1. Fish History & Classification
Chapter 11

2. Ostracoderms and Placoderms
Part I: Early Fishes
Ostracoderms and Placoderms

4. Refresher of geologic time scale
Paleozoic Era million years before present (mybp)
Cambrian mybp
Ordovician mybp
Silurian mybp
Devonian mybp
Carboniferous mybp
Permian mybp

5. Refresher of geologic time scale
Mesozoic Era mybp
Triassic mybp
Jurassic mybp
Cretaceous mybp
Cenozoic Era 63 mybp - present
Paleogene mybp
Neogene 24 mybp - present

6. What happened when? Notice where major evolutionary events occured.
Ancestors of fish did
not happen spontaneously.
Continuous development
was (is) working on each
taxonomic group all
the time.

7. Ostracoderms (shell-skins) - earliest vertebrates in fossil record
Originated in late Cambrian Period? (> 500 mybp) - first record is from Ordovician
Were abundant and diverse through the Ordovician and Silurian Periods (approx. 100 million years)
Became extinct by the late Devonian Period (approx mybp)

8. What group preceded Ostracoderms?
Earliest vertebrates probably like
modern Cephalochordates (Amphioxus)
Bilateral symmetry
Free-swimming (perhaps neotonous larva)
with cephalic sensory structures
with branchial gill apparatus
without bone, jaws or paired fins

9. Traits of Ostracoderms*
Boney armor - first record of bone in fossils - protection from predators
Internal skeleton - made of cartilage
Heterocercal tail
Lacked true jaws - were pump-filter feeders
Lacked paired fins - weak swimmers
Benthic habitat
Small size - none longer than 15 cm

10. Ostracoderm classification
Two classes:
Class Pteraspidomorphi (sp. diplorhina = “two nares”)
they literally had two separate olfactory bulbs in the brain.
those with a different shell, i.e. dermal armor

11. Ostracoderm classification
Class Cephalaspidomorphi (“single nostril”)
jawless fish

13. Success of Ostracoderms
First use of bone
for protection, not support
possibly used as auxiliary supply of Calcium?
Use of filter feeding to exploit common and abundant food source: plankton and suspended organic matter

14. Limitations (failures) of Ostracoderms
Habitat limitations
restricted to benthos (no kiddin’!?!)
weak swimmers due to heavy armor
weight
inflexibility
Food limitations
no jaws - restricted to plankton, suspended organics - slow growth

15. Fate of Ostracoderms*
Extinct within 100 million years - by late Devonian
Lineage debated...
possibly lampreys (Petromyzontiformes)
possibly Chondrichthyes
possibly Osteichthyes

16. How good were they? Ostracoderms were key to vertebrate evolution!
Gave rise to jawed vertebrates!
Jaw evoluation considered as the single "greatest development in vertebrate evolution“
(Romer (1962)
I know I like mine....

17. Placoderms - earliest gnathostomes (jawed vertebrates)
Originated after the Ostracoderms:
Originated in Silurian Period (440 mybp)
Abundant and Diverse in Devonian Period
Extinguished in Carboniferous Period (350 mybp)

18. Two key traits account for Placoderm success:
True jaws
opened new realm of food sources - larger prey items vs. filter-feeding - allowed faster growth to larger sizes
Paired fins
coevolved with acquisition of jaws:
greater control of movement
more effective pursuit and capture of prey

20. Additional traits first appeared in Placoderms
Bony dermal plates (produced by dermal cells) with three layers:
enamel layer - outer surface - hard & shiny
spongy layer - large vacuoles
lamellar layer - layered strata with flat vacuoles
Bony internal skeleton

21. Traits shared with Ostracoderms
Negatively buoyant (due to heavy plates)
Occupied benthic and near-benthic habitats (epi-benthic)
Dorsoventrally depressed (flat)--common among benthic fishes
Strictly marine

22. Differences from Ostracoderms
Placoderms reached larger sizes
up to 10 m (33 feet) in length
why? - food source, mobility
Placoderms had slightly
lighter and more flexible
(articulated) armor

23. Success of Placoderms Diversity: Duration:
greater than any other group of fishes present in Devonian
seven orders within single class
Duration:
mybp

24. Fate of Placoderms Probably evolutionary “dead-end”
Plesiomorphies with Chondrichthyes & Osteichthyes: jaws, paired fins, internal skeleton - suggest common ancestor
Apomorphies: armor, jaw structure, depressed form - suggest they are NOT ancestral to Chondrichthyes & Osteichthyes

25. Part II: Classification Specifics

26. REM:
Taxonomy – The theory and practice of describing, naming, and classifying organisms.
Systematics – The classification of living organisms into hierarchical series of groups emphasizing their phylogenetic interrelationships.
Nomenclature – The system of scientific names applied to taxa.

27. Why is Classification Important?
Communication- “Okay...I get that...”
Prediction- “Don’t get that...”

28. Why is Classification Important?
Communication
- apply consistent names to organisms
- Genus and species name for each organism is unique
- same name used everywhere
- important in keeping track of losses of biodiversity
- know which and how many species at certain time
to how severe the loss is in the present (management)
- Ex. Western United States = water habitats altered

29. Why is Classification Important?

30. Why is Classification Important?
Prediction
- reflects evolutionary history
- members of a group will share a more common
ancestor with each other than with members of
other groups
- will have inherited similar traits
- use shared history to infer that closely related
species share similar traits

31. Suborder Anabantoidei
Gouramies
- possess apparatus that allows extraction of O2
gulped air. (labyrinth organ)
Betta
- systematically classified as gouramies
- possess apparatus?....yep!
- able to live in low O2 environments

32. Why is Classification Important?
Prediction (cont.)
- environment
- can impose an adaptive regime on species that
live there
- results in shared features of unrelated species
-Gars and pikes
- similar body form due to ecological niche

33. Taxonomic Categories Phylum -- Chordata Subphylum Vertebrata
Superclass Gnathostomata
Grade Teleostomi
Class -- Osteichthyes
Subclass -- Actinopterygii
Infraclass -- Neopterygii
Division -- Teleostei
Order -- Perciformes
Family -- Centrarchidae
Genus -- Micropterus
Species -- salmoides

34. Taxonomic Categories Current system based exclusively on shared
common ancestry
All categories that taxonomists apply to fish are
artificial (descriptive) except for one…species
Species is a real entity (biological)…other
categories are artificial assemblages
- most biologists believe that species are
real entities that exist in nature.

35. What is a species? Biological Species Concept (Mayr 1940)
Biological Species : A group of actually (or potentially) interbreeding natural populations genetically isolated from other such groups by one or more reproductive isolating mechanisms.
most commonly applied species concept
- Some associated problems

36. What is a species? Problem: hybridization
-fish are notorious for this practice
-hybrids are often sterile (or thought to be! (HSB?)
-some are fertile and able to backcross with
either parent

37. Why are there so many species?
random genetic changes
differences in the selective environment
Anagenesis: change in a species over time
- species exist as a single population and whole species
will change over time and not branch off into
multiple discrete species.

38. Why are there so many species?
formation of multiple species from a single
ancestral species is due to isolation of the population into
distinct populations or gene pools.
Why?
- each population undergoes anagenesis and
eventually individual populations are distinct
enough to be recognized as separate species.

39. Why are there so many species?
What causes isolation?
- vicariant event: a geological or climactic event
- Isthmus of Panama = the most studied
-- around 3.5 mya
-- tropical Atlantic separated from tropical Pacific
Allopatric speciation: single species diverge into two species
in separate geographic locations
(squirrels/mountains)

40. Why are there so many species?
Sometimes populations of closely related species will
coexist in an environment.
Genetic independence achieved through premating
and postmating reproductive isolating mechanisms

41. Why are there so many species?
Postmating
- hybrid sterility
- inviability
Why is hybridization rampant among fish?
- external fertilization
- eggs and sperm drift in the proximity of gametes from
a different species
Avoidance:
- sexual selection: partners choose appropriate mates
- males are brightly colored to attract females

42. Why are there so many species?
Premating
- sexual selection: partners choose appropriate mates
- males are brightly colored to attract females
- males especially colorful in genera that have a large
number of coexisting species
Ecological differences
limits degree of competition
food preference, timing and location of spawning

43. African Cichlids

44. North American Darters

45. Linnaean Classification

46. Linnaean Classification
Rules of Nomenclature
still named the same way as Linnaeus and Artedi did in 1758
genus name is always capitalized
species name is lower case
oldest valid name sticks with the species
genus names are unique among all biota
species names are unique within a genus
International Code of Zoological Nomenclature
American Fisheries Society (US)

47. Phylogenetic Classification
Genus 1
Genus 3
Genus 2
Species 1
Species 3
Species 2
Species 2
Species 3
Species 2
Species 1
Species 1

49. Phylogenetic Classification
Cladistics
lumps together groups that are assumed to share a
common ancestor.
Clade: group that contains an ancestor and all of its
descendents.
- Guidelines defining clade from Willi Hennig (bug man)
- shared derived characters
derived character: is different from some primitive

50. Character State: Gas Bladder
Physoclistous
Physostomous

52. Phylogenetic Classification
Terms
- homologous: characters that are alike in state due to
shared common ancestry
- homoplasy: characters that are alike in state for other
reasons
- independent origin: a trait can evolve
independently in two different lineages
e.g., countercurrent heat exchangers in
mako sharks and tuna.

53. Phylogenetic Classification
Terms
- homoplasy (cont.)
- reversion: a character may revert to a more
primitive state.
e.g., some eels lack scales and paired fins
like primitive agnathans. Ancestors
of eels possessed scales and paired fins,
but were lost, thus reverting to the
more primitive state.