Many evolutionary trends are described in the post-Paleozoic echinoids and their functional advantages are discussed. In the ambulacra, the compound plate first appeared in the Late Triassic, becoming more pronounced during the Mesozoic, and reaching its zenith in the Cenozoic. Compounding enabled the echinoid to have more numerous tubefeet, strengthened the test, and increased the size of the ambulacral tubercles and spines. These larger spines provided greater protection from predators and faster locomotion. Petals first appeared in the Middle Jurassic and were developed for more efficient respiration. The first depressed petals occurred in the Late Jurassic, and by Late Cretaceous many echinoids had depressed petals culminating in deep petals in the Cenozoic. These depressions channeled water over the respiratory tubefeet, increased the width of the ambulacra and their tubefeet, and enabled these tubefeet to be protected from predators by the arching of spines over them. An anterior groove is slightly developed by the Middle Jurassic, distinct in the Cretaceous, and deepest in the Cenozoic. This groove provided a passage for food, and shelter for the large penicillate tubefeet. Phyllodes first occur in the Lower Jurassic in both the regular and irregular echinoids. During the Mesozoic the number of pores in the phyllodes in the irregular echinoids was reduced, and in most species one pore was eliminated of a porepair. The phyllodes provided a large number of feeding tubefeet near the peristome. In the apical system of the irregular echinoids, the periproct broke out during the Lower Jurassic. Its movement posteriorly served to separate the echinoid's excrement from its feeding and respiratory areas. The number of genital plates was reduced to a single plate in the cassiduloids by the Late Cretaceous, but this reduction occurred later in the holasteroids and spatangoids; many species living today have more than one genital plate. The Triassic and Early Jurassic echinoids were small; but during the latter part of the Jurassic, larger species occur, particularly among the irregulars and echinothurioids. All the Triassic echinoids except one were circular in marginal outline, but during the Jurassic the test in many irregulars became elongate enabling the echinoid to develop unidirectional movement. The flattening of the test permitted the echinoid to cover its test more easily, making the animal less conspicuous, less affected by wave motion, and placing more of the food-gathering tubefeet in contact with the seafloor. The Triassic lantern had grooved teeth and a shallow foramen, but by the Lower Jurassic some lanterns had a deeper foramen magnum. By the Middle Jurassic keeled teeth are present, and by the Late Cretaceous some lanterns have joined epiphyses. These changes permitted the lantern to be more mobile and strengthened the teeth and epiphyses. The lantern supports in all Triassic echinoids are outgrowths of interambulacral plates, but in the Lower Jurassic many species have ambulacral supports. By the Middle Jurassic these supports are joined together in some species to form an arch. These changes also increased the mobility and power of movement of the lantern. Gill notches first appeared in the Lower Jurassic (Hettangian) and were well developed by the Toarcian. The tubercles and their spines were large in the Triassic and gradually decreased in size in some species through the Mesozoic. This reduction enabled these echinoids with smaller spines to cover their tests with sediment. The rate of introduction of new plates was low in the Triassic, increasing during the Jurassic. This increase was mainly in the ambulacra and served to increase the number of tubefeet. Among the holasteroids-spatangoids some of the ventral interambulacral plates increased in size relative to adjacent plates during the Mesozoic and Cenozoic forming the labrum and plastron. These changes permitted the development of the “heart-shaped” test, and an anterior shift of the peristome. Diversity of echinoids increased since the Triassic with the development of different kinds of echinoids able to inhabit many varied habitats. All Triassic echinoids lived on top of the substrate, but in the Jurassic irregular echinoids began to burrow in the sediment. They increased in number of species during the Mesozoic and now are more numerous in species than the regular echinoids.
The difference between Jurassic and Triassic species is not as abrupt as formerly thought, and all Jurassic echinoids are considered to have had a cidaroid ancestor.
