Presence of the Earliest Vertebrate Hard Tissue in Conodonts

Abstract

From histological investigations into the microstructure of conodont elements, a number of tissue types characteristic of the phosphatic skeleton of vertebrates have been identified. These include cellular bone, two forms of hypermineralized enamel homologs, and globular calcified cartilage. The presence of cellular bone in conodont elements provides unequivocal evidence for their vertebrate affinities. Furthermore, the identification of vertebrate hard tissues in the oral elements of conodonts extends the earliest occurrence of vertebrate hard tissues back by around 40 million years, from the Middle Ordovician (475 million years ago) to the Late Cambrian (515 million years ago).

The material properties of acellular bone in a teleost fish

“…The dermal bones of several extinct jawless craniates (notably the Heterostraci, Anaspida and Thelodonti) were of aspidin, a type of acellular bone. However, the exoskeletal head shields of the Osteostraci (jawless vertebrates) were covered with cellular bone (Hanken and Hall, 1993), as were components of the feeding apparatus of Conodonts, the earliest known vertebrates (Sansom et al, 1992), and elasmoid fish scales (Meunier et al, 2003;Meunier et al, 2004). However, teleost fishes are the only vertebrates with a skeleton composed solely of acellular bone (Moss, 1961).…”

Phylogenetic analysis of vertebrate CXC chemokines reveals novel lineage specific groups in teleost fish

“…520 million years ago (Sansom et al, 1992;Kuroda et al, 2003). In mammals, over 50 chemokines have been characterised to date, that interact with approximately 20 G protein-coupled receptors (Allen et al, 2007).…”

The spatial and temporal diversification of Early Palaeozoic vertebrates

“…The vertebrate character of earliest euconodont hard tissues has been confirmed in a number of studies (e.g. Sansom et al 1992;Donoghue 1998). If a phylogenetic link with paraconodonts, initially proposed by Bengtson (1976), is demonstrated, then the origin of biomineralizing vertebrates would be extended back to the Middle Cambrian.…”

“…The presence of preserved soft tissues had already suggested to Aldridge et al (1986) that the affinities of this highly diverse group lay with the vertebrates, and a wealth of new data has now been advanced in support of the hypothesis. These include additional soft tissue finds (Smith et al 1987;Aldridge et al 1993;Gabbott et al 1995), the recognition of vertebrate hard tissue synapomorphies in conodont elements (Sansom et al 1992(Sansom et al , 1994Sansom 1996;Donoghue 1998;Donoghue & Aldridge 2001) and detailed cladistic analysis (Donoghue et al 2000). The inclusion of conodonts as the earliest vertebrates to possess a mineralized skeleton, more derived than either hagfishes or lampreys, increases the generic and specific biodiversity of CambroOrdovician vertebrates by two orders of magnitude.…”