Early animal evolution: a morphologist's view

TY - JOUR

T1 - Early animal evolution

T2 - a morphologist's view

AU - Nielsen, Claus

PY - 2019/7/1

Y1 - 2019/7/1

N2 - Two hypotheses for the early radiation of the metazoans are vividly discussed in recent phylogenomic studies, the 'Poriferafirst' hypothesis, which places the poriferans as the sister group of all other metazoans, and the 'Ctenophora-first' hypothesis, which places the ctenophores as the sister group to all other metazoans. It has been suggested that an analysis of morphological characters (including specific molecules) could throw additional light on the controversy, and this is the aim of this paper. Both hypotheses imply independent evolution of nervous systems in Planulozoa and Ctenophora. The Poriferafirst hypothesis implies no homoplasies or losses of major characters. The Ctenophora-first hypothesis shows no important synapomorphies of Porifera, Planulozoa and Placozoa. It implies either independent evolution, in Planulozoa and Ctenophora, of a new digestive system with a gut with extracellular digestion, which enables feeding on larger organisms, or the subsequent loss of this new gut in the Poriferans (and the re-evolution of the collar complex). The major losses implied in the Ctenophora-first theory show absolutely no adaptational advantages. Thus, morphology gives very strong support for the Porifera-first hypothesis.

AB - Two hypotheses for the early radiation of the metazoans are vividly discussed in recent phylogenomic studies, the 'Poriferafirst' hypothesis, which places the poriferans as the sister group of all other metazoans, and the 'Ctenophora-first' hypothesis, which places the ctenophores as the sister group to all other metazoans. It has been suggested that an analysis of morphological characters (including specific molecules) could throw additional light on the controversy, and this is the aim of this paper. Both hypotheses imply independent evolution of nervous systems in Planulozoa and Ctenophora. The Poriferafirst hypothesis implies no homoplasies or losses of major characters. The Ctenophora-first hypothesis shows no important synapomorphies of Porifera, Planulozoa and Placozoa. It implies either independent evolution, in Planulozoa and Ctenophora, of a new digestive system with a gut with extracellular digestion, which enables feeding on larger organisms, or the subsequent loss of this new gut in the Poriferans (and the re-evolution of the collar complex). The major losses implied in the Ctenophora-first theory show absolutely no adaptational advantages. Thus, morphology gives very strong support for the Porifera-first hypothesis.

U2 - 10.1098/rsos.190638

DO - 10.1098/rsos.190638

M3 - Journal article

C2 - 31417759

SN - 2054-5703

VL - 6

SP - 1

EP - 10

JO - Royal Society Open Science

JF - Royal Society Open Science

IS - 7

M1 - 190638

ER -