GEOL 331/BSCI 333 Principles of Paleontology

Fall Semester 2020
Deuterostomia III: Vertebrates

Tales from Topographic Oceans by Roger Dean featuring Eusthenopteron, and Ichthyostega

Key Points:
•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


The sister taxon of Ambulacraria. Contains:

Schematic of ancestral vertebrate (showing chordate characters plus some)
from Radinsky 1987. The Evolution of vertebrate Design.

Pikaia graciliens
Pikaia - Almost a chordate:

The Burgess Shale fossil Pikaia graciliens, known from hundreds of specimens, is often cited in popular literature as the "earliest chordate." In fact:

Conway-Morris and Caron plausibly regard it as a "stem-chordate," closer to Chordata than to Ambulacraria, but just barely. What Pikaia does have is this potential synapomorphy with proper chordates:

Branchiostoma sp. from Real Monstrosities
Cephalochordata: (Cam. - Rec.) "Headless fish."

Represented by the living Branchiostoma.

Confusing nomenclature: Earlier names have priority in scientific nomenclature. Unfortunately, the publication in which this animal was originally named (Branchiostoma) was largely ignored, and it was named a second time (Amphioxus). Additionally, there had been a common name (Lancelet). When the dust settled, Branchiostoma was acknowledged the proper scientific name, and amphioxus and lancelet are both considered common names. Live with it.

Branchiostoma sp.
Anatomy: In many ways, Branchiostoma is like a modified version of Pikaia. Similarities include: Where Branchiostoma differs significantly:

Branchiostoma life position from
Branchiostoma ecology: Branchiostoma opens a window onto the ecology of the "swimming pharynges" at the base of Deuterostomia. It does not spend much time actually swimming, but, instead, buries itself in soft sediment, with its front end projecting into the water suspension-feeding. When disturbed, it swims in a fish-like manner, but soon settles down, digs in, and resumes suspension-feeding.

Anatomical details:

Synapomorphy of Cephalochordata:

Cephalochordate fossil record: Sparse, but present as early as the Chengjiang lagerstätte in the form of Cathaymyrus diadexus.

Interpreting cephalochordates: For decades, cephalochordates have been presumed to be closely related to Chordata. They are so close morphologically. The Wizard need only give them hearts and brains. The next group, in contrast, seems more distant.

Clavelina moluccensis from
Urochordata: (Sparse record Cam. - Rec.)

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.


from University of South Carolina Aiken
  • 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!

    Larvacean in its "house" from
    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:

    What a difference twenty years make.

    When your instructors completed their PhDs, the consensus of deuterostome phylogenies held that:

    Discoveries that have changed that view:

    Chordates with heads:

    The remaining chordates are strongly distinct creatures. Synapomorphies include:

    Neurulation in Branchiostoma from A. S. Romer. 1977. The Vertebrate Body.

    Chordate Development:

    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.

    The special sense organs arise from the interaction of the neural tube with placodes of the outer ectoderm.

    Neural crest migration
    Additional derived features:

    These features enable "chordates with heads" to be:

    Their phylogeny

    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.

    Major groups:

    Craniate diversity:

    Hagfish - from Joseph Jameson-Gould, Real Monstrosities Blog


    (Myxinoidea and relatives - hagfish - Carboniferous - Quaternary). Synapomorphies of Hyperotreti: Hagfish display interesting behaviors and are endearing in a grotesque sort of way.

    Other characteristics:

    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.

    Pacific lamprey - from Brendan Maher, Natureblog


    (Petromyzontiformes and relatives - lampreys) - (Devonian - Recent): Characterized by:

    Priscomayon riniensis (Late Devonian)
    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.

    Mako shark - from Sam Cahir, Mail Online


    (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.

    "Craniate" Relationships

    "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:

    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

    Conodont elements
    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.

    Conodont apparatus
    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:

    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 earliest vertebrates:

    Chengjiang and the Burgess Shale have provided other records of basal vertebrates lacking conodont elements.

    Haikouichthys ercaicunensis from
    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:

    Metaspriggina walcotti from Science News
    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:

    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?

    Anatolepis armor

    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:

    Euphanerops longaevus
    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

    Thelodonts from Wikimedia Commons
    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:

    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:

    Pteraspis stensioei by Masato Hattori.
    Pteraspidomorphi (Ordovician- Devonian).

    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:

    Shuyu zhejiangensis Institute of
    Vertebrate Paleontology and Paleoanthropology
    Galeaspida: Restricted to southern China and Indochina, then a separate continent. (Silurian - Devonian)


    Synapomorphy with jawed-vertebrates: Perichondral bone in braincase.

    Cephalaspis Bionet Skola
    Osteostraci (Silurian - Devonian): Resemble galeaspids but with differences:


    Synapomorphies with jawed-vertebrates:

    Gnathostomata: (Sil - Rec.) The jawed vertebrates. Quantum leap forward.

  • Synapomorphies of Gnathostomata:


    Gnathostome head schematic from British Chalk Fossils
    Jaw anatomy: Up until now, when we have spoken of a "skull" we have meant only a plating of dermal bone around the head, or, in the case of Galeaspids and Osteostraci, that plus in endochondrally ossified neurocranium or braincase. With the appearance of the gnathostome jaws and branchial skeleton, the skull becomes a complex composite structure.

    Note: In vertebrates with bony skeletons, these components can ossify in different patterns and into different numbers of elements, but the underlying identity of the cartilage precursors remains the same.

    The following illustrations show its components using the fossil bony fish Eusthenopteron as an example.

    Gnathostome diversity:

    Traditionally, three major groups of unknown relationships were recognized:

    Heterosteus ingens by Dmitri Bogdanov from Palaeos
    Placodermi: Silurian to Devonian armored gnathostomes experienced a rapid worldwide diversification and sudden decline.

    Placoderms were very diverse and occupied a wide range of ecological roles. Their specializations included:

    Janvier, Phillipe. 1993. Early Vertebrates
    Fossil record: Fragmentary records of placoderms appear in the Middle Silurian. this is followed by a rapid diversification. During the Devonian, placoderms were the dominant vertebrate group. Both marine and fresh water forms are recorded with a worldwide distribution except for puzzling absence in South American sediments. Placoderm diversity was greatly reduced by an extinction event in the Late Devonian. They were completely extinguished by the mass extinction event at the end of the Devonian. Thus, entire radiation took up only about 50 million years, but while it lasted, it was spectacular.

    Placoderm surprise: Recently, Brazeau, 2009 and Zhu et al., 2013. have analyzed basal gnathostome phylogeny and found that placoderms are paraphyletic, with some groups, including the arthrodires being more closely related to living gnathostomes and others, including the antiarchs, being more basal.

    Eugnathostomata: The gnathostome crown-group. The last common ancestor of Chondrichthyes and osteichthyes and all of its descendants.

    Synapomorphies of Eugnathostomata include:

    Ctenacanthus from Julius T. Csotonyi web site
    Chondrichthyes - Cartilaginous fish (Silurian - Rec.) (sharks, chimaeras)

    All organisms more closely related to modern sharks and chimaeras than to any other gnathostomes.

    Synapomorphy: Prismatic calcification of the cartilage. Chondrichthyans do not lack other hard tissues. Various groups make teeth, fin spines, and dermal armor out of dentine and enamel.

    Osteichthyes: bony fish. (Sil - Rec) Not the first we review with bones, but it is here that we first see extensive, properly endochondral skeletons.


  • Major groups:

    Guiyu (Right. Silurian! Zhu, et al, 2009) is our closest well-known approximation of the last common ancestor of Osteichthyes. Remarkably, it is actually a basal sarcopterygian but also the oldest well-known gnathostome!

    This creature has features seen in both primitive sarcopterygians and actinopterygians, but retains others typical of primitive stem osteichthyans and stem gnathostomes, such as the stout pectoral spine.

    Cheirolepis canadensis from Latvijas Daba
    Actinopterygii: (Dev - Rec)

    The vast majority of "fishlike" bony fish.

    Osteolepis macrolepidota from Latvijas Daba
    Sarcopterygii: The lobe-finned fish and tetrapods.

    Aquatic sarcopterygian diversity:

    Of course there was a time when all sarcopterygians were aquatic. During the late Devonian, a few, in a series of unlikely steps, began emerging from the water.

    Extant tetropod phylogeny

    Tetrapod in brief. A survey of Tetrapoda, including all land vertebrates and some secondarily aquatic ones, exceeds our scope. Don't despair. There are many sources of information. Major living tetrapod groups include:

    Additional Reading:

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