BVert-Lec2 slide

A STEP BACK: PHYLOGENY OF THE ANIMALS

  • Fungi
  • Choanoflagellates
  • ANIMALIA
    • Multicellularity
    • Choanoflagellates (closest to animals in many depictions)
    • Porifera, Cnidaria, Ctenophora, Acoelomorpha
  • LOCHOTROCHOZOA (a major protostome grouping in some diagrams)
    • Bilateral symmetry?
    • Rotifera
    • Loss of coelom
    • Platyhelminthes
    • Segmentation
    • +Annelida
    • Bilateral symmetry
    • Protostome development
    • Molluska
  • ECDYSOZOA
    • Nematoda
    • Coelom, cephalization, CNS
    • Arthropoda
    • Segmentation
  • DEUTEROSTOMES
    • Radial symmetry in adults (Echinodermata)
    • Chordata
  • Segmentation
  • ANIMALIA
  • BILATERIA
  • PROTOSTOMES
  • DEUTEROSTOMES

MAJOR GROUPS OF LIVING CHORDATES

  • Chordates
  • Vertebrates
  • Tetrapods
  • Amniotes
  • Synapsids
  • Sauropsids

CHORDATA AND VERTEBRATA CHARACTERISTICS

  • Chordata: All chordates have at least some of these at some life stage:
    • A notochord
    • A dorsal hollow nerve cord
    • A post-anal tail
    • Pharyngeal pouches/slits
    • An endostyle on the floor of the pharynx
  • Generalized chordate body plan

NON-VERTEBRATE CHORDATES

  • Chordata is divided into three groups:
    • Urochordates (tunicates/sea squirts)
    • Cephalochordates (including lancelets like Amphioxus)
    • Vertebrates (hagfish, lamprey, cartilaginous fish, bony fish, amphibians, reptiles, birds, mammals)
  • Tunicate larvae: swim and display all chordate characteristics
  • Tunicate adults: sessile; most chordate features are lost
  • ~2000 species; nearly all are filter feeders

UROCHORDATES

  • Larvae: able to swim; have all chordate characteristics
  • Adults: sessile; most chordate features lost
  • ~2000 species; nearly all filter feeders
  • Visual: Adult tunicate and larval tunicate examples referenced

CEPHALOCHORDATES

  • Cephalochordates (lancelets/Amphioxus)
  • Filter-feeding; buried in sand
  • Fish-like segmented muscles
  • Fish-like movement patterns
  • Simple circulatory system and nervous system with primitive brain-like cerebral vesicle
  • Only 27 species recognized; most < 5 cm long

VERTEBRATE CHARACTERISTICS

  • Vertebrata is named after vertebrae, which form a centrum around the notochord during development
  • Only gnathostomes (jawed vertebrates) have true vertebrae
  • Vertebrates have:
    • Cranium, tripartite brains, and sensory organs
    • Complex endocrine organs
    • Mineralized tissues
    • Many other unique features (see handout)

VERTEBRATA OR CRANIATA?

  • Some researchers advocate renaming Vertebrata to Craniata
  • Hagfish and lampreys lack vertebrae but have crania
  • Hagfish have vestigial vertebrae — possible secondary loss?
  • For now: treat Craniata as a synonym of Vertebrata

HYPOTHETICAL PRIMITIVE VERTEBRATE ANCESTOR

  • Features highlighted in diagrams:
    • Primitive non-vertebrate chordate (A): Notochord, dorsal hollow nerve cord, endostyle, pharynx, myomeres, postanal tail, gut, anus, etc.
    • Ancestral vertebrate (B): Cranium, spinal cord, vertebrae, coelom, trilobed brain, neural structures, multi-chambered gut with gills, heart musculature, arches and slits, dorsal/ventral features
  • This illustrates the major transitions: from a notochord-dominated body plan to a craniate with a vertebral column, a more complex brain, and more elaborate organ systems

OTHERVERTEBRATE SPECIALIZATIONS: MANY HOX GENES

  • Homeobox (Hox) genes control much of embryonic development
  • Number of Hox genes varies widely:
    • Jellyfish: 1 or 2
    • Fruit flies: 8
    • Mice: 39 (in 4 sets)
  • More Hox genes likely allowed evolution of more complex structures in vertebrates
  • Hox gene knockout experiments in mice help us understand how each gene affects development
  • Example: Mouse axial skeleton model
  • More information: Nature Scitable resource linked in lecture notes

OTHERVERTEBRATE SPECIALIZATIONS: NEURAL CREST TISSUE

  • Neural crest is a tissue layer present only in vertebrates (a fourth germ layer after endoderm, mesoderm, and ectoderm)
  • Develops into many diverse structures:
    • Craniofacial cartilage, bone, and muscle
    • Parts of the brain (e.g., glial cells)
    • Most of the peripheral nervous system
    • Trachea and larynx
    • Head and neck glands (e.g., pituitary and salivary glands)
    • Secretory cells in gut
    • Smooth muscle lining in the aorta
    • Endocrine cells
    • Pigment cells in skin and iris
  • Neural crest tissue is likely the single most important evolutionary innovation in the vertebrate lineage
  • Know a few key derivatives

THE EARLIEST TRUE VERTEBRATES

  • The vertebrate fossil record is rich, but bone preservation is partial
  • Evidence from shale fossil sites in the last ~50 years pushes origin of vertebrates back to
    • 525extMYA525 ext{ MYA} in the Cambrian Period
  • Myllokunmingia: has vertebrate features like segmented muscles and notochord
  • Earliest “vertebrate”: features include W-shaped myomeres, gill pouches, cranium
  • Video resource: Triumph of the Vertebrates (6:10)
  • Note: MYA stands for millions of years ago; here we reference the time scale as 525extMYA525 ext{ MYA}

EVOLUTION OF JAWLESS FISH

  • Key lineages depicted:
    • Common chordate ancestor
    • Vertebrata (craniata)
    • Ostracoderms (Ordovician)
    • Agnatha (jawless fishes)
    • Hagfishes (Myxini) and Lampreys (Petromyzontiformes)
  • Major groups involved in early evolution: Ostracoderms, Agnathans, Chondrichthyes, Osteichthyes, Actinopterygii, Sarcopterygii, etc.
  • Timeline references: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Mesozoic, Cenozoic

OSTRACODERMS OF THE ORDOVICIAN

  • Bony jawless vertebrates armored in the Ordovician (ostracoderms)
  • Armor made of thousands of fused tooth-like structures
  • Arandaspis: example of armored jawless Ordovician ostracoderm with many sensory organs in the cranium
  • Ostracoderms are diverse and heavily armored but are paraphyletic (not a single clade)
  • This group is an example of ongoing debates about agnathan phylogeny; some researchers place lampreys and hagfish as sisters to each other rather than with ostracoderms
  • Concept: Ostracoderm Explosion (diversity pulse) in Ordovician

HAGFISH (MYXINIFORMES)

  • Evolution and ecology:
    • 82 living species
    • Have evolved independently from other vertebrates for hundreds of millions of years
    • Inhabit cold marine waters
    • Able to tie themselves in knots to escape predators and to tear flesh via slime
  • Notable traits:
    • Jawless
    • Tongue with keratinized teeth
    • No paired appendages
    • No true vertebrae, only paired cartilages in the tail
  • Interactions with humans:
    • Slime used in eelskin leather products
    • Eaten in some cultures
    • Hagfish slime incidents reported (e.g., Oregon, July 2017)
  • Image reference: Fig. 3.11

LAMPREYS (PETROMYZONTIFORMES)

  • Evolution and ecology:
    • 47 living species
    • Found in marine and freshwater
    • Many are parasitic: latch onto hosts and suck fluids
  • Notable traits:
    • Jawless
    • No paired appendages
    • No true vertebrae, only paired cartilages in the tail
  • Interactions with humans:
    • Problems in the Great Lakes
    • Parasites of commercially important fish
  • Note: Taxonomic shift: textbook’s assignment of lampreys to Cephalaspidomorphi is outdated; current views differ
  • Figure reference: Fig. 3.12

LECTURE 2 READING GUIDE

  • Focus reading on the following sections:
    • Chapter 3: Early Chordates and Jawless Fish
    • Introduction
    • Early Cambrian Fishlike Fossils
    • Evolution
    • Morphology
    • Integumentary System
    • Skeletal System

REMARKS AND CONNECTIONS

  • Big-picture themes:
    • Transition from non-vertebrate chordates to vertebrates with cranium and vertebral column
    • Emergence of novel structures (neural crest tissue, expanded Hox gene family) enabling more complex morphology
    • Early vertebrate fossils broaden understanding of when vertebrate traits first evolved (Cambrian–Ordovician timeline)
  • Real-world relevance:
    • Modern jawless fish (hagfish and lampreys) illuminate primitive vertebrate conditions
    • Understanding Hox genes and neural crest helps explain vertebrate diversity and evolution
  • Ethical/philosophical notes:
    • Fossil interpretation is contingent on available specimens and phylogenetic frameworks; debates (paraphyly, monophyly) persist
    • Studying early vertebrates informs biomedical research and developmental biology by revealing ancestral states and developmental gene networks
  • Key numerical references to remember:
    • Approx. 525extMYA525 ext{ MYA}: origin of vertebrates (Cambrian)
    • 20002000 species of Urochordates (tunicates)
    • 2727 species of Cephalochordates
    • Hagfishes: 8282 living species
    • Lampreys: 4747 living species
    • Mice have 3939 Hox genes (in 4 sets)
  • Concepts to internalize:
    • Distinction between Vertebrata and Craniata (and why some propose renaming)
    • The role of neural crest as a major vertebrate innovation
    • The armor of ostracoderms as an important but paraphyletic stage in vertebrate evolution
  • Visual and media references (for study):
    • Video: Triumph of the Vertebrates (6:10)
    • Visuals of myllokunmingia, ostracoderms, hagfishes, lampreys