•Chordata, sister taxon of Ambulacraria, contains Cephalochordata, Urochordata, and Vertebrata.
•Characterized by notochord, hollow dorsal nerve cord, endostyle, myomeres, and post-anal tail.
•Pikaia of the Burgess Shale is just outside of Chordaata.
•Cephalochordata comprises suspension-feeders employing their large pharynx, but capable of fish-like locomotion.
•Urochordata comprises creatures that, as adults, are mostly pharynx, but as larvae display the synapomorphies of Chordata.
•Vertebrata comprises chordates with heads that contain brains and three pairs of special sense organs.
•Developmentally, chordates add the formation of the neural tube to the sequence discussed earlier for Bilateria.
•Vertebrates add to this the formation of neural crest tissue - arguably a consequence of the duplication of HOX genes.
•Urochordata is the sister taxon of Vertebrata, forming Olfactores.
•Hyperotreti (hagfish), Hyperoartia (lampreys) and Gnathostomata are the major living groups of vertebrates.
•Their relationship is unsettled, with some favoring the "Vertebrata" hypothesis in which lampreys group with gnathostomes, and others the "Cyclostomata" hypothesis in which lampreys and hagfish are sister taxa.
•After a century of study, conodont elements were shown to belong to basal vertebrates, now named Euconodonta.
•The evolution of jawless vertebrates tracks the proliferation of bony tissue in the vertebrate body.
•Gnathostomata has a long list of derived characters, including jaws, that form from mandibular and hyoid arches. They, too, display a Hox gene duplication giving them four hot clusters.
•Placodermi has been shown to be a paraphyletic group of basal gnathostomes within which Eugnathostomata, the crown group is nested.
•For more, take GEOL431 in spring 2021.
"Say, what a lot of fish there are!"
Theodore Seuss Geisel
Also known as "tunicates" or "sea squirts." As adults, the body is dominated by a huge pharynx used for suspension-feeding. Adults of readily accessible species are typically attached to some hard surface, however many are free-swimming. They may be solitary or colonial.
Larvae: All urochordates begin life as a non-feeding planktonic "tadpole" larva that swims using a muscular tail, then undergo a metamorphosis into their feeding adult form. In attached forms, that metamorphosis happens when the larva settles onto a hard surface (after which the tail which degenerates). But note, the tail has:
Synapomorphies of Chordata!
- a notochord
- a hollow dorsal nerve cord
- muscles arranged into myomeres
Indeed, one sub-group - Larvacea maintain their tadpole-like morphology as they metamorphose into feeding adults. (Freaky stuff - Adults secrete a gelatinous structure, the house (that's the technical term) which they inhabit and push around as they swim.
Urochordate fossil record:
- A sparse fossil record from Chengjiang: Shanlouclava shankouense (right) from Chen et al., 2003 and Cheungkongella from Shu et al. 2001 b have been proposed as fossil urochordates. Each claim has its proponents and critics. As reconstructed, Shankouclava looks like an ascidiacean that never completely resorbed its tail.
- But Garcia-Bellido et al., 2014 regard vetulicolians as stem urochordates.
What a difference twenty years make.
When your instructors completed their PhDs, the consensus of deuterostome phylogenies held that:
- Hemichordata was the sister taxon to Chordata. Synapomorphy: Pharyngeal openings.
- Cephalochordata was the sister taxon of Craniata. Synapomorphies:
- The Burgess shale fossil Pikaia was a cephalochordate because it resembled Branchiostoma and had no distinct head.
- Urochordates we considered to represent an ancestral form, from which cephalochordates and craniates might have evolved by pedomorphosis.
Discoveries that have changed that view:
- Vetulicolians and Yunnanozoans provide a picture of the ancestral deuterostome as a "swimming pharynx." Thus:
- The swimming morphology and segmentation of craniates, cephalochordates, larval urochordates, and Pikaia are plesiomorphies.
- Same for the presence of pharyngeal openings in hemichrodates and chordates.
- If Larvacea represents the basal condition for Urochordata (Swalla et al. 2000) then urochordates also arose from "swimming phayrnges." (Indeed, Garcia-Bellido et al. 2014 propose that vetulicolians are actually stem urochordates. We await confirmation.)
- Strong molecular evidence for Olfactores and Ambulacraria that doesn't imply improbable reversals and convergences.
- Re-description and "demotion" of Pikaia.
Chordates with heads:
The remaining chordates are strongly distinct creatures. Synapomorphies include:
- Head including:
- Anterior expansion of nerve cord into brain.
- Special sense organs:
- Olfactory capsules for olfaction
- Eyes for vision
- Otic capsules for hearing
Neurulation in Branchiostoma
from A. S. Romer. 1977. The Vertebrate Body
Neurulation: The indeterminate nature of deuterostome development, in which the fates of specific cells are influenced by inductive relationships with other cells, is illustrated by the next big step chordate development - the formation of the neural tube that gives rise to the central nervous system. In this process, the activity of mesoderm cells triggers a cascade of events.
- Cells on the mid line of the mesoderm form a cylinder that becomes the notochord.
- Mesoderm to either side folds outward and expands down the side of the gastrula's inner surface.
- An inductive relationship causes the ectoderm directly above the notochord to sink downward, forming a trough, and ectoderm to either side to rise up forming crests to either side. The trough collapses into a hollow tube lying above the precursor to the notochord. This is the neural tube - the precursor to the brain and spinal cord.
The special sense organs arise from the interaction of the neural tube with placodes of the outer ectoderm.
Additional derived features:
Neural crest migration
These features enable "chordates with heads" to be:
- Neural crest tissue forms significant portions of the head, gill-arch skeleton, peripheral nervous system, and pigmentation.
- Water pumped through pharynx by muscles instead of my ciliary action.
- The bars separating the slits of the pharynx support special gas-exchange organs - gills.
- Branchial apparatus supported by cartiligenous brachial arches (gill bars) external to
- Pharyngeal muscles pump water through pharynx
- Larval endostyle transforms into adult thyroid gland
- In the gut:
- Liver and pancreas present (formed through endoderm-mesoderm induction)
- Digestive system invested with smooth muscular lining (rather than cilia)
- Circulatory System
- Two chambered heart
- Hemoglobin for oxygen transport
- Erythrocytes (red blood cells) to contain hemoglobin
- Caudal fin stregthened by cartiligenous radials
- Arcualia - cartilaginous precursors to vertebrae.
- Increased size (an order of magnitude)
- Cartilaginous endoskeleton: including fin rays and brachial arches
- W - shaped myomeres - an elaboration of the simple chevrons of Chordata.
- Orders of magnitude bigger than any non-craniate chordate
- Much more active than non-craniate chordates.
First question: Who is the sister taxon of "chordates with heads?"
Olfactores - Urochordate - craniate synapomorphies:
Thus, paradoxically, urochordates seem to share more synapomorphies with craniates while being ecologically less similar. Recent molecular studies (E.G. Delsuc et al., 2006) have also recovered Urochordata and vertebrates as sister-taxa.
- Although according to Holland et al., 1996, neural crest homologs (marked by gene expression) may be present in Branchiostoma, morphologically identifiable neural crest is absent, as are ectodermal placodes.
- In urochordates, migratory neural crest cells are distinctly present, but they only control the distribution of pigment in the skin. Although there are no special sense organs, the atrium arises through the invagination of an ectodermal placode.
First, consider the living groups:
(Myxinoidea and relatives - hagfish - Carboniferous - Quaternary). Synapomorphies of Hyperotreti:
Hagfish display interesting behaviors and are endearing in a grotesque sort of way.
- The skeleton consists of the notochord and specialized cartilages of mouth. The latter take the form of a mid-line rod-and-pulley upon which the tooth-plates are protracted and retracted.
- Otic (inner ear) capsules have only one semicircular canal (as opposed to three in jawed vertebrates).
- An adult hagfish is much too big to achieve gas exchange by simple diffusion. Its gill slits are, therefore, lined with thin pleats of heavily vascularized tissue - proper gills - which serve as breathing organs.
- Five to fifteen pairs of gill openings.
But hagfish lack many features we see in other craniates:
Not surprisingly, hagfish have almost no fossil record. Potential fossils of hagfish are entirely from the Carboniferous, including Myxinikela (Carboniferous) from Mazon Creek. Distinct tentacles and nasal basket on a rather plump body.
- Any suggestion of arcualia outside of the tail. (In hagfish, the only arcualia form in the position of the hemal arches of more derived vertebrates.)
- A brain-case as part of their skeleton. Theirs is largely membranous.
- Extrinsic eye muscles: Hagfish eyes are simple and cannot be rotated in the head.
Hyperoartia: (Petromyzontiformes and relatives - lampreys) - (Devonian - Recent): Characterized by:
- Undergoes metamorphosis from suspension-feeding ammocoetes larva that looks like a brainy version of Branchiostoma to a parasitic adult.
- In adult, annular cartilage supports a large sucker surrounding the mouth armed with keratin "teeth".
- Piston cartilage supporting a protrusible "tongue" armed with more keratinous denticles. Although distinct, this is also a rod-and-pulley arrangement similar to that of hagfish.
- Branchial skeleton: the lamprey gill pouches are supported by a branchial basket whose cartilagenous elements are external to the gills.
- Seven pairs of gill openings.
- Tail bends downward slightly (barely hypocercal)
- Living taxa have two distinct dorsal fins, although fossils don't.
vertical semicircular canals in the otic capsule. (In this and in other respects considered below, lampreys appear to have much in common with the final craniate group.)
Fossil record: Predictably sparse for creatures with no hard parts but known from Devonian and Carboniferous. The really cool part: According to Miyashita, 2018, a solid growth series for Priscomyzon shows that it lacks the ammocoetes phase, and hatches out as a tiny parasite. Could the ammocoetes ontogenetic stage be a derived feature of modern lampreys? This challenges our basic assumptions about the ancestral vertebrate being a vetulicolian-ike suspension feeder.
(Silurian - Quaternary) The last common ancestor of jawed vertebrates and all of its descendants. Living gnathostomes are distinct from living either Hyperotreti or Hyperoartia in many respects. Conspicuously, they have jaws, but also many other features to be discussed later.
"All I can say is that if cyclostomes form a clade, either hagfishes are the most extraordinary example of reversion among vertebrates, or lampreys and gnathostomes are the most extraordinary example of evolutionary convergence."
Philippe Janvier - 2007.
Until now we have been coy about the name of the clade containing chordates with heads. Now know the reason: Illuminating the relationship between Hyperotreti, Hyperoartia, and Gnathostomata is difficult and contentious. Two major hypotheses predominate that take their names from the positions they imply for lampreys:
- The Cyclostome hypothesis, in which Hyperotreti and Hyperoartia form a clade, Cyclostomi. Together, Cyclostomi and Gnathostomata make up Vertebrata - the chordates with heads.
- The Vertebrata hypothesis, in which Hyperoartia group with Gnathostomata - Vertebrata. This omits hagfish. Together, Vertebrata and Hypertreti make up a monophyletic "Craniata." If the "cyclostome hypothesis" is correct, then "Craniata" is the obsolete junior synonym of "Vertebrata." If it isn't, then the chordates with heads should properly be called "Craniata."
This is a huge problem in vertebrate systematics that cries out for resolution. Janvier's quote is apt, but opinion is swinging toward the Cyclostomata hyopthesis. What we really need is for a Cambrian konzervat-lagerstätte to cough up a fossil basal hagfish or lamprey. Of course, maybe they have and we haven't recognized it. ;-)
In GEOL331, we reluctantly adopt the terminology of the cyclostome hypothesis.
Intimations of the unseen - conodonts
Euconodonta: (Cambrian - Triassic) Since 1856, paleontologists have been aware of minute (0.1 - 0.5 mm.) fossils made of apatite (calcium phosphate), the same mineral as vertebrate bone and teeth.
- Highly diverse and rapidly evolving, thus excellent index fossils.
- Originally proposed to be the teeth of some unknown fish, but paleontologists soon determined they were were clueless about:
Thus, the word "conodont" was used to refer to the elements, themselves. The unknown creatures that made them were called "conodont animals."
- What kind of animal they were from
- What part of the animal they represented.
In the 1960s clarification came by the discovery of articulated groups of conodonts. For the first time it became clear that these elements (or most of them) worked together as part of a conodont apparatus.
The conodont animal: Briggs et al., 1983, described Clydagnathus, an Early Carboniferous age eel-shaped creature in which he noted:
- Chordate-like V-shaped segmented muscle blocks
- Midline fins supported by fin rays
- The conodont apparatus in an anterior position, suitable for use in feeding.
- A head a brain and two capsules for special senses, thought to be very large eyes and smaller otic capsules.
We now have an emerging consensus on what the "conodont animal," now the monophyletic group Euconodonta, looked like - a small, eel-shaped chordate.
But where does Euconodonta go on the chordate cladogram? The presence of phosphatic hard parts arguably places it, closer to Gnathostomata than to hagfish or lampreys, but there are concerns:
Goudemand et al. 2011 describe the well-preserved conodont apparatus of Novispathodus, concluding that it was protracted during feeding by a rod-and-pulley arrangement similar to that of lampreys and hagfish. It seems plausible that such an arrangement is plesiomorphic (ancestral) for craniates. Link to animation.
- The conodont elements, as elements, are not homologous to any other phosphatic vertebrate feature.
- Jawless vertebrates might have secondarily lost hard tissues as a reversal.
- Some morphological interpretations of euconodonts, especially the huge eyes, seem to cry out for revision.
- The assumption of euconodont monophyly hasn't really been rigorously tested.
The earliest vertebrates:
Chengjiang and the Burgess Shale have provided other records of basal vertebrates lacking conodont elements.
Myllokunmingia and Haikouichthys: Chengjiang gives us the best picture picture of what the ancestral vertebrate might have looked like with Haikouichthys ercaicunensis (Shu et al., 1999) H. ercaicunensis may be junior synonym of Myllokunmingia fengjiaoa). These seem to preserve:
- W-shaped myomeres
- placode-derived special sense organs
- a branchial skeleton resembling that of lampreys
- Dorsal fin with possible fin rays
- possible arcualia the anterior notochord
Metaspriggina: And from the Burgess shale, Conway-Morris and Caron, 2014 describe Metaspriggina walcotti (right). In this case, the presence of eucondont-like eyes and W-shaped myomeres is unambiguous.
The evolution of Paleozoic vertebrates presents a paradox:
We have such a copious record of heavily armored Early Paleozoic forms that it is tempting to forget that the group's most basal members (like Haikouichthys and Myllokunmingia) essentially lacked hard tissues. Indeed, the early evolution of Vertebrata is marked by the diversification of bony tissues and their proliferation through the body. This pattern was recently illuminated by Sansom et al., 2010 and Miyashita et al., 2019. (Composite synopsis at right.) So, we start with a review of bony evolution.
Hagfish and lampreys, as the only living jawless vertebrates, provide an interesting glimpse of early vertebrate evolution, however they lack the proper hard tissues by which we know the vast diversity of early vertebrates - bone.
Fossil vertbrates are mostly known from hard tissues - bone and teeth. Bone is composed of:
- A mineral component - made of calcium phosphate (i.e. the mineral hydroxyapatite - Ca5(PO4)3OH)).
- A protein component - made mostly of the fibrous protein collagen.
Bone in any form only occurs among members of Vertebrata, although we know there are some vertebrates who lack it. What does the study of fossil organisms tell us about the distribution of bony tissue?
A rogue's gallery of early Paleozoic vertebrates:
The earliest vertebrate hard tissues are small acellular elements: conodont elements, which show outer layers of enamel covering layers of dentin. Conodonts were not the only representation of craniate hard tissues in the Cambrian, however. Enigmatic, scale-like plates of bony armor called Anatolepis were also present. In this and similar creatures, histologically tooth-like denticles complete with enamel and dentin formed a composite superficial body armor.
Indeed, in many early vertebrates, there seems to have been little difference between teeth and scales, which took the form of little denticles with a pulp cavity, dentin, and enamel.
The most basal vertebrates, however, lacked any hard tissues (except for conodont elements.) A survey of early vertebrate evolution should focus on their acquisition:
Anaspida (Silurian), Euphanerops (Devonian), and Jamoytius (Silurian)
These form a clade in analysis of Miyashita et al., 2019. Illuminated by well-preserved Jamoytius and Euphanerops. These were cylindrical, vaguely lamprey-like creatures with varying amounts of acellular dermal bone plates, but without specialize mouthparts. They lack obvious adaptations to suspension feeding or to taking large prey. Deposit feeders? Many morphological details reinforce the proto-lamprey impression
Thelodonti (Ordovician - Devonian)
Morphology: Hard tissue Entirely consists of small scales that usually disarticulate when the animal dies. These scales are distinctive, consisting of enamel and dentin layers around a pulp cavity, like a vertebrate tooth. Note: from this point on the tree onward, aquatic vertebrates generally retain scales of this sort or their derivatives, regardless of any other kind of skeletal ossification they may have.
Synapomorphies with jawed-vertebrates:
- Dermal denticles with distinct root, crown, and pulp-cavity.
- Paired nostrils and nasal capsules.
Issue: True Bone:
At this point we pick up a new kind of bone in which the cells that secrete and maintain hard tissue may be locked within it, yielding cellular bone. Seen in larger bony elements. Cellular bone forms in two ways:
- Dermal bone: Laid down as a two-dimensional membrane. (E.g. human cranial bones). Ancestrally these formed near the body surface, but in derived vertebrates, their derivatives may invade deeper parts of the body.
- Cartilage bone: Three-dimensional bone that is preformed in cartilage. (E.g. human limb bones). The cartilage, in turn, is preformed by condensations of mesenchyme cells, amoeboid mesodermal connective tissue of the embryo. (Remember them?) Cartilage bone makes its evolutionary debut in the skeleton of the braincase but is widespread in vertebral columns and the skeleton of the limbs.
However cartilage bones in living vertebrates actually form in two ways:
- Perichondral ossification: Bone formation begins at the perichondrium - the membrane surrounding the cartilage precursor.
- Endochondral ossification: Bone formation begins in the interior of the cartilage precursor.
Note: Among living vertebrates, both types of ossification occur in cartilage bone, so neontologists and human anatomists use "cartilage bone" and "endochondral bone" interchangeably. Paleontologists must distinguish them, however, because perichondral ossification evolved first and many fossil vertebrates have perichondrally ossified bone without the endochondral component.
Pteraspidomorphi (Ordovician- Devonian).
by Masato Hattori.
The earliest well-preserved vertebrate, the Ordovician form Sacabambaspis, ironically represents a more derived form of hard tissue, in which individual denticles are integrated into broad head-shield composite elements and joined to one another through dermal layers of aspidin, a composite of thelodont-like denticles, lamina of dentin, and cellular dermal bone. Note: It was not an internal skeleton.
Synapomorphy with jawed-vertebrates:
- Plate-like cellular dermal bone.
Galeaspida: Restricted to southern China and Indochina, then a separate continent. (Silurian - Devonian)
- large flat head shield enclosing an perichondral bony braincase with pteraspidomoprh-like head shield .
- large opening in upper front of head shield (maybe a large median nostril) leads to paired olfactory capsules and pharynx
- mouth is ventral
- Pharynx is large with many gill openings, suggesting suspension feeding.
Synapomorphy with jawed-vertebrates: Perichondral bone in braincase.
Osteostraci (Silurian - Devonian): Resemble galeaspids but with differences:
Synapomorphies with jawed-vertebrates:
- Paired pectoral appendages
- Heterocercal tail
- Distinct dorsal fin
Gnathostomata: (Sil - Rec.) The jawed vertebrates. Quantum leap forward.