A mixed-age classed 'pelycosaur' aggregation from South Africa: earliest evidence of parental care in amniotes?

Abstract

Living species of mammals, crocodiles and most species of birds exhibit parental care, but evidence of this behaviour is extremely rare in the fossil record. Here, we present a new specimen of varanopid 'pelycosaur' from the Middle Permian of South Africa. The specimen is an aggregation, consisting of five articulated individuals preserved in undisturbed, close, lifelike, dorsal-up, subparallel positions, indicating burial in 'life position'. Two size classes are represented. One is 50% larger than the others, is well ossified, has fused neurocentral sutures and is distinguished by a coat of dermal ossifications that covers the neck and shoulder regions. We regard this individual to be an adult. The remaining four skeletons are considered to be juveniles as they are approximately the same size, are poorly ossified, have open neurocentral sutures and lack dermal ossifications. Aggregates of juvenile amniotes are usually siblings. Extant analogues of adult and juvenile groupings suggest that the adult is one of the parents, leading us to regard the aggregation as a family group. The Late Middle Permian age of the varanopid family predates the previously known oldest fossil evidence of parental care in terrestrial vertebrates by 140 Myr.

Figure 1

The varanopid specimen SAM-PK-K8305. Numerals indicate individuals: leader lines point to the skulls of individuals 1–4, whereas that for individual 5 points to its hip region because the anterior half of its skeleton, including the skull, is missing. as, astragulus; c, cleithrum; ca, calcaneum; cl, clavicle; cv, caudal vertebra; f, frontal; fe, femur; fi, fibula; h, humerus; il, ilium; j, jugal; m, maxilla; ma, manus; n, nasal; o, osteoderm; op, opisthotic; p, parietal; pf, postfrontal; pm, premaxilla; po, postorbital; pp, postparietal; prf, prefrontal; pu, pubis; r, radius; s, stapes; scc, scapulocoracoid; sco, scleral ossicles; so, supraoccipital; sq, squamosal; st, supratemporal; ti, tibia; u, ulna.

Figure 2

Close up of the skulls of varanopid individuals 1–4. (a) Individuals 1 and 2, (b) individual 3 and (c) individual 4. Varanopid synapomorphies include a posterodorsally expanded external naris, a slender subtemporal bar and narrow quadratojugal, reduced occipital shelf of the squamosal, anteriorly extending parietal over the orbital region and a reduced tabular bone.