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Comparative Study
. 2011 Apr 22;278(1709):1150-7.
doi: 10.1098/rspb.2010.1641. Epub 2010 Oct 14.

Decay of vertebrate characters in hagfish and lamprey (Cyclostomata) and the implications for the vertebrate fossil record

Robert S Sansom  1 , Sarah E Gabbott, Mark A Purnell
Affiliations
Comparative Study

Decay of vertebrate characters in hagfish and lamprey (Cyclostomata) and the implications for the vertebrate fossil record

Robert S Sansom et al. Proc Biol Sci. .

Abstract

The timing and sequence of events underlying the origin and early evolution of vertebrates remains poorly understood. The palaeontological evidence should shed light on these issues, but difficulties in interpretation of the non-biomineralized fossil record make this problematic. Here we present an experimental analysis of decay of vertebrate characters based on the extant jawless vertebrates (Lampetra and Myxine). This provides a framework for the interpretation of the anatomy of soft-bodied fossil vertebrates and putative cyclostomes, and a context for reading the fossil record of non-biomineralized vertebrate characters. Decay results in transformation and non-random loss of characters. In both lamprey and hagfish, different types of cartilage decay at different rates, resulting in taphonomic bias towards loss of 'soft' cartilages containing vertebrate-specific Col2α1 extracellular matrix proteins; phylogenetically informative soft-tissue characters decay before more plesiomorphic characters. As such, synapomorphic decay bias, previously recognized in early chordates, is more pervasive, and needs to be taken into account when interpreting the anatomy of any non-biomineralized fossil vertebrate, such as Haikouichthys, Mayomyzon and Hardistiella.

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Figures

Figure 1.
Figure 1.
Phylogeny of vertebrates and hierarchical character classifications. Dashed lines represent alternative resolutions (cyclostome monophyly/paraphyly).
Figure 2.
Figure 2.
Anatomy and decay of cyclostomes. (a) Adult lamprey (left) and hagfish (right) anatomy with reconstructions of the morphology at the end of decay stages 1–5. Red represents hard cartilage, blue is soft cartilage, purple is undetermined cartilage type and green is keratinous tissues. (b) Decay sequences for adult lamprey (left) and hagfish (right) through time (days). Observations of the decay of each character (yellow, pristine; orange, decaying; red, onset of loss; terminal point, complete loss) are used to rank them according to last occurrences. For each character, the decay ranks (left) and synapomorphic ranks (right) used to test synapomorphic biases are shown. Asterisk marks skeletal characters with H for hard or S for soft cartilage type where relevant. Profiles of putative fossil vertebrates and cyclostomes are illustrated on the right. Green bars represent described preserved characters in fossil taxon. Thin blue bars represent absent characters which we would expect to find given the decay rank and synapomorphic rank of other characters preserved (light blue for potentially phylogenetically absent synapomorphies, dark blue for absent plesiomorphies).
Figure 3.
Figure 3.
(a) ammocoete decay stages. Stage 1 (day 1); stage 2 (day 8); stage 3 (day 15); stage 4 (days 28 and 35); stage 5 (day 90); stage 6 (day 200). ba, branchial arch; bo, branchial opening; en, endostyle; es, eye spot; li, liver; my, myomeres; ng, notochord groove; no, notochord; oc, otic capsule; tr, trabecular cartilage (skull). (b) Adult lamprey decay stages. Stage 1 (day 2); stage 2 (day 28); stage 3 (day 92); stage 4 (day 207); stage 5 (day 300); stage 6 (day 300). ac, annular cartilage; bo, branchial opening; cc, copular cartilage; cr, cranial cartilage; el, eye lens; gl, gill lamellae; gp, gill pouch; no, notochord; oc, otic capsule; od, oral disc; or, orbit; pc, piston cartilage; sc, stylet cartilage; sl, sensory line; te, teeth; tr, trematic ring; ve, velum. (c) Hagfish decay stages. Stage 1 (day 2); stage 2 (day 6); stage 3 (day 15); stage 4 (day 35); stage 5 (day 63); stage 6 (days 130 and 200). br, brain; dc, dentigerous cartilage; dn, dorsal nerve cord; fr, fin rays; lc, lingual cartilage; li, liver; lm, lingual musculature; mc, median cartilage; my, myomeres; no, notochord; oc, otic capsule; pa, palatine cartilage (skull); sg, slime glands; sn, subnasal cartilage; te, teeth; tn, tentacles. For scale, mesh apertures are 2 mm × 2 mm and white grid is 10 mm × 10 mm.
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