Fish
Lecture 1
Early Fish
 The earliest known bony fish were a group referred to
as the ostracoderms which were fish encased in bony
armor. These fish are from the early Cambrian period,
500 mya.
Early Fish
 There are two fossils from China, the Myllokumingia
and the Haikouichthys, these two fossils show a small
(3 cm) fish-shaped vertebrate with fish like body
structure, they do lack evidence of bone or
mineralized scale like ostracoderms. Both species had
a dorsal fin and ribbon like ventral fins.
 There are complete articulated skeletons from
jawless fishes from the late Ordovician time period.
These fish were torpedo shaped, ranging from 12-35
cm in length. These early fish were heavily armored
with many small, close-fitting, polygonal bony plates
3-5 mm long. These plates abutted to each other in
the head and gill region, forming a head shield. These
bony plates overlap like modern day scales. There are
up to eight gill openings and extra protection around
the eyes as well as the presence of sensory canals.
Agnathans
 Hagfish and Lampreys
 Hagfish lack vertebral elements completely
 Lampreys have only cartilage flanking the nerve chord
 To be classified as Agnathans the extant species must have no
jaws, notochord during embryo and adult phase, lack paired
fins and have 7 or more paired gill slits. All Agnathans extant
and ancestral have no identifiable stomach or appendages.
 Agnathans are the jawless fishes and date back to the
Cambrian time period, and are monophyletic
Jawless Fishes Today
 Modern day (or extant) jawless fishes are lampreys
and hagfishes. We have a lamprey that is native to
Idaho, this is the pacific lamprey or Entospenus
tridentatus.
From jawless to jawed:
Gnathostomes
 Considered to have originated within the Aganthan
lineage.
 Known for certain from the early Silurian time period
(although there is some evidence that they date back
to the mid-Ordovician)
 Derived characteristics are jaws that bear teeth (teeth
came later), two sets of paired fins or limbs
Placoderms
 Placoderms were some of the earliest jawed fishes,
that did not have teeth
 They had bony plates covering ½ to 1/3 of their body,
however this bony plate was divided into separate head
and trunk portions linked by a mobile joint that allowed
the head to be lifted up during feeding.
 Lived from the Silurian to the
Devonian time period
Chondrichthyes
 Evolved around 530 MYA (mid Devonian)
 Have cartilaginous skeletons
 This group includes the sharks, rays, and ratfishes
 Sharks have 403 species, ranging from 15cm to 10 meters
 Rays have 534 species, dorsoventrally flattened & typically
bottom dwellers that swim with undulations of their pectoral fins
 Ratfishes or Chimaeras have 33 species, have a single gill cover
that extends over all 4 gill openings
Osteichthyes
 The bony fishes (late Devonian)
 Two broad categories are recognized
1.
Actinopterygians – Ray finned fishes
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Includes the sturgeon
More than 27,000 species in fresh & salt water
More await discovery, Census of Marine Life project is describing
150-200 previously unknown species annually
Broken into 2 main groups
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Chondrostei (43 species)
Neopterygii – splint into three lineages (over 27,000)
2. Sarcopterygians – lobe finned or fleshy finned fishes
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Includes the coelacanth (2 species)
8 species still survive (6 species of lung fish)