No bones about it: an enigmatic Devonian fossil reveals a new skeletal framework--a potential role of loss of gene regulation.
Palaeospondylus gunni (Devonian, Scotland) is an enigmatic vertebrate, assigned to various jawless and jawed groups since its original description. New sections through the whole body allow description of a novel skeletal tissue for Palaeospondylus, comprising the entire skeleton. This tissue is mineralized cartilage and is characterized by large cell spaces embedded in minimal matrix. Bone is completely absent. Calcium phosphate mineralization has a differential topography of deposition within the cartilage that reflects a biogenic origin, despite subsequent diagenetic modification. This combination of hypertrophied cell spaces surrounded by regionalized mineralized matrix differs from all other cartilage in fossil and extant vertebrates. However, it compares most closely to gnathostome endochondral bone in early developmental stages. For example, Palaeospondylus skeletal histology differs from the Devonian agnathan Euphanerops and extant lamprey cartilage. Comparison with mineralized cartilage of armored fossil agnathans and placoderms shows the histology is not comparable to globular calcified cartilage. It also differs from that in extant chondrichthyan mineralized tesserae, which is restricted to a subperichondral zone. Amongst this diversity of calcified cartilage types we discuss various interpretations, including one that implicates tissue either in developmental stasis, before osteoblasts can deposit bone, or at a phylogenetic stage when this step has not evolved. These very different hypotheses highlight difficulties in interpreting fossil ontogenies when phylogenetic relationships are uncertain. Nevertheless, we propose that the composition of the Palaeospondylus skeleton represents a fossilized ontogenetic stage of endochondral bone, a type of bone characteristic of osteichthyan vertebrates.
Johanson
Zerina
Z
Department of Palaeontology, Natural History Museum, Cromwell Road, London SW7 5BD, UK. z.johanson@nhm.ac.uk
Kearsley
Anton
A
den Blaauwen
Jan
J
Newman
Michael
M
Smith
Moya Meredith
MM
D000818:Animals; D005075:Biological Evolution; D001842:Bone and Bones / Q000033:anatomy & histology / Q000737:chemistry / Q000502:physiology; D002113:Calcification, Physiologic; D005580:Fossils; D005786:Gene Expression Regulation; D010802:Phylogeny; D012863:Skeleton; D014714:Vertebrates / Q000033:anatomy & histology / Q000145:classification / Q000502:physiology