Origin and Phylogeny of Chordates - Protochordates and Related Groups

A comprehensive set of Q&A flashcards covering chordate origins, protochordates, invertebrate chordate relatives, and major hypotheses on chordate evolution.

What are the two major evolutionary lines of Bilateria and which groups belong to Deuterostomes?

Protostomes and Deuterostomes; Deuterostomes include echinoderms, hemichordates, and chordates.

In Protostomes, from which structure does the mouth form?

The mouth forms from the blastopore (spiral cleavage is typical).

In Deuterostomes, what becomes the anus and what is the fate of the mouth?

The blastopore becomes the anus; the mouth is formed secondarily.

Name the three germ layers and what each primarily gives rise to.

Ectoderm (outer layer), Mesoderm (middle layer; forms schizocoelom or enterocoelom), Endoderm (inner layer; forms the gut).

Which cleavage pattern is associated with Protostomes and which with Deuterostomes?

Protostomes: Spiral cleavage; Deuterostomes: Radial cleavage.

What is the fundamental difference in blastopore fate between Protostomes and Deuterostomes (per Table 2.1)?

Protostomes: blastopore becomes mouth; Deuterostomes: blastopore becomes anus; mouth forms secondarily.

List the Protostome vs Deuterostome patterns summarized in Table 2.1.

Protostomes: mouth from blastopore, spiral cleavage, schizocoelic coelom, ectodermal skeleton. Deuterostomes: anus from blastopore, radial cleavage, enterocoelic coelom, mesodermal skeleton.

What skeletal features differentiate vertebrates from cephalochordates and urochordates?

Vertebrates have an endoskeleton (bone/cartilage); cephalochordates and urochordates lack a bony/cartilaginous skeleton and rely on collagen rods for support.

Define the notochord and its primary function.

A slender rod from the mesoderm; a hydrostatic locomotor organ that bends the body side-to-side and resists lengthwise compression.

What happens to the notochord in most primitive chordates versus vertebrates and mammals?

In primitive chordates it remains functional; in vertebrates it is often replaced by the vertebral column; in mammals, remnants become the nucleus pulposus of intervertebral disks.

What are pharyngeal slits and when are they present in chordates?

Openings in the pharynx behind the mouth; present at some stage in all chordates and may be present in adults in some species, or may disappear in some vertebrates.

What is the later role of pharyngeal slits in respiration and feeding across chordates?

In aquatic vertebrates, slits support gill ventilation; primitive chordates use a pharyngeal basket; terrestrial vertebrates form slits that do not open to the outside in adults.

What is the Endostyle and how is it related to the Thyroid Gland?

Endostyle: a glandular groove in the pharyngeal floor that secretes mucus for filter feeding and is involved in iodine metabolism; Thyroid Gland: endocrine gland producing thyroxine and triiodothyronine, arising from the pharyngeal floor and evolutionarily related to the endostyle.

What evidence shows the Endostyle transforming into the Thyroid in vertebrates?

lamprey larvae possess an endostyle that transforms into a thyroid gland in adulthood; endostyle is present in urochordates and cephalochordates, while thyroid is in vertebrates.

Define the Dorsal Hollow Nerve Cord and its formation.

A hollow nerve tube located dorsal to the gut, formed by neurulation: neural plate folds inward to form a hollow neural tube, the precursor to the CNS.

What is a Postanal Tail and its locomotor function?

A body extension posterior to the anus containing the notochord and segmented musculature; important for locomotion in aquatic forms.

Which groups are included in Protochordates?

Hemichordates, Cephalochordates, and Urochordates.

What is the modern view of Protochordate relationships regarding Ambulacraria?

Echinoderms and Hemichordates form Ambulacraria; Cephalochordates are basal chordates; Urochordates are more derived and closer to vertebrates.

Describe the lifestyle and feeding method of protochordates.

Marine animals; feeding by cilia and mucus; life stages include pelagic larvae and benthic adults; reproduction can be dioecious or monoecious.

Describe key features of Hemichordates.

Marine worms with chordate-like pharyngeal slits and a collar cord; tail may resemble a postanal appendage but lack a true postanal tail and notochord.

What is tornaria and why is it significant in Hemichordates?

A tornaria larva; planktonic and ciliated; resembles echinoderm auricularia larva, supporting close relationship (Ambulacraria) among echinoderms and hemichordates.

What are the two main taxonomic groups of Hemichordates?

Enteropneusta (burrowing acorn worms) and Pterobranchia (colonial, tube-dwelling).

Describe Enteropneusta body regions and their functions.

Proboscis (locomotion, feeding, burrow shaping), Collar (houses collar cord), Trunk (pharyngeal slits, gonads) — body is segmented into these regions.

How do Enteropneusta feed and digest?

Deposit feeding and suspension feeding; water flows Mouth → Pharyngeal slits → Branchial pouch → Branchial pore → outside; mucus traps food.

What is the general lifestyle and feeding mode of Pterobranchia?

Colonial, small; sessile in self-secreted tubes; suspension feeders using tentacles from the collar; glomerulus in proboscis for excretion.

What is the nervous system organization in Pterobranchia?

Simpler than acorn worms; no tubular nerve cord; collar ganglion acts as the main nerve center.

Describe Cephalocordata’s lifestyle and true chordate features.

Burrowing suspension feeders in warm seas; possess true chordate traits (notochord, dorsal hollow nerve cord, pharyngeal slits, postanal tail); body dorsoventrally inverted.

How do Cephalocordata feed and filter particles?

Suspension feeding via a ciliary current through pharyngeal slits; oral hood with buccal cirri; Hatschek’s pit may have endocrine function.

Explain Cephalocordata circulation and excretion.

Colorless blood plasma with no cells or pigments; no heart; circulation via contractile vessels; excretion via paired nephridia and Hatschek’s nephridium; podocytes resemble vertebrate kidney cells.

What metamorphosis occurs in Cephalocordata?

Larva is planktonic for 75–200+ days; metamorphosis forms atrium from metapleural folds, and organs like oral cirri, wheel organ, and velum tentacles develop; larva settles to become an adult burrower.

Describe Urochordata’s chordate traits and main feature in adults vs larvae.

Larvae (tadpole-like) possess all five chordate traits; adults are typically sessile (Ascidiacea) or pelagic; branchial basket for feeding; tunic outer covering.

List the main classes of Urochordata and a key characteristic of each.

Ascidiacea: sessile adults with swimming tadpole-like larvae; Larvacea: permanently pelagic; Thaliacea: pelagic, often colonial (salps and pyrosomes).

What is the Auricularian Hypothesis and its starting point?

A hypothesis that chordates evolved from a dipleurula-like echinoderm larva (bilateral, gut, adoral and circumoral cilia) leading to chordate features like notochord, dorsal nerve cord, pharyngeal slits, and postanal tail.

What selective pressures drive the Auricularian Transformation?

Increased planktonic stage and larval size to avoid predators and settle successfully; cilia become insufficient for locomotion and feeding as body mass grows.

Explain Paedomorphosis in the context of echinoderm-to-chordate evolution.

Retention of larval characteristics in a sexually mature adult; used to explain how a larval echinoderm form could give rise to chordate-like and later vertebrate forms.

Outline the proposed evolutionary pathway from echinoderm ancestors to vertebrates via paedomorphosis.

Echinoderm ancestor → paedomorphic larva → hemichordate-like form → urochordate-like form → vertebrates; paedomorphosis provides mobility advantages for planktonic larvae.