Vertebrate Evolution: Notochord & Early Vertebrates

• Topic 2 focuses on vertebrate evolution and diversity.

Fate of Notochord in Agnathans

• In Lampreys, the notochord grows, and animals feature paired lateral neural cartilages.
• These cartilages are positioned laterally to the spinal cord within the notochord.
• Whether this structure represents a primitive vertebra or vestigial vertebrae is currently unknown.
• Hagfishes retain the notochord into adulthood, serving as a strong, flexible cartilage rod.

Early Vertebrates

• Haikouichthys is one of the oldest vertebrate fossils discovered in Chengjiang, China.
• It dates back to the Early Cambrian period, approximately 518 to 530 million years ago (mya).
• The Haikouichthys had a small, tapered body with eyes, a brain, pharyngeal arches, a notochord, and rudimentary vertebrae.
• It bears a close resemblance to the living hagfish.
• Ostracoderms appeared around 450 mya, during the late Ordovician period.
• The "Age of Fishes" began approximately 400 mya, in the middle Silurian period.
• Early cartilaginous and bony fish emerged about 360 mya, in the Late Devonian period.
• Tetrapods first appeared around 360 mya, later in the Late Devonian period.
• Amphibians appeared approximately 330 mya.
• Known amniotes date back to about 310 mya.
• One vertebrate lineage colonized land about 365 million years ago.
• There are currently 52,000 known vertebrate species.

Chordate Characteristics

• Chordates possess both a notochord and a dorsal, hollow nerve cord.
• Deuterostomia exhibits bilateral symmetry.
• Vertebrates, urochordates, and cephalochordates all exhibit a notochord.
• The notochord provides structural support.
• Gnathostomes are vertebrates characterized by jaws.
• Tetrapods are gnathostomes that possess limbs.
• Amniotes are tetrapods that produce a terrestrially adapted egg.
• Mammals are amniotes distinguished by hair and milk production.
• Humans are mammals with large brains and bipedal locomotion.

Chordate Ancestry

• Ancestral chordates may have resembled lancelets.
• Hox genes, which organize the vertebrate brain, are also expressed in the lancelet's nerve cord tip.
• The vertebrate brain may have evolved from an ancestral structure similar to the lancelet's nerve cord tip.
• Homologous and orthologous hox genes are transcription factors that specify body region.
• Homologous genes descend from a shared ancestral DNA sequence.
• Orthologous genes evolved from a common ancestral gene through speciation, maintaining similar functions across different species.

Subphylum Cephalochordata

• Lancelets, also known as Amphioxus (Genus Branchiostoma), retain chordate characteristics into adulthood.
• These marine suspension feeders burrow in sediment, using cirri for filter feeding.
• Muscle segments called myomeres exhibit metamerism and develop from blocks of mesoderm called somites.
• Myomeres facilitate lateral undulations needed for swimming.
• Pharynx is used for respiration, filter feeding, and water regulation.
• Lancelets have separate sexes.

Subphylum Urochordata

• Tunicates or "sea squirts" display chordate features in their larval stage only, but tail and notochord are retained throughout life in Larvaceans.
• Tunicates exist either as solitary individuals or as colonial structures.
• Tunicates have 3 subclasses.
  • Ascidians are the most diverse of the 3.
• Filter feeders get water from the incurrent siphon to the mouth and expel it through the excurrent siphon.
• Notochord and tail disappear gradually; the dorsal nerve cord is reduced to a single ganglion, and pharyngeal gill slits persist into adulthood

Class Agnatha: Jawless Fish

• Class Agnatha includes two groups of jawless fishes, ostracoderms and cyclostomes
• Ostracoderms were covered with bony dermal armor and had small mouth openings.
• Ostracoderms lacked jaws and paired fins and had four openings on their dorsal surface of the head
• In the 4 orders of Ostracoderms, Order Osteostraci is the likely ancestors of jawed fishes
• Cyclostomes features extant eellike boneless lampreys and hagfishes with “round mouths."

Hagfishes

• Hagfishes are in the Class Agnatha with cartilaginous skull but lack jaws and vertebrae
• A flexible cartilage notochord is retained its entire adulthood
• They have small brains, eyes, ears, and a shallow buccal funnel lined with tooth-like structures
• Are mostly marine scavengers that feed on worms and dead fish, with some parasitic species, and features slime glands that secretes mucus to repel predators

Lampreys

• Lampreys are jawless vertebrates with denticle-lined buccal funnel in the Class Agnatha, with a cartilage based protein filled skeleton
• Many are parasitic as adults.
• Notochord serves as their main axial skelton with dorsal cartilaginous projections
• The skeletons are cartilage based with no collagen but made of a protein matrix
• All lampreys ascend to freshwater streams to breed
• Marine forms are anadromous and freshwater forms move between lakes and streams
• Ammocoetes are larval forms that are blind, toothless and filter feeders that live to up to 7 years and get to >5 inches.

Conodonts

• Conodonts is an extinct group and the first vertebrates with mineralized sketal elements

Gnathostomes: Jawed Vertebrates

• Jaws arose from modification of gill arches (third and fourth).
• Gnathostomes have an additional duplication of hox genes and an enlarged forebrain associated with enhanced smell and vision
• Aquatic gnathostomes have a lateral line system, which is sensitive to vibrations
• Gnathostomes includes sharks and their relationships, ray-finned fishes, lobe-finned fishes, amphibians, reptiles and mammals

Class Acanthodii

• Acanthodii is an extinct class known as "spiny sharks" and the oldest known jawed vertebrates
• They appeared in the SIlurian period and have stout fins with stout spines supporting their fins
• head and body is protected by dermal armor of bony plates and scales and is possibly the sister group of bony fishes

Class Placodermi

• Placodermi is an extinct class with paired fins who were swift predators
• They have head and neck armored with heavy bony plates and sharp bony blades
• Dunkleosteous was 10 meters lengths and 3.6 tons weight

Class Chondrichthyes

• Chondrichthyes is split into 2 subclasses - Elasmobranchii and Holocephali.
• Chondrichthyes (Class Chondrichthyes) have a skeleton composed primarily of cartilage
• largest and most diverse group consist of sharks, rays and skates, whose sharks eggs are fertilized internally and empty into a cloaca.
• Subclass Elasmobranchii have fusiform body with denticles, heterocercal tail and 2 dorsal fins and exposed without operculum for gill slits
• • Sharks mostly carnivores
• • Reproduction: Internal claspers that are viviparous, ovoviviparous, Oviparous
• Subclass Holocephali features body dorsoventrally compressed that lives on or near the bottom
  • Dorsal spiracles, gills slits on ventral surface and pectoral fins that are flat and infused with the head
• Holocephali (Chimaeras) don't have body scales, a single pair of gill slits covered by operculum
• Their upper jaw is attached to the skull, no spiracle closed
• They have bony plates and and no teeth and eat crustaceans, mollusks, fishes

Osteichthyans (Class Osteichthyes)

• Osteichthyans include the bony fish and the clade of tetrapods
• Osteichthyans that are aquatic are the common fishes which includes ray finned (Actinopterygii) and lobe finned fishes (Sarcopterygii)