Animal Evolution: The Hard Problem of Cartilage Origins Thibaut Brunet, Detlev Arendt  Current Biology  Volume 26, Issue 14, Pages R685-R688 (July 2016) DOI: 10.1016/j.cub.2016.05.062 Copyright © 2016 Elsevier Ltd Terms and Conditions

Figure 1 The scattered occurrence of cartilage in bilaterians. (A) Patten’s hypothesis on the origin of vertebrates from horseshoe crabs. A is Limulus and D is the fossil ostracoderm Cephalaspis. B and C are hypothetical intermediates. From [2]. (B) Phylogenetic distribution of cartilage in bilaterians. Limulus and Sepia from [1]. Boxes indicate evolutionary innovations: (1) Collagen-secreting soxD+ soxE+ cells; (2) true cartilage. Asterisks indicate phyla where only a subset of species have cartilage. Adapted with permission from Macmillan Publishers Ltd: Nature [1], copyright 2016. (C) Assembly of glycosaminoglycans (red), proteoglycans (green) and hyaluronan (blue) in proteoglycan complexes characteristic of cartilage. Current Biology 2016 26, R685-R688DOI: (10.1016/j.cub.2016.05.062) Copyright © 2016 Elsevier Ltd Terms and Conditions

Figure 2 Parallel evolution of cartilage in bilaterian animals from an ancestral collagenous tissue. The ancestral soxD+ soxE+ colA+ ventral mesentery is assumed to have given rise to both the chordate sclerotome and the chelicerate endosternite. A non-cartilaginous ‘Dohrn’s septum’ is still found in today’s pycnogonids, the sister group of all other chelicerates. Unlike real cartilage, a ventral mesentery was likely present in the last common protostome/deuterostome ancestors (as it is found in today’s deuterostomes, chaetognaths, and a number of spiralian and ecdysozoan phyla). The origin of the cuttlefish funnel cartilage is unknown and might have involved parallel evolution or convergence. Deuterostome drawings modified from [18], horseshoe crab and stem-chelicerate redrawn from [19], and scorpion redrawn from [20]. Vnc: ventral nerve cord. Current Biology 2016 26, R685-R688DOI: (10.1016/j.cub.2016.05.062) Copyright © 2016 Elsevier Ltd Terms and Conditions