The evolution of jawed vertebrates
Among living vertebrates, we have only vertebrates with jaws and lampreys. During the Early Paleozoic, however, a wide range of jawless vertebrates occupied numerous ecological niches. Traditionaly, these were termed Ostracoderms. Although you will find this term in older literature, it is sadly paraphyletic.
But first, consider an attribute of almost all vertebrates:
Vertebrate hard tissues:
Fossil veretbrates are mostly known from hard tissues - bone, dentine, and enamel. Bone is composed of:
In contrast, dentine (aka "acellular bone") lacks the living component, and enamel consists entirely of interlocking crystals of the mineral component.
- A mineral component - made of calcium phosphate (i.e. the mineral hyrdoxylapatite).
- A protein component - made mostly of the fibrous protein collagen
- A living component - various living cells that reside within and remodel the extracellular matrix of the bone.
Bone is secreted by living cells that inhabit some preexisting connective tissue - either:
- a membrane
- a piece of cartilage.
As a result, two corresponding types of bone are known:
These distinctions are significant because dermal and endochondral bone have distinct, separate evolutionary origins.
- Dermal bone - formed by the ossification of a membrane. Dermal bones tend to be flat and plate-like. E.g. the bones of the top of the human braincase.
- Endochondral bone - preformed in cartilage which then becomes ossified. Endochondral bones are usually three dimensional, such as the long bones of your limbs.
Our earliest suggestion of vertebrate bone is Anatolepis from the Late Cambrian - isolated plates of scale-like dermal bone.
By the Silurian we see a great diversity of jawless fossil vertebrates, lacking paired fins and endochondral bone, including:
Pteraspidomorphi (Cambrian (assuming Anatolepis) - Devonian). Includes Sacabambaspis (right), the first well-preserved vertebrate fossil, and a significant diversity of others.
- in the form of acellular dermal armor. We see two armor shields, one dorsal and one ventral. Each gill opening is protected by an individual bone plate. All of this bone is dermal and forms external armor over the creature. It has no internal bone.
- The mouth is opened and closed by row of narrow parallel plates in the lower "lip."
- Sacabambaspis. had no fins except for a symmetrical tail or caudal fin.
- A head with small eyes and otic capsules above the mouth.
- Impressions of otic capsules on the underside of the dorsal shield indicate the presence of two sets of semicircular canals.
- Notochord is either straight or invades lower lobe of tail.
- Of note to paleofanatics, Pteraspidomorphi included Heterostraci, diverse specialized forms in which the pharyngeal (= gill) openings were covered by single plates.
Without a living analog, speculations about the ecology of these creatures is very limited. We not the following:
- If the distribution of pharyngeal slits is any indicator, they had a substantial pharynx.
- They lacked jaws, or even the keratinous structures that hagfish and lampreys use to process food. The implication was that they were somehow filtering particulate matter. Two general speculations in the literature:
The latter seems more likely given that their oral plates never seem to suffer from abrasion or wear.
- Mud sievers: They ingested fine soft sediment, expelled the indigestible material, and digested the rest.
- Suspension feeders: They lived like a larval lamprey, resting on or in the bottom and suspension feeding from the water.
Leads to the general picture of pteraspidomorphs as creatures that rested on the bottom to suspension feed, but would swim haphazardly when disturbed, only to settle to the bottom again. Their probably poor control over swimming orientation and direction makes it unlikely that they could perform ram suspension feeding.
- Pteraspidomorphs lacked the surfaces that modern aquatic vertebrates use to control their direction in the water. They could, at best, propel themselves Branchiostoma-style.
- The extremely convex shape of their ventral shields is reminiscent of brachiopods that raft on soft substrates.
- Experiments with models indicate that when not being actively propelled, the bodies of pteraspidomorphs would settle onto the substrate without diving or rolling.
- in the form of acellular scales. No significant armor
- Body is cylindrical with notochord supporting lower lobe of caudal fin.
- Pharyngeal (gill) openings are small and form a short, posteriorly slanting row.
- While there are no paired fins, there are paired triangular spines in a vaguely "pectoral" position.
Overall, anaspids seem adapted for active swimming. exactly how they ate is mysterious, but they lack the obvious adptations to suspension feeding of pteraspidomorphs. Phillipe Janvier has argued persuasively that anaspids might be the sister taxon to lampreys (Hyperoartia). that would make lampreys secondarily cartilagenous.
Thelodonti (Ordovician - Devonian)
- Entirely consists of small scales that usually disarticulate when the animal dies. These scales are distinctive, consisting of enamel and dentine layers around a pulp cavity, like a vertebrate tooth.
- In some cases, these tooth-like scales line the oral cavity and mouth.
- Again, no paired fins, there are paired triangular "fin flaps" without skeleton or muscle.
- Notochord supports lower lobe of caudal fin.
One interesting group of thelodonts is the Silurian - Devonian Furcacaudiformes. These show the presence of a stomach, along with some very surprising morphologies resembling those of highly maneuverable reef fish, but without paired fins, how did they maneuver. Strains the abilities of the comparative method.
In all of the above taxa, there is no consensus about their interrelationships or closeness to jawed vertebrates. But then we get two groups that share a major synapomorphy with gnathostomes: Endochondral bone
Galeaspida: Restricted to southern China and Indochina, then a separate continent. (Silurian - Devonian)
- large flat head shield enclosing an endochondral bony braincase.
- large opening in upper front of head shield leads to olfactory capsules and pharynx
- mouth is ventral
- Pharynx is large with many gill openings, suggesting suspension feeding.
- No suggestion of paired fins.
- Overall, although the equipment is different, the life-style looks like that of pteraspidomorphs.
Osteostraci (Silurian - Devonian): Resemble galeaspids but with differences:
- Head shield contains large sensory fields.
- Head shield fused with endochondrally ossified braincase.
- Notochord invades upper lobe of tail
- Dorsal fin
- But the big thing: Paired pectoral fins (complete with endochondral skeleton elements and muscle) present.
Gnathostomata: The jawed vertebrates. Quantum leap forward.
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 braincase. With the appearance of the gnathostome jaws and branchial skeleton, the skull becomes a complex composite structure. The following illustrations show its components using the fossil bony fish Eusthenopteron as an example.
- Jaws consisting of upper elements - the palatoquadrates and lower elements - Meckel's cartilages.
- A specialized hyoid arch helps connect the skull to the braincase.
- Branchial skeleton internal to gills.
- Enlarged forebrain that is flexed ventrally with respect to hind brain.
- Pharyngeal calcifications involving endodermal interaction with neural crest ectoderm - i.e. phayrngeal teeth.
- Paired pelvic appendages
- Horizontal (third) semicircular canal
- Start with the neurocranium:
- Add the branchial arches:
- Add the hyoid arch:
- Add the mandibular arch - palatoquadrate and Meckel's cartilage:
- Finally, add the dermal bone of the skull roof:
Origin of jaws: Many gnathostome synapomorphies are continuations of longstanding vertebrate trends toward increased skeletal ossification, brain enlargement, and improvement of swimming. Big exceptions are changes to the pharynx, especially jaws. We have:
The biggest problem is that these complex structures appear "overnight" in the fossil record. Many decades of searching have not revealed a transitional form. Four general hypotheses account for this radical transformation. In all probability they are inductively derived from the interaction of the endoderm of the pharynx and neural crest ectodermal tissue when the downturn and expansion of the forebrain brought the pharyngeal endoderm and neural tube ectoderm into close proximity. This has the advantage of explaining the absence of intermediate forms and the simultaneity of the appearance of jaws and the increase in brain size. Possible cause - yet another hox gene duplication. All gnathostomes have one more cluster than lampreys or hagfish.
- Jaws themselves - palatoquadrate and Meckel's cartilage.
- Internal branchial skeleton.
- Inductive formation of pharyngeal calcifications.