Bone is often thought to be a feature of highly evolved vertebrates with teleosts (modern bony fish) being the first animals with a bony skeleton. Fish bone has been shown to possess both active osteoblasts and osteoclasts (Witten, et al. J Morphol 250:197).
All sharks, rays and jawless fish have a cartilaginous skeleton but have the same circulating levels of calcium as all other vertebrates (2.2-2.6 mmol/L).
Tight calcium regulation delineates vertebrates from invertebrates. Fish and higher vertebrates have both hypocalcemic (calcitonin & stanniocalcin) and hypercalcemic (parathyroid hormone & parathyroid hormone-related protein) factors. Stanniocalcin (STC) was first found in fish and then identified in mammals. Calcitonin (CT), parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP) were all described in mammals and subsequently isolated from fish.
Some of the teleost species have multiple genes for both PTH and PTHrP. There were also two copies of the PTH gene in elephant sharks (Callorhinchus milii) and one of these has not persisted in higher vertebrates indicating that one PTH gene might have accumulated a number of deleterious mutations and has been lost in the process. The recent sequencing of the Japanese lamprey genome (Lethenteron japonicum) (Mehta, et al. Proc Natl Acad Sci U S A 110: 16044, 2013) is instructive. Lampreys have a pivotal position in evolutionary history, having undergone two whole genome duplications when compared to invertebrates. Like sharks they have a cartilaginous skeleton but have the ability to move from seawater to freshwater. The Japanese lamprey genome database has been interrogated for the presence of PTH and PTHrP. Certainly two receptors for PTH and PTHrP (pth1r and pth2r) are present in agnathan genome (Petromyzon marinus) (Pinheiro, et al. BMC Evol Biol 12:110, 2012). Two PTH receptors have also been identified in invertebrates (Ciona intestinalis) but the ligands have not been found (Kamesh, et al. BMC Evol Biol 8: 129, 2008).
Localization and physiological studies in a number of vertebrate species have demonstrated that some, if not all, of calcium regulating factors are important in skeletal formation and maintenance.