Fossil Jawless Vertebrates

John Merck
Link to cladogram and phylogram cheat-sheets

Chengjiang and the Burgess Shale provide an early glimpse of the first craniate chordates - stem vertebrates.

Haikouichthys ercaicunensis from Feinberg and Mallatt, 2013
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.

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.

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 vertebrate evolutionary pattern:

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 is illuminated by Sansom et al., 2010 and Miyashita et al., 2019. So, we start with a review of bony evolution as revealed in these studies.

Note: The taxa depicted fall into three broad categories:



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 vertebrates 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

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:

Total Group Cyclostomi

Total Group Gnathostomata

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

Major Outstanding Issues:

Additional Reading: