Agnathans Flashcards
Agnatha etymology
“No jaws”
Agnatha is a _________ group
Paraphyletic
_________ and ___________ are EXTANT representatives
Hagfishes; lampreys
Is Agnatha a clade when Hagfish and Lamprey combined?
It may or may not be a clade when combined
Ostracaderms
Diversity of EXTINCT lineages with some sharing characteristics with early jawed vertebrates
Paraphyletic by themselves
NO internal ossification of endoskeleton
Most had ossified EXOSKELETON (external armor)
Most lacked paired fins
–But see OSTEOSTRACANS
Filtered small prey/food by pumping water with MUSCULARIZED pharynx
Hagfish and lampreys:
Endoskeleton:
-Notochord PERSISTS
- Fibrous/cartilaginous elements
- -NO internal ossification
- -Bone comes later
- Vertebrae absent/rudimentary
- -Vertebrae are NOT a vert synapomorphy
Pore-like gill openings
Tripartite brain (vert synap)
Monophyly of Extant Agnathans:
Hagfishes and lampreys superficially similar but ancient lineage divergence and extant forms quite specialized
2 hypotheses:
- -Lampreys sister to jawed verts and hagfish basal to verts -> HISTORICAL
- -CLADE: CYCLOSTOMATA (most recent common ancestor»_space;400 mya) -> CURRENT
Class Myxini
Hagfishes:
- Marine
- ~70 species
- Rudimentary skull
- No vertebrae
- One pair of semicircular canals (role in orientation/balance)
- Eyes likely degenerate (i.e., reduced - derived)
- Body fluids isosmotic with seawater (like most marine invertebrates)
- EXTERNAL fertilization
- Females produce SMALL numbers of LARGE eggs
- NO larval stage
- Predators and scavengers:
- -Feed on annelids, molluscs, and crustaceans
- -Scavenge dead/dying fish and whales (can be numerous on a carcass)
- GOOD sense of smell helps find food
- Keratinized “teeth”
- -NOT true teeth yet
- -NOT homologous to true teeth
- -ANALAGOUS to true teeth
- Produce a LOT of slime
- Attach to prey with keratinized plates and rasp with tongue
- Will tie knot in tail and slide it forward for leverage when feeding
Class Petromyzontiformes
Lampreys
- Suction mouth with keritinized “teeth”
- ~38 species
- Simple cartilaginous vertebrae
- 2 pairs of semicircular canals
- Well developed eyes
- Marine AND/OR freshwater
- Larvae (ammocoetes) hatch and filter feed in freshwater
- Metamorphosis into parasitic/non-feeding adults (~50/50 of species)
- Parasitic adults migrate to ocean or lake then return to FW to breed (migrations can be >1000miles)
- Adults of parasitic forms attach to fish, rasp flesh with keratinized “teeth,” suck body fluids
- -Wounds can be fatal, cause infection, etc.
- Invasive species effects fisheries
Condodonta
Complete fossils found in 1980s
Previously known from tooth-like fossils
Early chordate group (>520 mya)
Currently considered part of vertebrate clade (agnathans)
Phylogenetic HYPOTHESIS: Sister lineage to clade including all vertebrates EXCLUSIVE of Cyclostomata
Mineralized (phosphate) tooth-like “condont elements” (derived)
Fin rays (derived)
Cranium, notochord, paired eye and otic capsules, W-shaped myomeres (ancestral)
Ostracaderms
NO internal ossification of endoskeleton
Named for bone in dermis (“shell skin”)
-Most had ossified EXOSKELETON (external armor)
LACK paired appendages (EXCEPT osteostracans
Lived for 150 million years (>500 to ~350mya)
Found in variety of marine and FW habitats
Filtered small prey/food by PUMPING WATER WITH MUSCULARIZED PHARYNX
Includes:
- -Anaspida
- -Heterostraci
- -Osteostraci
Anaspida
EXTINCT lineage
444-419mya
Compressed with hypocercal tail
Small weakly mineralized scales
Heterostraci
433-359mya
Most with 2 large bony plates (dorsal and ventral) with dentine but unique histology
Osteostraci
433-359mya
PAIRED pectoral appendage (fins)
2 pairs of semi-circular canals (like lampreys)
Dermal bone histology similar to that of gnathostomes
HYPOTHESIZED sister lineage to Gnathostomes
Gnathostomata
“Jaw mouth”
Key innovation (also synaps)
- -Origin of jaws from pharyngeal arches (neural crest derivative) allowed predation on large and active prey
- -Origin of pair pectoral and pelvic appendages increased maneuverability
