The origin of ascophoran bryozoans was historically contingent but likely

Abstract

The degree to which evolutionary outcomes are historically contingent remains unresolved, with studies at different levels of the biological hierarchy reaching different conclusions. Here we examine historical contingency in the origin of two evolutionary novelties in bryozoans, a phylum of colonial animals whose fossil record is as complete as that of any major group. In cheilostomes, the dominant living bryozoans, key innovations were the costal shield and ascus, which first appeared in the Cretaceous 85 -95 Myr ago. We establish the parallel origin of these structures less than 12 Myr ago in an extant bryozoan genus, Cauloramphus, with transitional stages remarkably similar to those inferred for a Cretaceous clade. By one measure, long lag times in the first origins of costal shield and ascus suggest a high degree of historical contingency. This, however, does not equate with dependence on a narrow set of initial conditions or a low probability of evolution. More than one set of initial conditions may lead to an evolutionary outcome, and alternative sets are not entirely independent. We argue that, although historically contingent, the origin of ascus and costal shield was highly likely with sufficient possibilities afforded by time.

Cribrimorph and OtherCauloramphusSpecies (Bryozoa: Cheilostomata) from the Northwestern Pacific

“…2E herein), from marginal pore chambers. Circum-opesial spines present; orificial spines with a flexible joint at base; opesial spines with or without a flexible joint (Dick et al, 2009). Reproductive zooids have a reduced, vestigial kenozooidal ooecium (e.g., Figs.…”

“…Dick et al (2009) identified this species as C. disjunctus due to the large, paired avicularia and somewhat disjunct zooids with small lacunae evident between them in SEM images; at that time, C. disjunctus was the only known species with similar characters. Cauloramphus cheliferoides and C. disjunctus are very similar in opesia and zooid size, but differ in many other characters (Table 2), including avicularium length/depth ratio; avicularium position; and number of orificial, opesial, and total spines.…”

“…Internally, there is an ascus ( fig. 1h in Dick et al, 2009). Around distal margin of orifice are four or five (97% and 3%, respectively; n = 60, 2) short, thick, erect orificial spines.…”

“…Some, however, have circumpolar Arctic (e.g., C. cymbaeformis) or amphi-Boreal (e.g., C. spinifer) distributions, and a few range into the Subtropics or Tropics, usually in deeper water, e.g., C. brunea Canu and Bassler, 1930, in the Galapagos and C. opertus Canu and Bassler, 1928, in the Gulf of Mexico. Dick et al (2009) illustrated and provided brief morphological descriptions of what they identified as Cauloramphus disjunctus and three other, undescribed Cauloramphus species from the western Aleutian Islands, Alaska; two of these species have a cribrimorph costal shield and provide a remarkable example of the parallel evolution of complex characters. Upon examination of the type series for C. disjunctus, we concluded that none of the Aleutian or the Commander species is C. disjunctus, but that several of them (including the cribrimorph species) having hypertrophied, paired avicularia like those in C. disjunctus comprise a monophyletic group within Cauloramphus, which we term the 'C.…”

“…disjunctus clade. ' Here we formally describe eight new species of Cauloramphus from the shelf zone at depths of 80-415 m in the western Aleutian Islands, Alaska (Dick, 2008), including the species briefly described and illustrated in Dick et al (2009); describe one new species collected subtidally from the Commander Islands to the west; and redescribe C. disjunctus Canu and Bassler, 1929. We also present a hypothesis on the phylogeny of the C. disjunctus clade, based on morphology and previous molecular data.…”