Regular echinoids are common in Flood deposits. Within the secular model, the ‘earliest’ sea urchins are found in the Palaeozoic. Eotiaris guadalupensis, for example, is a Permian echinoid believed to be a stem group cidaroid (Thompson et al. 2015). Some irregular echinoids are also found in Flood deposits (e.g. the Jurassic Plesiechinus (Kier 1982)), though monophyly is often debated (Saucède et al. 2007). Within the Luminacea, the Cassiduloida are found in Cretaceous strata (Souto et al. 2019), which would be Flood deposits. Sand dollars as a morphotype, however, appear to be primarily from post-Flood deposits.
[There have been a couple of records of Scutelliformes being found in Cretaceous strata. Within the Dendrasteridae, Scutellaster cretaceus was described in 1895 from Colorado, but later determined to be a common Pliocene species, Anorthoscutum interlineatum (Durham 1953). The alleged locality of discovery in Colorado Springs yielded no additional specimens when searched later. In another case, the California Academy of Sciences notes a specimen (Scutella sp.) in their collection deposited by Waring from the Chico Formation in California. Waring (1917) described it as "Scutella (?)", "a single fragment," with the plate noting "top of imperfect specimen." This is probably a misinterpreted specimen. Martinez-Melo (2019) examined several cases of alleged sand dollars in Mexican Cretaceous deposits, and determined they were based on erroneous reports.]
The enigmatic flattened Togocyamus appears in West African Paleocene deposits, while a number of ‘true’ sand dollars appear worldwide (‘rapid evolution’ per Kier 1982) during the Eocene. These appearances were not necessarily related; independent convergent adaptations, from different baraminic lineages or within the same lineage, are plausible in a creationist perspective where organisms are engineered for adaptation and morphological change. This may suggest that the break-up of the continents during the Flood provided more niche opportunities for irregular urchins to adapt to a sand dollar morphology. Some of the morphological changes may be due to paedomorphosis (Mooi 1990), accompanying a change to a burrowing lifestyle (Kier 1982).
There have been numerous secular attempts to determine an origin for the sand dollars and kin (Smith 2001; Kroh and Smith 2010). One recent phylogenomic study suggested that sand dollars and sea biscuits developed in the Cretaceous (despite no fossil evidence there), while crown-group echinoids “originated in the Permian and diversified rapidly in the Triassic, despite the relative lack of fossil evidence for this early diversification” (Koch et al. 2022). That seems an unlikely pathway for a creationist model; again, a post-Flood derivation for sand dollars from one or more related ovate irregular echinoid species remains the best scenario given the fossil evidence.
