History of fishes - Structural Patterns and Trends in Diversification AGNATHANS = Jawless • Class – Pteraspidomorphi • Class – Myxini?? (living) • Class – Cephalaspidomorphi – Osteostraci – Anaspidiformes – Petromyzontiformes (living) Major Groups of Agnathans • 1. Osteostracida 2. Anaspida 3. Pteraspidomorphida • Hagfish and Lamprey = traditionally together in cyclostomata Jaws = GNATHOSTOMES • Gnathostomes: the jawed fishes -good evidence for gnathostome monophyly. • 4 major groups of jawed vertebrates: Extinct Acanthodii and Placodermi (know) Living Chondrichthyes and Osteichthyes • Living Chondrichthyans - usually divided into Selachii or Elasmobranchi (sharks and rays) and Holocephali (chimeroids). • • Living Osteichthyans commonly regarded as forming two major groups ‑ – Actinopterygii – Ray finned fish – Sarcopterygii (coelacanths, lungfish, Tetrapods). • SARCOPTERYGII = Coelacanths + (Dipnoi = Lung-fish) + Rhipidistian (Osteolepimorphi) = Tetrapod Ancestors (Eusthenopteron) Close to tetrapods Lungfish - Dipnoi • Three genera, Africa+Australian+South American ACTINOPTERYGII Bichirs – Cladistia = POLYPTERIFORMES Notable exception = Cladistia – Polypterus (bichirs) - Represented by 10 FW species - tropical Africa and one species - Erpetoichthys calabaricus – reedfish. Highly aberrant Cladistia - numerous uniquely derived features – long, independent evolution: – Strange dorsal finlets, Series spiracular ossicles, Peculiar urohyal bone and parasphenoid • But retain # primitive Actinopterygian features = heavy ganoid scales (external layer of ganoine), gular plates, spiral valve in the intestine and lung-like swim bladderHistorical Context Agassiz (1833-44) compendium fossil fish & Muller's (1845) classification living actinopterygians - 3 major assemblages: Chondrostei (sturgeon/paddlefish), Holostei (Gars/Amia), and Teleostei - "Interrelationships of fishes” (1973) - by Greenwood, P. H., Miles, S., Patterson, C. eds, J. Linn. Soc. (London) 53 Supplement 1. Academic Press, New York, Basal actinopterygians – (1) FYI – not required (fossil Paleonisciformes), (2) Polypteriformes (Cladistia), (3) Acipenseriformes=Chondrosteans=sturgeon/paddlefish, (4) Neopterygians. 1+2 = Most taxa represented are fossil - Polypterids living. (3) Acipenseriformes=Chondrosteans – 2 living = Acipenseridae - sturgeons (25 recent species) and Polyodontidae - paddlefishes 4) Neopteryii - Three major groups: Lepisosteiformes (Ginglymodi) - gars (Lepisosteidae – 2 genera and 7 spp.; ganoin scales) and Amiiformes=Halecostomi - halecomorphi - several fossils, and amia (bowfin=cycloid scales) – traditionally Holostei plus Teleosteii Teleostei - subject of much of rest diversity survey – 20,000 + species. Evolution of particular function in Actinopterygian fishes in context of historical sequence of structural changes within clade • Function = feeding; specifically the elaboration of inertial suction feeding mechanisms. • Remember - aquatic medium (800 times more dense and 80 times more viscous than air). Also oxygen less, temperature stable, hydrate, buoyancy, viscosity (more drag). • Food acquisition on land is a very different matter than that underwater. • Understand fundamental differences in aquatic versus terrestrial feeding modes • Functionally Actinopterygian feeding can be grouped into three major categories: • 1) Ram feeding, 2) Inertial suction and 3) Manipulation • Actinopterygian feeding mechanism - mouth opening in primitive Actinopterygians mediated by two musculoskeletal couplings: • 1. Dorsal epaxial - neurocranial coupling; Elevates the head. • 2. Ventral coupling - hypaxial musculature, cleithrum, sternohyoideus and hyoid apparatus = causes mandibular depression Where did jaws come from? How to study feeding? - High speed cinematography, Electromyographic analyses, Measurement of buccal cavity pressure differentials, Strain gauge analyses Chondrichthyes – Sharks, Skates, Rays MOUTH OPENING - Lift head with epaxial muscles, Mandible pulled down by ventral muscles between hyoid/mandible and pectoral girdle, Enlargement of mouth/pharynx (suction) = BIG GAPE MOUTH CLOSING – ADDUCTION; Formidable bite – Large adductor mandibulae (AM) and preorbitalis muscles = optimized out-force at teeth Evolution of Feeding• Actinopterygian feeding mechanism = increase structural complexity. Cladistically derived lineages structural complex - number of connections in the structural network - increases. Feeding in Primitive Actinopterygians - Anatomy 1. Dorsal epaxial muscles-neurocranial coupling; Elevates head 2. Ventral coupling – (hypaxial musculature, cleithrum, sternohyoideus and hyoid apparatus) = causes mandibular depression via insertion of mandibulohyoid ligament Found also in lungfish, coelacanths and Sharks Primitive Actinopterygians - living Polypterids - jaw adapted to "gripping" and "biting" habit. Amiiformes=Halecostomes Feeding Mechanism = Amiidae 2 independent biomechanical pathways for mandibular depression: • 1) Primitive ventral coupling retained. • 2) New coupling = opercular series. • Why have two biomechanically independent pathways of mandibular depression? Next Innovation - Halecostomes 1. Maxilla - is free, pivots on medially directed process posterior to vomer. Food capture accomplished by highly effective suction mechanism. Theoretically any kind of prey in water can effectively be collected by this basic feeding mechanism. ‘Corollary’ - no marked specializations or adaptations in trophic apparatus necessary to collect wide variety of aquatic organisms Fundamental dichotomy between this aquatic feeding model and basic terrestrial model Teleosts Teleost feeding mechanism - 3 major changes occurred in structural network of head. • 1) Insertion of mandibulohyoid ligament to interoperculum - effectively shifts action of hyoid and opercular couplings onto interoperculum. • 2) Elongate ascending process on premaxilla - associated with jaw protrusibility. • 3) Adductor mandibulae muscle complex - associated with evolution of jaw protrusion. Teleosts – Other Morphological Innovations • Pharyngeal Jaws (Cyprinids, Cichlids, Labrids etc) - Functional Decoupling • Modular Multiplicity - Hardware vs Software (Patterns Neuromuscular Firing) Modular Multiplicity •