Jamieson, B.G.M. 1995e. Evolution of tetrapod spermatozoa with particular reference to amniotes. In Advances in Spermatozoal Phylogeny and Taxonomy. Jamieson, B.G.M., Ausio, J. and Justine, J.-L. eds. pp. 343-358. Mémoires du Muséum national d'Histoire Naturelle, vol. 166, Paris.
Synapomorphies of tetrapod sperm appear to be: nuclear 'shoulders'; elongation, relative to dipnoans, of two longitudinal elements (dense fibres) peripheral to the axoneme adjacent to doublets 3 and 8; and, questionably, development of an annulus. A lissamphibian synapomorphy relative to Neoceratodus may have been loss of one undulating membrane, leaving a single undulating membrane adjacent to the fibre of doublet 3. Amniote synapomorphies (retained in Chelonia and Sphenodontida) include: elongation of the distal centriole through the entire length of the moderately elongate midpiece; subspheroidal mitochondria, with concentric cristae; a fibrous sheath; nine peripheral axonemal fibres; inward projections (longitudinal columns) of the fibrous sheath aligned with fibres 3 and 8; loss or transformation of the retronuclear body, present in dipnoans and (as the neck structure) urodeles. A possible crocodilian synapomorphy is a thick dense sheath around the singlets of the axoneme or the distal centriole. Synapomorphies of birds are loss of the subacrosomal cone and, less certainly derived, adhesion of all nine dense fibres to their axonemal doublets (also in monotremes). The conical acrosome, fibrous sheath, and elongate centriole of ratites are symplesiomorphies not proving monophyly. Restriction of the endonuclear canal to the anterior region of the nucleus in other non-passerines and passerines may be a synapomorphy of these, homoplasic with crocodiles and derived ratites (emu). Squamate synapomorphies are: loss of endonuclear canals with restriction of the perforatorial rod to a prenuclear location; intermitochondrial bodies; forward extension of the fibrous sheath into the midpiece; a paracrystalline subacrosomal cone; and, homoplasically, shortening of the centriole. Mammal sperm are distinguished by loss of the perforatorium (and canal), homoplasic with some non-ratite birds, great reduction of the centrioles, and, in therians, (apomorphic?) detachment of peripheral fibres, except sometimes 3 and 8, from the doublets.