Ontogenetic development of an exceptionally preserved Devonian cartilaginous skeleton

Abstract

Cartilaginous vertebrate skeletons leave few records as fossils, unless mineralized. Here, we report outstanding preservation of early stages of cartilage differentiation, present in the Devonian vertebrate Palaeospondylus gunni. In large specimens of Palaeospondylus, enlarged, hypertrophic cell spaces (lacunae) are dominant in the cartilage matrix, each defined by thin mineralized matrix, where phosphorus and calcium co-occur. This is comparable to living endochondral cartilage, where cell hypertrophy and matrix mineralization mark the end of an ontogenetic process of cell growth and division before bone formation. New information from small individuals of Palaeospondylus demonstrates that the skeleton comprises mostly unmineralized organic matrix with fewer hypertrophic cell spaces, these occurring only in the central regions of each element. Only here has the surrounding matrix begun to mineralize, differing from the larger specimens in that phosphorus is dominant with little associated calcium at these earlier stages. This reflects cellular control of mineralization in living tissues through phosphate accumulation around hypertrophic cells, with later increase in calcium in the cartilaginous matrix. These features are always associated with endochondral bone development, but in the Palaeospondylus skeleton, this bone never develops. This skeletal state is thus far unique among vertebrates, with two alternative explanations: either later stages of endochondral bone development have been lost in Palaeospondylus, or, in a stepwise acquisition of the mineralized skeleton, these late stages have not yet evolved.

Palaeospondylus as a primitive hagfish

“…These features, however, may have evolved from cyclostome-like conditions. The calcified cartilage of Palaeospondylus lacked perichondral ossification [ 48 ], which is specific to the gnathostomes [ 38 ], and it is conceivable that the calcified cartilage of Palaeospondylus evolved either through loss of perichondral ossification in gnathostomes, or through acquisition of biomineralization in cyclostomes. The latter possibility appears more plausible, as the hypertrophied cell lacunae in the cartilages of Palaeospondylus [ 11 , 48 ] are reminiscent of cyclostome cartilages in their thin layers of extracellular matrix [ 21 , 49 ].…”

“…The calcified cartilage of Palaeospondylus lacked perichondral ossification [ 48 ], which is specific to the gnathostomes [ 38 ], and it is conceivable that the calcified cartilage of Palaeospondylus evolved either through loss of perichondral ossification in gnathostomes, or through acquisition of biomineralization in cyclostomes. The latter possibility appears more plausible, as the hypertrophied cell lacunae in the cartilages of Palaeospondylus [ 11 , 48 ] are reminiscent of cyclostome cartilages in their thin layers of extracellular matrix [ 21 , 49 ]. A recent study also suggested that the vertebral element is synapomorphic to the vertebrates [ 21 , 50 ], and the cylindrical vertebral column in Palaeospondylus (Additional file 1 : Figure S1) may have evolved secondarily from arcualia in basal cyclostomes, through the invasion of cartilaginous cells into the fibrous sheath within the elastica externa [ 51 ].…”

Questioning hagfish affinities of the enigmatic Devonian vertebratePalaeospondylus

“…Also problematic with regards to a chondrichthyan association is the composition of the Palaeospondylus cartilaginous skeleton that includes hypertrophied chondrocyte lacunae surrounded by mineralized matrix, previously interpreted as representing an early stage in endochondral bone development [ 10 , 47 ], a type of bone found in bony fishes (Osteichthyes) [ 48 ]. The presence of hypertrophied cells in chondrichthyan cartilage is disputed [ 49 , 50 ], with mineralization characteristically only in the perichondral tesserae [ 51 ].…”

“…The fishes occur throughout several beds, with Palaeospondylus co-occurring with the lungfish Dipterus, the acanthodian Mesacanthus, and the placoderm Pterichthyodes [5]. Previously, Palaeospondylus has been assigned to almost every major jawless and jawed vertebrate group and identified as both larval and adult [6][7][8][9][10][11]. Most recently, Hirasawa et al [12] described similarities between Palaeospondylus and larvae of the extant hagfish Eptatretus burgeri, suggesting a hagfish affinity, and more particularly as a stem hagfish.…”

Divergent Expression of SPARC, SPARC-L, and SCPP Genes During Jawed Vertebrate Cartilage Mineralization

“…Also known to mineralize their matrix are the chondrocytes, not only at the ossification front of endochondral bone growth (in the case of hyaline cartilage), but also in stable forms of mineralized cartilage such as fibrocartilages and other forms of cartilage displaying striking similarities to bony tissues ( Beresford, 1981 ; Dyment et al, 2015 ; Paul et al, 2016 ; Pears et al, 2020 ; Berio et al, 2021 ). Even though both perichondral bones and cartilaginous tissues displayed mineralization in the earliest forms of mineralized internal skeletons ( Ørvig, 1951 ; Min and Janvier, 1998 ; Donoghue et al, 2006 ; Johanson et al, 2010 , 2012 ; Pears et al, 2020 ), mineralizing cartilages have been understudied from a genetic and evolutionary perspective in extant vertebrates. A better understanding of the genetic underpinning of the mineralizing chondrocytes is therefore necessary to understand the early steps of the evolution of endoskeletal mineralization in vertebrates.…”