Joseph:

This link could be of some help:

Best

Syed

Thank you. It is inner ear structures and their precursors that I am interested in.

Hi Joseph,

A perichondrally ossified braincase first appears in galeaspids and osteostracans, which are 'ostracoderms' (jawless stem gnathostomes), and in both groups it incorporates the otic capsule.

All the best,  Per

Thank you. I appears, then, that an ossified otic capsule is coeval with a bony head shield, or a "skull," broadly defined. Are there any publications that illustrate these ancient otic capsules, or the histology of the otic capsule itself or the head shield in general? I'm curious about the morphology of the ostocytes. Do they have the rounded shape that in mammals is characteristic of bone derived from neural crest?

Not exactly: some of the ostracoderm groups that have dermal bony headshields, heterostracans for example, have an unossified endoskeleton. This is probably the primitive condition.

Unfortunately these early otic capsules are only perichondrally ossified, so there is a thin film of bone lining the labyrinth cavity but the capsules were otherwise cartilaginous. I am not aware of any publications describing their osteocyte lacunae.

An at least partly ossified otic capsule is coeval with endochondral/perichondral ossification, then? Also, apart from the otic capsule, what other peri/endochondral ossification is seen in galeaspids and osteostracans? 

The inner ear plays an interesting role in regulating remodeling and mineral balance in endochondral bone that is independent of mechanical loading, and may explain why spaceflight causes more bone loss than bed rest, which is used experimentally to simulate the effects of microgravity on the skeleton. There also are intriguing differences in the behavior of bone derived from neural crest and bone of mesodremal origin, with the former being immune to osteoporosis and not requiring mechanical loading to maintain mass and density. This all seems to me to be one of those rare areas where a better understanding of the early evolution of these systems could be of practical medical value. It's also personally interesting because I long ago followed a research thread that led me far away from my intended career in vertebrate paleontology, and is now leading me back.

Didn't Stensio do serial sections on an osteostracan, in addition to those he did on Eusthenopteron? Perhaps those would show bone microstructure.

Peri-, but NOT endochondral. Endochondral ossification comes much later, it is unique to osteichthyans (bony fishes + tetrapods). In osteostracans and galeaspids, the otic capsule is part of a big co-ossified braincase that includes the entire roof of the pharynx (complete with impressions of gill pouches) as well as an 'occipital region' that is certainly post-otic but looks suspiciously unsegmented to me. In osteostracans the pericard is co-ossified with the pharynx. There are also scapulocoracoids which - strange to relate - are attached to the occipital region of the braincase by way of cartilage bridges, all encompassed by the same continuous perichondral ossification. Galeaspids have no scapulocoracoids and their pericard is unknown.

I would love to know more about the differences between crest-derived and mesoderm-derived bone: as you may know from my past work I'm interested in such questions (see Matsuoka et al. 2005) and with synchrotron microtomography we are now able to retrieve the exact shapes of osteocyte lacunae in fossil bones (see Sanchez et al. 2012, 2013, 2014).

Stensiö did make grinding series of several osteostracans, but the resolution is quite low and you can't see the osteocyte lacunae.

See: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0008358

Yes, I am aware of your work, and see the connection. I'd love to discuss this further.

There are tantalizing scattered observations that scream for a deep evolutionary treatment. It appears that the inner ear, acting through the sympathetic nervous system, plays a role in maintaining bone mineral balance that is partly or wholly independent of the osteocyte mechanosensory system. Bone of the otic capsule also may have an effect, as an endocrine organ. Interestingly, extracellualr fluid in otic capsule lacunae and canaliculi is continuous with CSF. Microgravity stimulated osteoclast activity in a teleost flown on the ISS (http://www.nature.com/articles/srep14172)-- the same thing that happens to humans, where it is attributed entirely to lack of mechanical loading.

One interesting question is whether the prevailing assumption that there is no way of regulating skeletal mass and mineral content as a whole-- that the body does not "know" how big its skeleton is-- may be wrong. Could a (possibly ancestral) vestibular/otic capsule-controlled mechanism of maintaining bone mass act, in part, to keep and regulate inventory of skeletal Ca and phosphate?

"In osteostracans the pericard is co-ossified with the pharynx."

That's fascinating!