Ancestral state reconstruction of ontogeny supports a bilaterian affinity for Dickinsonia

Abstract

Despite numerous attempts, classification of the Precambrian fossil Dickinsonia has eluded scientific consensus. This is largely because Dickinsonia and its relatives are structurally simple, lacking morphological synapomorphies to clarify their relationship to modern taxa. However, there is increasing precedence for using ontogeny to constrain enigmatic fossils, and growth of the type species Dickinsonia costata is well understood. This study formalizes the connection between ontogeny in Dickinsonia-which grows by the addition of metameric units onto one end of its primary axis-with terminal addition, defined as growth and patterning from a posterior, subtermial growth zone. We employ ancestral state reconstruction and stochastic character mapping to conclude that terminal addition is a synapomorphy of bilaterian animals. Thus, terminal addition allies Dickinsonia with the bilaterians, providing evidence that large stem- or crown-group bilaterians made up a significant proportion of the Precambrian biota. This study also illustrates the potential for combining developmental and phylogenetic data in constraining the placement of ancient problematic fossil taxa on the evolutionary tree.

Ediacaran developmental biology

“…Gold et al . (2015) follow Jacobs et al . (2005) in their definition of ‘terminal addition’, but figure a truly terminal generative zone (fig.…”

“…Morphogenesis has been considered most commonly in Dickinsonia costata (e.g. Runnegar, 1982), a taxon that has been discussed in debates surrounding the evolution of bilaterality (Malakhov, 2004; Gold et al ., 2015). …”

“…It exhibits an approximately oval outline, with distally expanding units emanating from a visible central midline. Units are continuous across the midline (Runnegar, 1982; Gold et al ., 2015), imparting a bilateral symmetry. D. costata in Australia range from ∼6–250 mm in length (Reid et al ., 2017), with size variants commonly considered to represent different ontogenetic stages (e.g.Evans et al ., 2017 ; Hoekzema et al ., 2017).…”

“…The largest units are located close to the middle of the organism, not at either pole (Sperling & Vinther, 2010; Hoekzema et al ., 2017). The position of the smallest units has often been used to infer the position of a growth zone (Runnegar, 1982; Ivantsov, 2007; Evans et al ., 2017), which has been described as being in a ‘posterior’ position (Ivantsov, 2007) with units added terminally (Gold et al ., 2015; Evans et al ., 2017). Gold et al .…”

“…The little work that has been done, based on the premise that ontogenetic characters are considered to have been conserved across evolutionary time, demonstrates the potential power of morphogenesis in testing established hypotheses of affinity (e.g. Antcliffe & Brasier, 2007: Gold et al ., 2015). Investigation of morphogenesis in Ediacaran taxa also has the potential to constrain hypotheses of developmental evolution associated with the evolutionary emergence of animals, and to test models of trait evolution that are currently based only on theoretical predictions.…”