Detailed Notes on Deuterostomes: Vertebrates and Their Closest Relatives

Deuterostomes Overview

• Deuterostomes are categorized into three main groups:

  • Phylum Echinodermata (invertebrates)

  • Phylum Hemichordata (invertebrates)

  • Phylum Chordata (vertebrates)

• Embryological and molecular analyses indicate that these three phyla are closely related.

Phylum Echinodermata

• Key Characteristics:

  • Includes species such as sea stars, sea urchins, sea cucumbers, brittle stars, and sea lilies.

  • Larvae are bilaterally symmetrical and free-swimming.

  • Adults exhibit radial symmetry, often being slow-moving or sessile.

  • They have a well-defined coelom and a complete digestive system but lack excretory and respiratory systems as well as heads or brains.

  • Reproduction can be sexual or asexual (e.g., via splitting).

• Water Vascular System:

  • A unique locomotor system consisting of fluid-filled canals.

  • Each radial canal connects to tube feet that extend through ossicle holes.

  • Function:

  • Ampulla contraction forces fluid into tube feet, allowing attachment to surfaces.

  • Tube feet lengthen or shorten for movement and grip.

• Clades of Echinodermata:

  • Six major clades: Concentricycloidea, Asteroidea, Ophiuroidea, Echinoidea, Holothuroidea, Crinoidea.

Phylum Hemichordata

• Key Organism:

  • Acorn worms, which are sedentary marine animals residing in U-shaped tubes or burrows.

• Body Structure:

  • Comprised of three main parts: proboscis, tentacled collar, and elongate trunk.

  • Feature branchial slits in the pharynx for feeding and respiration.

Phylum Chordata

• Key Innovations:

  • Presence of a notochord which supports the embryo from head to tail.

  • Segmental muscles that allow independent contractions.

  • Dorsal hollow nerve cord that develops into the central nervous system; anterior ganglion or brain forms.

  • Perforated pharynx, which may have outpocketings or slits at some life stage.

Invertebrate Chordates

• Subphylum Cephalochordata (Lancelets):

  • Long, pointed bodies with light receptors and chemical sense organs for feeding via ciliary action.

• Subphylum Urochordata (Tunicates):

  • Larvae display chordate features and adults are sessile, secreting a tunic.

Vertebrates

• Characteristics:

  • Possess an internal skeleton for muscle attachment and organ protection.

  • Unique structural features: vertebral column surrounding the nerve cord, bony cranium enclosing the brain.

  • Coordinate Systems:

  • Axial skeleton: Cranium, vertebral column, ribs, and sternum.

  • Appendicular skeleton: Pectoral and pelvic girdles with appendages.

• Neural Crest Cells:

  • Give rise to distinctive vertebrate structures including the cranium, teeth, and sensory organs.

• Key Morphological Innovations:

  • Oldest vertebrate fossils date to about 550 million years ago and include species like Myllokunmingia and Haikouichthys.

Major Vertebrate Lineages

• Agnathans: Jawless vertebrates including extinct Conodonts and Ostracoderms as well as living Myxinoidea (hagfishes) and Petromyzontoidea (lampreys).

• Gnathostomata: Jawed vertebrates, further divided into:

  • Chondrichthyes: Cartilaginous fishes like sharks and rays.

  • Actinopterygii: Ray-finned fishes.

  • Sarcopterygii: Lobe-finned fishes; crucial for land transition (tetrapoda).

Evolution of Jaws

• Jaws evolved from cartilaginous gill arches, allowing greater feeding efficiency.

Fish Adaptations

• Fins: Different fin types for stability and propulsion (dorsal, caudal, pectoral, anal, pelvic).

• Chondrichthyes (Cartilaginous Fishes):

  • Cartilaginous skeleton; feeding adaptations include spirals in digestive systems.

• Actinopterygii (Ray-finned Fishes):

  • Bony internal skeletons; swim bladders for buoyancy and diverse feeding behaviors.

Sarcopterygii: Transition to Tetrapoda

• Include two main groups: lobefins and lungfishes, emphasizing adaptations for life in low-oxygen environments.

• Key Features: Evolved from water to land involving support structures for body weight and new respiratory adaptations.

Modern Amphibians

• Feature adaptations for both aquatic and terrestrial environments, including moist skin for gas exchange. Eggs laid in water, leading to metamorphosis from larvae.

• Three lineages: Anura (frogs), Caudata (salamanders), Gymnophiona (caecilians).

Amniota Evolution

• Key adaptations allow life in arid environments include waterproof skin, amniotic eggs, and uric acid excretion.

• Two major lineages:

  • Synapsida: Mammals with fur/hair.

  • Reptilia: Includes turtles, crocodilians, and birds.

Mammalia

• Key adaptations:

  • High metabolic rates and specialized teeth.

  • Parental care with prolonged development in the uterus.

  • Complex brain structures for learning and behavior.

Major Groups of Mammals

• Monotremes: Egg-laying mammals.

• Marsupials: Carry young in pouches.

• Placental Mammals: Diverse group with prolonged gestation and nutritional support through the placenta.

The Evolution of Humans

• Key Characteristics of Hominins: Upright posture and bipedal locomotion.

  • Lineages include Australopithecus and Homo erectus, showcasing tool-use development and social behavior traits.

• Modern humans (Homo sapiens) evolved from African hominoids, gaining adaptations for language and complex societies.