Deuterostomes and Echinoderm Phylogeny

Chapter 34: Deuterostomes

• Deuterostomes include two major groups: echinoderms and chordates.

Echinoderm Phylogeny

• Echinodermata are closely related to Chordata.

Phylum Echinodermata

• Exclusively marine organisms
• Deuterostomes with an endoskeleton of calcium carbonate plates covered by living tissue
• Exhibit pentaradial symmetry
• Includes sea stars, brittle stars, sea urchins, sand dollars, and sea cucumbers
• Thought to have evolved from bilaterally symmetrical ancestors because larvae are bilaterally symmetrical
• Free-swimming bilateral larva of the common sea star, Asterias rubens
• Adult sea stars exhibit pentaradial symmetry
• Echinoderm skeleton is internal but directly under the skin and is not suited for muscle attachment

Echinoderm Body Plan

• Features a hydraulic system aiding in movement or feeding
• Water-vascular system is a modification of coelomic spaces with a central ring canal and extending radial canals.
• The madreporite is the opening/entrance of the water vascular system.
• Radial canals transport liquid to the tube feet.
• The ampulla in each tube foot contracts, the tube foot extends and can attach to the substrate.
• Muscles in the tube feet contract, and the tube foot bends, pulling the animal forward.

Symmetry

• Exhibit pentaradial symmetry as adults and bilateral symmetry as larvae.
• The oral surface defines the mouth.
• All systems are organized with branches radiating from the center.
• Nervous system consists of a nerve ring with branches, lacking centralization of function.

Phylum Chordata

• Chordate endoskeleton is truly internal
• Chordate endoskeleton is functionally similar to arthropod exoskeleton.
• Includes fishes, amphibians, reptiles, birds, and mammals

The Chordates

• Chordata are more closely related to Echinodermata than other phyla.

Characteristics of Chordates

• All chordates exhibit four characteristics at some time in their lives: nerve cord, notochord, pharyngeal slits, and postanal tail.
• The notochord is a structure made of specialized cells that guides the development of other cells, especially forming the nervous system and vertebral column, acting as a signaling center during development.
• May be replaced by the vertebral column
• Pharyngeal slits become structures of the throat and ears
• Postanal tail extends past the terminal anus

Four Principal Features of Chordates

• Hollow dorsal nerve cord
• Pharyngeal pouches
• Notochord
• Postanal tail

Other Chordate Characteristics

• Chordate muscles are arranged in segmented blocks called somites
• Most chordates have an internal skeleton against which the muscles work.
• Somites are more prominent in embryos than adults

The 3 Chordate Subphyla

• Two nonvertebrate subphyla
• Urochordata (~Tunicata): Larvae are tadpole-like with a notochord and nerve cord
• Adults typically lose the tail, notochord, and nerve cord, e.g., sea squirts
• Cephalochordata: Notochord persists throughout the animal's life, providing structural support and flexibility

Characteristics of Chordates: Example of the Lancelet

• Vertebrates, tunicates, and lancelets are chordates, coelomate animals with a flexible rod, the notochord
• Notochord provides resistance to muscle contraction and permits rapid lateral body movements
• Pharyngeal pouches or slits reflect the animal's aquatic ancestry and current habitat in some
• All chordates have a hollow dorsal nerve cord. In vertebrates, the notochord is replaced during embryonic development by the vertebral column

Subphylum Vertebrata

• Vertebrates are chordates with a spinal column
• Distinguished from nonvertebrates by:
• Vertebral column: Encloses and protects the dorsal nerve cord.
• Head: Distinct and well-differentiated possession of sensory organs.

Vertebrates Also Have

• Neural crest: This unique group of embryonic cells forms many vertebrate structures and migrates from the neural tube
• Internal organs: Such as liver, kidneys, endocrine glands, heart, and a closed circulatory system
• Endoskeleton: Made of cartilage or bone, supporting large size and extraordinary movement

Major Characteristics of Vertebrates

• Head with brain encased in skull
• Vertebral column (part of skeletal system)
• Heart-powered closed circulatory system
• Dorsal nerve cord and Kidneys
• Liver, Limbs (or fins)
• Postanal tail

History of the Vertebrates

• The first vertebrates appeared in the oceans about 545 MYA
• They had a mouth at one end and a fin at the other.
• Jawed fishes later became dominant.
• Amphibians invaded the land.
• Reptiles replaced amphibians as the dominant land vertebrates.
• Birds and mammals became dominant after the Cretaceous mass extinction (65 MYA)

Phylogeny of the Living Vertebrates

• Key characteristics evolved among different vertebrate groups

Fishes

• They are the most diverse vertebrate group
• Over half of all vertebrates are fishes.
• Fishes includes species in different classes
• Provided the evolutionary base for the invasion of land by amphibians.

Fishes Characteristics

• Majority have a vertebral column but hagfish and lamprey are exceptions
• Possess jaws and paired appendages, with exceptions in hagfish and lamprey.
• Have internal gills that help with complete separation between O2 and no O2 blood
• Utilize a single-loop for blood circulation
• Affected by specific nutritional deficiencies, which has impacted all their vertebrate descendants

History of the Fishes

• Agnatha exist today as hagfish (class Myxini) and lampreys (class Cephalaspidomorphi)
• Ostracoderms are now extinct.
• Jaw development occurred in the late Silurian period
• Jaws evolved from the anterior gill arches made of cartilage.

Evolution of the Jaw

• Derived from anterior gill arches of ancient jawless fishes.

Two Major Groups of Bony Fishes

• Ray-finned fishes (class Actinopterygii) have parallel bony rays support and stiffen each fin; there are no muscles within the fins.
• Lobe-finned fishes (class Sarcopterygii) have fins consisting of a long fleshy muscular lobe, supported by a central core of bones with fully articulated joints
• They are ancestors to amphibians
• The coelacanth, a lobe-finned fish discovered in 1938, represents a group of fishes thought to have been extinct for about 70 million years.

Class Amphibia

• Includes the first vertebrates to walk on land
• Are direct descendants of fishes

Orders of Amphibians

• Anura (frogs and toads): compact, tailless body, a large head fused to the trunk, and rear limbs specialized for jumping
• Caudata (salamanders and newts): slender body, long tail, and limbs set out at right angles to the body
• Apoda (caecilians): tropical group with a snakelike body, no limbs, and little or no tail

Distinguishing Amphibian Features

• Possess legs as an adaptation to life on land (except apoda)
• Lungs exist, but are not diaphragmatic
• Cutaneous respiration supplements lungs
• Pulmonary veins separate the pulmonary circuit increasing blood pressure
• Partially divided heart improves separation of pulmonary and systemic circuits

Successful Invasion of Land by Vertebrates

• Required several adaptations • • • • Legs to support body's weight. Lungs to extract oxygen from air. Redesigned heart and circulatory system to drive larger muscles. Reproduction still in water to prevent egg drying ie. they need to live near water System to prevent whole body desiccation.

Ichthyostega

• It was one of the first amphibians
• Had sturdy forelegs and flipper-shaped hindlimbs and did not lift its legs,
• Moved like a seal
• Long, broad, overlapping ribs formed a solid cage for the lungs and heart and can be called a "transitional form" in evolution

Tiktaalik

• Transitional fossil found between fish and Ichthyostega in 2006
• Possessed gills and scales like a fish, but had a neck like an amphibian
• Shoulder, forearm, and wrist bones resembled amphibians, yet the limb ended in a lobed fin

Class Reptilia

• There are over 10,000 living species
• All living reptiles exhibit three key features: amniotic eggs, dry skin, and thoracic breathing.

Amniotic Eggs

• All reptiles, birds, and mammals are amniotes.
• Amniotic egg has four membranes:
• Chorion: Outermost layer, allows gas exchange.
• Amnion: Encases embryo in fluid-filled cavity, providing cushionning
• Yolk sac: Provides food.
• Allantois: Contains excreted wastes from embryo.

Amniotic Vs Non-Amniotic Embryos

• Amniote embryos develop a series of protective extraembryonic membranes, including the amnion, chorion, yolk sac, and allantois.
• Nonamniote embryos, such as those of amphibians and fish do not.

Major Orders of Reptiles

• Squamata (suborder Sauria): Lizards possess limbs set at right angles to their body; anus is in transverse (sideways) slit; most are terrestrial.
• Squamata (suborder serpentes): Snakes have no legs, move by slithering, scaly skin is shed periodically in a single piece; most are terrestrial; snakes evolved from lizards, making lizards a paraphyletic group.
• Rhynchocephalia: Tuataras are sole survivors of a once successful group that largely disappeared before dinosaurs; fused, wedgelike, socketless teeth; primitive third eye under skin of forehead.
• Chelonia: Turtles, tortoises, and sea turtles are armored reptiles with a shell of bony plates to which vertebrae and ribs are fused; sharp, horny beak without teeth.

Amniote Evolutionary Tree

• Relationships between reptiles, birds, and mammals

Anapsids, Synapsids, and Diapsids

• Reptiles dominated earth for 250 million years
• Reptiles are distinguished by the number of holes on the side of the skull behind the eye orbit:
• 2 holes on each side Diapsids had 2 holes e.g. dinosaurs/birds
• 1 hole on each side Synapsids had 1 hole e.g. mammal like reptiles
• 0 holes on each side Anapsids had 0 holes e.g. turtles

Dinosaurs

• They dominated earth for over 150 million years.
• Mammal ancestors co-evolved with dinosaurs around 220 M years ago.
• They became extinct 65 MYA, except for birds, descendants.
• This can be atributed to a asteroids impact

Modern Reptiles

• Modern reptiles developed two important characteristics:
  • Internal fertilization ensuring successful fertilization
  • Improved circulation ensuring provision of oxygen is more efficient to the body; crocodiles, birds, and mammals have completely divided 4-chambered heart.

Birds (Class Aves*)

• Birds are the most diverse of all terrestrial vertebrates