Vertebrate Origins

Vertebrates are but a single subphylum within the chordates.

What defines a chordate? Notochord at some stage of

development. Dorsal hollow nerve cord. Pharyngeal gill slits present at some

stage of development Endostyle (becomes thyroid gland in

vertebrates). It is a ciliated glandular groove on the floor of the pharynx, that aids in filter feeding by secreting mucus, and just as in the thyroid, it is able to concentrate iodine.

Vertebrates are but a single subphylum within the chordates.

Muscular postanal tail Ventral heart with a closed circulatory

system. Living bony or cartilaginous

endoskeleton.

Chordata Include: a. Urochordata – Tunicates

As envisioned by Pough et al.

As envisioned originally by Romer.

Chordates include:Cephalochordates

Cross section through Amphioxus, a cephalochordate.

Cephalochordates include: Vertebrates

What defines a vertebrate? Presence of vertebrae!

They are cartilaginous in some fishes. They are absent in hagfishes. Lampreys possess only rudimentary

cartilaginous elements around the nerve cord. Note, at one time these organisms were considered to be degenerate.

Presence of a Cranium (hence the original name of the group: Craniata.

Vertebrae and cranium for the group Vertebrata. Note: these mammalian structures are highly derived.

What defines a vertebrate? Presence of duplicated Hox gene

(homeobox gene) Presence of embryonic tissue

called the neural crest, which give rise to epidermal placodes. These are the origin of the complicated sensory tissue characteristic of vertebrates.

An interesting observtion about vertebrates:

While most animals are small, vertebrates are relatively large. Thus diffusion is no longer sufficient for most bodily functions.

This necessitates specialized structures and systems in vertebrates.

Basal metabolic rates in vertebrates are higher than other animals.

Vertebrates are easily capable of anaerobic metabolism.

What is the evolutionary history of the vertebrates?

3 hypotheses Arthropod hypothesis

Arthropods are a major animal group – common and therefore likely to have daughter groups.

They share some characteristics with the vertebrates.

If you turn an arthropod upside down, you have the basic vertebrate body plan.

What is the evolutionary history of the vertebrates?

3 hypotheses Arthropod hypothesis

The body is segmented. There is a ventral nerve cord and a dorsal

heart. Problem – the exoskeleton. This idea dates to 1818 by St. Hilaire.

What is the evolutionary history of the vertebrates?

3 hypotheses Annelid hypothesis

Semper and Dohrn noted in 1875 that annelidshave the same basic body plan as vertebrates, only upside down, and they have an excretory system that is remarkable similar to that of some chordates.

Problem – the nerve cord is ventral and bifurcates to go around the pharyngeal tube to a dorsal brain. If you turn the organism upside down, the brain is ventral and the mouth dorsal … a situation which does not show up in any vertebrate.

What is the evolutionary history of the vertebrates?

3 hypotheses Echinoderm - Hemichordate –

Chordate Hypothesis hypothesis Both of the above hypotheses suffer from

the fact that annelids and arthropods have spiral determinate cleavage while chordates have radial indeterminate cleavage.

What is the evolutionary history of the vertebrates?

3 hypotheses Both annelids and arthropods are

protostomes while chordates are deuterostomes.

Arthropods and annelids have shizocoelous coelom formation while chordates have enterocoelous coelom formation.

What is the evolutionary history of the vertebrates?

3 hypotheses Echinoderms have precisely the same

characters as the chordates: radial indeterminate cleavage, deuterostomes, and enterocoelous coelom formation.

Also, some echinoderm bipinnaria larvae resemble closely the tornaria-like larvae of some chordates in that both have sensory cilia at the anterior end, both have a complete digestive system with ventral mouth and posterior anus, and both have ciliated bands in loops.

Diagramatic side views of larvae of A: acorn worm, B: starfish, and C: sea cucumber. Black lines represent ciliated bands. The digestive tracts are stipled. All are bilaterally symmetric.

What is the evolutionary history of the vertebrates?

3 hypotheses It is important to remember that the

echinoderms we see today are probably very dissimilar from the echinoderms that were the actual ancestors to the chordates. Early echinoderms for example were not pentaradial. The diversity of echinoderms today is but a fraction of what was once there.

Not all basal deuterostomes were asymmetrical or pentaradial. The calcichordata were bilaterally symmetrical, and may in fact be specialized echinoderms.

Calichordate – fossil deuterostome

Possible early chordate from Mid-Cambrian Burgess Shale Pikaia

Non-vertebrate Chordates

UrochordatesTunicates (sea squirts)

Sea squirts have sessile filter feeding adults and free swimming planktonic larvae. Larvae look similar to amphioxus – basic vertebrate body plan. Have pharyngeal gill slits, notochord, dorsal hollow nerve cord, muscular post anal tail

Urochordates

Adults however, look very different. How could this lead to vertebrates? Paedomorphosis – retention of juvenile

morphology in the reproductive adult. This is an example of heterochrony.

Alternatively, we may be derived from the sessile adult stage.

Tunicates and Hemichordates: A & B Pterobranchs, C: colonial tunicate, D tunicate, E free swimming tunicate, and F acorn worm.

Longitudinal section through the head of an acorn worm (Hemichordata)

Urochordates Chordates are unique in having innervation of 2

types: segmented innervation and non-segmented innervation. It may be that we were originally nonsegmented (like the sessile adults) and later our morphology was over-run by the newly derived segmented components.                                                                              

Also, chordates have allorecognition. Invertebrates do not. However, echinoderms have allorecognition, as do some colonial organisms. Perhaps it is a means of preventing fusion of non-identical organisms. The ancestors of echinoderms may have been colonial and sedentary.

Contrast between visceral and somatic components. Tunicate like larvae w/ somatic component retained in adult, and true vertebrate w/ visceral in black.

Cephalochordates

Fish-like in appearance and totally marine.

Best know example is amphioxus (lancelet).

Has segmented myomeres, and many homologies with vertebrates.

Generalized non-vertebrate chordate design compared with hypothetical primitive vertebrate.

So, the vertebrate family tree looks something like this … in Romer’s depiction.

Who are the earliest Vertebrates? Probably the conodonts, although

the issue has not been resolved. Conodont fragments are abundant

– probably ‘teeth.’ These are referred to as ‘conodont elements.’ Impressions of complete conodont animals have been found.

This raises the issue of what becomes a fossil.

Fossilization? Most fossils are mineralized tissue.

Bone is an excellent site for mineralization, as are odontodes in the skin. Teeth too are good.

There is some question about when bone evolves as a vertebrate character.

Hagfish and lampreys have no bone (they do have inner ear ossicle)

Nature of early bone has some implications for physiology – ion & fluid regulation.

What is the function of early bone?

May serve a protective function. There were large aquatic invertebrate predators, and the armor of ostracoderms and placoderms may have prevented predation. Unfortunately, the bony armor is below the

skin and thus susceptible to injury Perhaps it was used as a mineral sink? This

is related to an early hypothesis about where vertebrates evolved.

Did vertebrates have a freshwater or marine origin?

Romer and Smith argued for a freshwater origin. Bone may represent a mineral sink. Phosphates and calcium were probably

a ‘hot’ commodity in the Silurian. Bone armor may have prevented

osmosis. Although all fossils were found in

marine sediments, they argued the fossils washed into the sea.

Did vertebrates have a freshwater or marine origin?

All fossils are marine. All old vertebrate groups are marine. Kidney function was probably co-

opted from other mineral regulation functions. (Do fish drink?)

Prevailing view today is that vertebrates have a marine origin.

Vertebrate Ancestry Ostracoderms

Oldest fossil vertebrates except conodonts.

First discovered in Ordovivian rock in Russia and the U.S.

Belong to agnathan/cyclostome group. Major radiation in the Silurian and

Devonian, but extinct by the end of the Devonian.

Vertebrate Ancestry Ostracoderm morphology

No jaws No paired fins. Heavy bone armor.

Vertebrate Ancestry Placoderms

Less developed bony armor Paired fins and thus probably more

active swimmers. Had jaws and were capable of

predaceous life-style First appeared in Silurian, major

radiation in Devonian, extinct by end of Permian.

Vertebrate Ancestry One Placoderm group (acanthodians)

had bony scales like modern fishes. Placoderms may have given rise to, or

had a common ancestor with 2 major groups: the Chondrichthyes and the Osteichthyes.

Vertebrate Ancestry Chondrichthyes

No bone, probably underwent reduction from Placoderm condition, or may represent true underived condition. Could this be an example of neoteny or paedomorphosis? They have a living endoskeleton, but it is made of cartilage.

Completely predaceous life-style.

Vertebrate Ancestry They have a spiracle. They have internal fertilization. The holocephalans (chimeras)

have an upper jaw that is fused to the brain case, and a flap of skin that covers the gill region.

Vertebrate ancestry Osteichthyes

They have a bony endoskeleton, probably a retention of the ostracoderm or placoderm condition.

They have bony scales and opercula Origin was the Devonian, they split

almost immediately into 2 groups: the Actinopterygians and the Sarcopterygins

Vertebrate Ancestry Actinopterygians.

Chondrosteans (sturgeons), Holosteans (bowfins and garpikes) and Teleosts (modern bony fishes).

Sarcopterygians. Dipneusti (lungfish), crossopterygians

and ceolocanths.