Chordates

Protostomes

  • The protostomes have been divided into lophotrochozoans and ecdysozoans based on phylogenetic analysis.

    • Ecdysozoans include nematodes (roundworms) and arthropods.

    • Arthropods comprise the majority of animal species.

Deuterostomes

  • Deuterostomes include echinoderms and chordates.

    • This is a traditional grouping.

Echinoderms

  • Echinoderms include sea stars, sea urchins, sea cucumbers, and brittle stars.

    • Analysis suggests they are closely related to chordates.

Chordates

  • Chordates are characterized by four key features present at some point during their development:

    • Notochord: A flexible, supporting rod for muscle attachment and mobility.

      • In vertebrates, the notochord is segmented into vertebrae.

    • Dorsal Hollow Nerve Cord: A nerve tract that develops into the central nervous system.

      • The anterior portion typically enlarges to form the brain, and the rest forms the spinal cord.

    • Pharyngeal Gill Slits: Slits or clefts in the pharynx.

      • Allow passage of water and nutrients in filter-feeding organisms.

      • May change later in development in other organisms.

    • Post-Anal Tail

Chordate Evolution

  • Chordate evolution dates back to the Cambrian period.

    • Picaia: A specimen from the Burgess Shale, hypothesized to be one of the earliest chordates.

      • Bilaterally symmetric, coelomate with segmented bodies.

  • A new Cambrian deposit was discovered in China, potentially yielding more early chordate material.

Basic Chordate Features

  • Lancelets exemplify chordate characteristics:

    • Dorsal hollow nerve cord.

    • Notochord.

    • Pharynx with gill slits.

    • Post-anal tail.

    • Muscle segments.

  • Segmentation in chordates:

    • Vertebrates exhibit muscle segmentation, body segmentation, and segmentation of the vertebral column.

  • Novel chordate characteristics include:

    • Nerve cord.

    • Notochord.

    • Pharyngeal gill slits.

  • Development and changes in characteristics, especially pharyngeal gill slits and the pharynx, are important in understanding the evolution of structures like jaws in vertebrates.

Phylogenetic Tree

  • Ancestral lineages include sponges and cnidarians.

  • The "protostomes" are now the lophotrochozoans and ecdysozoans.

  • Chordates branch off from the deuterostomes, with echinoderms being the closest related group.

Deuterostome Phylogeny

  • Echinoderms are the outgroup and most closely related to chordates.

  • Phylum Chordata includes three main groups:

    • Cephalochordates (lancelets).

    • Urochordates (tunicates or sea squirts).

    • Craniates.

      • Craniates include vertebrates and slime hags.

        • Hagfish have a cranium but not a vertebral column.

  • Vertebrates include cartilaginous fish, bony fish, ray-finned fishes (actinopterygians), lobe-finned fishes (coelacanths and lungfish), and tetrapods.

    • Tetrapods include amphibians, reptiles (including birds), and mammals (amniotes).

Chordate Groups

  • Chordates include invertebrate and vertebrate chordates.

    • Vertebrata is a subphylum.

    • Other subphyla include cephalochordates and urochordates.

  • Craniates are intermediate between cephalochordates/urochordates and vertebrates.

    • Hagfish have a skull (craniate) but lack vertebrae and hinged jaws.

Cephalochordates (Lancelets)

  • Most basal chordate group.

  • Develop all four chordate traits as larvae and retain most traits as adults.

  • Small, eel-like creatures with tentacles around their mouth.

  • Larvae are free-swimming and feed on plankton.

  • Adults are mostly sessile and filter feed, buried in the sand.

    • The pharynx produces mucus to trap organic debris.

Urochordates (Tunicates or Sea Squirts)

  • Develop all four chordate characteristics during the larval stage.

  • Larval stage is short and free-swimming, with a notochord, dorsal hollow nerve cord, muscle segments in the tail, and a pharynx with gill slits.

  • Undergo metamorphosis into a sessile organism attached to a substrate.

  • Adults retain only the pharynx and digestive system.

    • Incurrent and excurrent siphons for filter feeding.

  • Nervous system degenerates to control basic functions.

  • Sea squirts shoot water out of the excurrent siphon when startled.

  • Tunicates have nine Hox genes, while other chordates (including lancelets) have 13.

    • Tunicates appear to have lost four Hox genes.

  • Examples of Washington native tunicates include hairy stalked and Monterey sea squirts.

  • Adults attach to sediment and filter feed, some living in colonies.

  • Can be transported around and cause problems as invasive species via debris transport, bilge water, etc.

Hox Genes

  • Hox genes control development within the notochord.

  • In lancelets, HOX genes control development of the head end and the rest of the notochord.

  • In vertebrates, Hox genes control the development of a more developed brain, including the forebrain, midbrain, and hindbrain.

Neural Crest Cells

  • Development of a cranium may be related to gene duplication producing multiple sets of Hox genes.

  • Neural crest cells are involved with the development of facial and skull bones.

  • The notochord forms from a flat sheet that rolls into a tube, and neural crest cells develop in the folds.

  • Neural crest cells migrate and form a wide variety of tissues, including facial bones, jaw, and temporal bones.

Fossil Evidence

  • Haikuala: A more derived fossil from the Cambrian period in China, believed to be an early craniate.

    • Had eyes and a brain.

  • Another fossil from the Cambrian of China had eye capsules and ear capsules, indicating the development of sensory structures.

  • Craniates have clefts or slits in the pharynx that evolved into gills and gill slits.

  • They have higher metabolism and are more mobile than tunicates and lancelets.

  • Craniates typically have a heart with multiple chambers, blood with an oxygen-carrying molecule (like hemoglobin), and kidneys.

Myxini (Hagfish)

  • Craniates without vertebrae or hinged jaws.

  • Scavengers that feed at the bottom of the ocean on whale carcasses, sick or dying organisms, etc.

  • Produce slime as a defense mechanism.

  • Have a cartilaginous skull and rod derived from the notochord.

  • Small brain, small eyes, and ears.

  • Tooth-like formations for scraping flesh.

  • Deep-water organisms with a mouth but no hinged jaws.

Lampreys

  • Similar to hagfish but have structures that hagfish lack.

  • Rasping mouth with mineralized tooth-like structures used to attach to fish and suck blood.

The protostomes have been divided into lophotrochozoans and ecdysozoans based on phylogenetic analysis. Ecdysozoans include nematodes (roundworms) and arthropods, with arthropods comprising the majority of animal species. Deuterostomes include echinoderms and chordates, a traditional grouping, and echinoderms such as sea stars, sea urchins, sea cucumbers, and brittle stars are closely related to chordates. Chordates are characterized by a notochord, dorsal hollow nerve cord, pharyngeal gill slits, and a post-anal tail. Chordate evolution dates back to the Cambrian period, exemplified by Picaia, and basic chordate features are seen in lancelets. Segmentation is evident in vertebrates, and novel chordate characteristics include the nerve cord, notochord, and pharyngeal gill slits. Phylogenetic analysis shows chordates branching off from deuterostomes, with echinoderms as the closest related group. The phylum Chordata includes cephalochordates, urochordates, and craniates, with vertebrates being a subphylum. Cephalochordates are the most basal chordate group, while urochordates undergo metamorphosis into sessile organisms. Hox genes control development, with neural crest cells involved in the development of facial and skull bones. Fossil evidence, such as Haikuala, indicates early craniates had eyes and a brain. Myxini (hagfish) are craniates without vertebrae, while lampreys have a rasping mouth with mineralized tooth-like structures.