulping air but cannot thrive out of water (facultative air breather).
South American and African Lungfish: Adapted for longer survival out of water; can estivate by burrowing into mud, breaking a metabolic dormancy until water returns.

Loss of certain skeletal features: Loss of toothed premaxillary, maxillary, and dentary bones; teeth now manifest as fused plates adapted for a diet of hardy foods.
Adaptations include: Strong muscular linkage between jaw and primitive chondrocranium, and a jaw suspension evolution resembling tetrapods.
Fins: A notable morphological change includes the fusion of dorsal, caudal, and anal fins into a continuous fin around the body.

Concept: Specific transformation affects scale and dermal bone structure; retains juvenile traits in adults.
Historical perception mistaken lungfish for salamander-like species due to externally gilled hatchlings.
Factoid: Lungfish exhibit the largest genome among vertebrates, confirming their unique evolutionary path.

Notable anatomical similarities with other lung-breathing vertebrates include:
- Pulmonary vein facilitating oxygenated blood delivery to the left atrium.
- Presence of a vomeronasal organ for enhanced chemosensation.