Vertebrate Evolution and Earth's History

Agnathans and Gnathostomes

• Agnathans are jawless fish, including hagfish and lampreys
• Gnathostomes are jawed fishes
• Gnathostomes evolved from Agnathans
• Differences between the two groups include the presence of jaws in Gnathostomes

Geologic Time and the Fossil Record

• Earth formed over many years
• Periods/epochs are distinguished by changes in landmasses, organisms, or asteroid impacts
• "-zoic" means "animal," referring to the appearance of animal life after Precambrian
• Ordovician, Silurian, and Devonian periods are known as the "Age of the Fishes"
• Permian, Triassic, Jurassic, and Cretaceous periods are known as the "Age of the Dinosaurs"
• Tertiary period marked the start of mammals
• Quaternary period marks the appearance of the genus Homo - Pleistocene era is the caveman era
• Holocene era is the modern human era

What Did the Earliest Vertebrates Look Like?

• Pikaia, from the Burgess Shale beds in British Columbia, was an invertebrate chordate
• It possessed a notochord and V-shaped myomeres, placing it in the Chordata clade
• Possibly a cephalochordate

Early Vertebrate Evidence

• Haikouella, a small fish-like animal from early Cambrian beds in Haikou, China, displayed chordate synapomorphies (dorsal nerve cord, notochord, pharynx)
• It also exhibited vertebrate synapomorphies (pharyngeal muscles, paired eyes, enlarged brain)
• However, Haikouella lacked a cranium and a distinct forebrain, suggesting it is not a vertebrate, but possibly a sister group

Conodonts

• Conodonts were odd tooth-like fossils discovered in Paleozoic marine sediments
• Fossils in the 1980's showed myomeres, teeth, paired eyes, notochord
• These are almost certainly members of the clade Vertebrata, though their exact position remains uncertain

Environment of Early Vertebrate Evolution

• Earliest fossils were found in marine sediments
• Nonvertebrate chordates and deuterostome invertebrates originated in the marine environment, displaying body fluid concentrations similar to their surroundings
• Hagfishes also show these characteristics

Simplified Phylogeny of Early Vertebrates

• Phylogeny diagram showing extinct and extant jawless (Agnatha) and jawed (Gnathostomes) vertebrate groups
• Extinct groups are indicated with + signs

Cyclostomes - Extant Jawless Fish

• Agnatha (jawless) include hagfish and lampreys
• Lack of internal ossification, paired fins, scales, and jaws
• Lack complex reproductive structures
• Single nostril
• Have a velum (pump)-pharynx
• Have rudimentary vertebral precursors (arcualia)
• Single circulation
• Muscular tongue with keratinous teeth

Myxiniformes - Hagfishes

• ~75 species, ~0.5m long marine cold water scavengers
• Slime glands (90-200)
• Single opening for water intake (olfaction and gill ventilation)
• Gill openings moved posteriorly (burying head in prey)
• 5-16 pairs of gill openings
• Females outnumber males 100:1

Petromyzontida - Lampreys

• ~40 species, both freshwater and marine, free-living and parasitic
• 10 cm to 1 m in length
• Temperate waters

Lamprey Spawning Cycle

• Adults migrate from lakes or oceans to small streams in late winter/spring
• Build nests, eggs hatch in ~2 weeks, releasing ammocoetes (larval stage)
• Ammocoetes filter feed and remain in the larval stage for 3-7 years
• Free-living lampreys don't feed as adults and die after spawning
• Parasitic species migrate back to lakes/oceans and live as parasites

Humans vs. Lampreys

• Sea lamprey (Petromyzon marinus) moved into Lake Ontario
• Niagara Falls used to be a barrier
• Construction of canals in 1913-1918 allowed the lamprey to enter Lake Erie and then spread
• Lamprey invasion resulted in a collapse of Lake Trout fisheries by 1950
• Control measures were implemented

Ostracoderms

• First unambiguous vertebrate fossils
• Jawless, possessing true bone
• Median fins (dorsal, anal, caudal)
• Heavy armor plating

Gnathostome Derived Characters

• Jaws and teeth, allowing for a firmer grip and increased food access
• Two sets of paired limbs (pectoral and pelvic), improving movement
• Jointed gill arches for double-pump ventilation
• Vertebral column with segmented vertebrae (protecting the spinal cord/ providing structural support)
• Nerve fibers possessing myelin sheaths to increase their efficiency
• Epaxial and hypaxial muscles enabling efficient movement
• Ribs connecting myomeres for more efficient locomotion
• Heart with additional parts, such as the conus arteriosus, improving circulation

Gnathostome Body Plan

• Horizontal septum divides trunk muscles
• Three semicircular canals in the inner ear
• Two olfactory tracts, leading to two nostrils
• Jaws with teeth
• Jointed gill arches
• Internal gill rakers preventing food from entering the gills
• Hypobranchial muscles connecting to the pectoral girdle
• Heart with conus arteriosus between ventricle and ventral aorta

Extant Gnathostome Teeth

• Differences in tooth structure (placement, and attachment style) across distinct vertebrate groups (e.g., chondrichthyans, bony fish, amphibians, reptiles, mammals)

Origin of Jaws

• Gnathostome jaws formed from the mandibular arch; the hyoid arch forming the jaw supports.
• HOX genes duplication event allowed complex jaw structures

Cyclostome vs. Gnathostome Gill Placement

• Cyclostomes have gills located internally, while Gnathostomes have gills in a more external position

Vertebrate Nose

• Cyclostomes have single nostrils (monorhiny)
• Gnathostomes have double nostrils (diplorhiny)
• Nasal capsules are close to the forebrain in cyclostomes, making it short; in Gnathostomes, the nasal capsules are more anterior, resulting in a longer brain.

Origin of Paired Appendages

• Pectoral fins were the first paired appendages
• Fish swim by flexing their bodies and tail in a wave motion with lateral fin movements

Advantages of Fins

• Caudal fin propels the fish through water
• Other fins (e.g., pectoral and pelvic) control direction and act as brakes

Explore the fascinating evolution of vertebrates from jawless Agnathans to jawed Gnathostomes. This quiz covers significant geologic periods and the characteristics of early vertebrates. Test your knowledge on the timeline of Earth and the major transitions in the history of life.