Trilobite size‐frequency distributions, recognition of instars, and phyletic size changes

Size-frequency analysis of over 5,000 Ordovician trilobites from the Teretiusculus Shales of the Builth inlier, central Wales, has revealed size distributions with counter intuitive shapes. Not only do most species show normal or slightly skewed distributions, despite the preponderance of moults, but there is no evidence of instar peaks. Such features can, however, be explained by reference to steady-state population structures of Recent marine arthropods, in which small individuals often form only a minor proportion of the post-larval population structure. Trilobite steady-state population structures would have differed in detail from species to species, but certain distribution shapes may have been characteristic of particular environments. These findings necessitate a reappraisal of previous work on trilobite size-frequency distributions, survivorship and recognition of instars. The Builth data also show the first clear evidence of phyletic size increase and parallel size changes in trilobites.