Euconodonta and Jawless Vertebrates

John Merck

Hints of unseen diversity - 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 types
Conodonts came in four general shape classes (right) and were always found as disarticulated clasts in marine sediment. This situation led to the very reasonable but unsupported assumption that each type of element represented a different taxon.

At this point, speculation raged about:



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.

Moreover, different elements began to be distinguished by the manner in which they were formed developmentally.

Since these seemed to appear in a stratigraphic (chronological) sequence, speculation was that these growth forms represented evolutionary stages. Not quite. In 1988, single cone elements were eliminated from the roster of euconodonts by the discovery that they were the fossils of early chaetognaths. In living chaetognaths, similar elements are made of calcium carbonate and are used in prey capture.



Clydagnathus
The conodont animal: Briggs et al., 1983, described Clydagnathus, an Early Carboniferous age eel-shaped creature in which he noted:



Euconodonts
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 Absolute China Tours
Myllokunmingia and Haikouichthys: Chengjiang gives us the best picture picture of what the ancestral vertebrate might have looked like with Haikouichthys ercaicunensis (right - possibly junior synonym of Myllokunmingia fengjiaoa) (Shu et al., 1999) which seem to preserve: These have been boldly interpreted as basal vertebrates, in which the paired cartilages are arcualia and the fin texture is from cartilaginous fin-rays; and conservatively viewed as stem craniates, with the paired cartilages being branchial elements and the fin-rays being preservation artifacts.


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. More interesting is the presence of a branchial skeleton in which the skeleton is: In both respects, Metaspriggina resembles jawed vertebrates more than lampreys. Which state is plesiomorphic? Right now, GORK.

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 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. (Synopsis at right.) So, we start with a review of bony evolution.


From disciplineorregret.com

Bones

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:

This material is secreted and maintained by living cells:

Acellular (left) and cellular (right) bone from Dean and Shahar, 2012
Bony tissue can be:

What is its history?

The earliest known phosphatic hard tissues were acellular, and were tooth-like in being made of:

Among living craniates, bone in any form only occurs among members of Vertebrata - craniates with vertebral elements protecting their spinal cords. What does the study of fossil organisms tell us about the distribution of bony tissue?


Anatolepis armor from University of Toronto, BIO356

A rogue's gallery of early Paleozoic vertebrates:

As we do this survey, be aware of these issues:

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 from Christian Science Monitor

Euphanerops (Devonian right) and Jamoytius (Silurian)

The phylogeny of Sansom et al., 2010 was made possible by reexamination of the important early vertebrate Jamoytius, often previously cited as the ancestral vertebrate or close to Hyperoartia. It seems to represent the first appearance of hard tissue elements outside the mouth: W-shaped bony acellular scales - composites of dentin and enamel. Other characteristics:

Branchial openings: In Sansom et al., 2010, Jamoytius is found to be the sister taxon of the similar but soft-bodied Euphanerops (right) , which has a branchial array of roughly thirty openings that extends almost to the anus. Janvier et al., 2006 equate Euphanerops with Endeiolepis, and describe exceptionally preserved specimens of each, concluding that Euphanerops' gills were organized in pouches like those of a lamprey.

Fin folds: hagfish and lampreys have no suggestion of paired appendages. In Euphanerops, however, a pair of long lateral pleats extend down the ventral margins of the animal's trunk. They contain neither cartilage nor muscle, but suggest a starting point for the evolution of paired appendages.


Pharygolepis from BIOL202 - Susquehanna University

Anaspida (Silurian)

Morphology:

Overall, anaspids seem adapted for active swimming. Exactly how they ate is mysterious, but they lack obvious adptations to suspension feeding or to taking large prey.

Synapomorphy of Anaspida and Gnathostomata: Dermal skeleton of head.


Thelodonts from Wikimedia Commons

Thelodonti (Ordovician - Devonian)

Morphology:

Living chondrichthyans preserve a similar pattern of denticles.


Furcacauda from PaleoStories
Furcacaudaformes: (Silurian - Devonian) While most thelodonts were slightly flattened dorso-ventrally, this group is morphologically distinctive in being:

Thelodont monophyly: Is not assured. Recent analyses have (nearly) supported monophyletic Thelodonti:

Nevertheless, there is a cottage industry in identifying potential synapomorphies between sub-taxa within Thelodonti and other vertebrate groups. These will be noted below.

Teeth without jaws? In several thelodonts, internal and external odontodes are clearly preserved inside the mouth and pharynx as well as in the skin. Rücklin et al., 2011 describe the odontodes of Loganellia scotica in detail. At present, these "internal" odontodes are the earliest plausible manifestations of teeth, although debate rages about:

Stay tuned. We will note additional landmarks in the appearance of proper teeth in later lectures.

Synapomorphies of Thelodonti and jawed-vertebrates:



Sacabambaspis janvieri Centre Nationale de la Recherche Scientifique - Evolution

Pteraspidomorphi

(Cambrian (assuming Anatolepis) minimally 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, dentin, and acellular bone. These shields are the first vertebrate elements that are bone-like in outward structure. Their possible functions include:

Note: It was not an internal skeleton.


Astraspis desiderata History of Life, Richard Cowen, University of California, Davis
By the Silurian, many Sacabambaspis - like creatures are known, E.G. Astraspis (right).

Morphology:



Pteraspis stensioei History of Life, Richard Cowen, University of California, Davis
Heteroscraci: The great majority of pteraspidomorph diversity consists of Heterostraci, such as Pteraspis right.

Their synapomorphy:

The largest pteraspidomorphs approached 1.5 m.


Drepanaspis gemuendenensis Palaeos
Without a living analog, speculations about the ecology of these creatures is very limited. We note the following:

Feeding:



Athenaegis chattertoni from PaleoStories
Janvier, 1997 suggests that the tail structure of basal pteraspidomorphs like Athenaegis (right) resembles that of furcacaudate thelodonts sufficiently to be a possible synapomorphy, suggesting that pteraspidomorphs might be derived from within Thelodonti. (According to Pradel et al., 2007, the tail of Sacabambaspis was strongly hypocercal, however.)

Synapomorphy of Pteraspidomorphi and jawed-vertebrates:



Interpretation of thelodont Turinia pagei (Early Devonian)
by Donaghue and Smith, 2001 from Palaeos

Issue: Transformation of feeding mechanics:

Remember the rod-and-pulley oral mechanism found in hagfish and lampreys, and inferred in euconodonts? Although the internal feeding apparatus of Jamoytius, Euphanerops, anaspids, thelodonts, and pteraspidomorphs is poorly know, so far, we have not surveyed any animal that could not possibly have had such a feeding mechanism: We now move on to creatures whose feeding apparatus is definitely different feeding strategies.

Issue: True Bone:

Before proceeding, a note: 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:


Shuyu zhejiangensis Institute of
Vertebrate Paleontology and Paleoanthropology

Galeaspida:

Restricted to southern China and Indochina, then a separate continent. (Silurian - Devonian)

Morphology:

Overall, although the equipment is different, the life-style looks like that of pteraspidomorphs: Bottom dwelling suspension/deposit feeding. But galeaspids are the first vertebrates in which the nasal capsules are separated from the hypophyseal duct. (See Gai et al., 2011 for details.)

Synapomorphy of Galeaspida and jawed-vertebrates:



Ateleaspis tesselata (Late Silurian) from The Fossil Fishes of Great Britain

Osteostraci

(Silurian - Devonian): Endemic to Euramerica. Resemble galeaspids in having large horseshoe-shaped head shields that include an perichondrally ossified brain-case, but with major differences:

Synapomorphies of Osteostraci:

Other characteristics:


Cephalaspis Bionet Skola
Osteostraci are both informative and misleading. Their well-ossified brain-cases enable us to know their anatomy as in no other fossil jawless vertebrate, but aspects of their anatomy are enigmatic:

Synapomorphies of Osteostraci and jawed-vertebrates In any case, Osteostraci shares conspicuous synapomorphies with Gnathostomata:



Pituriaspis doylei and Neeyambaspis enigmaticaPaleoStories

Pituriaspida:

(Devonian) A poorly known group with two known genera: Pituriaspis and Neeyambaspis from Queensland. Known only from their well-integrated, solid head shields. Generally similar to Osteostraci. Characterized by: More and better fossils, please.

Coping with history: You will encounter the term ostracoderms in the literature. This referred to the vast paraphyletic grade-group of armored jawless vertebrates whose diversity we have surveyed.

Major Outstanding Issues:

Additional Reading: