An eutactic star, in a n-dimensional space, is a set of N vectors which can be viewed as the projection of N orthogonal vectors in a N-dimensional space. By adequately associating a star of vectors to a particular sea urchin we propose that a measure of the eutacticity of the star constitutes a measure of the regularity of the sea urchin. Then we study changes of regularity (eutacticity) in a macroevolutive and taxonomic level of sea urchins belonging to the Echinoidea Class. An analysis considering changes through geological time suggests a high degree of regularity in the shape of these organisms through their evolution. Rare deviations from regularity measured in Holasteroida order are discussed.
This work is dealing with regularity, which is a property with deep implications in organisms. From the biological point of view regularity has been related with radial symmetry, and irregularity with bilateral symmetry (Coen, 1999). The heuristic value of radial and bilateral symmetry in biology account for taxonomic issues, however, symmetry as well as disruption symmetry have been an empirical and intuitive approach accounting for structural properties in organisms (Holland, 1988;Smith, 1997;Jan et al., 1999;Rasskin-Gutman et al., 2004;Knoblich, 2001).
From a mathematical point of view, the property of regularity of a geometric form has not been formalized. Based in previous results by Torres et al. (2002), we hypothesize that eutacticity provides a measure of regularity based in the following argument. A set of N vectors in R n , with a common origin, is called a star and a star is said to be eutactic if it can be viewed as the projection of N orthogonal vectors in R N . It turns out that stars associated with regular polygons, polyhedra or, in general, polytopes, are eutactic (Coxeter, 1973) and thus regularity and eutacticity are closely linked. A disadvantage of using eutacticity as a measure of regularity is that a star vector must be associated with the geometrical form under study. As we shall see, this is not a problem with echinoids. In fact, Torres et al. (2002) found that the flower-like patterns formed by the five ambulacral petals in 104 specimens of plane irregular echinoids (from Clypeasteroidea) are eutactic. Here we present a deeper study that overcome the restriction to plane irregular echinoids, using the five ocular plates (OP) to define the star vector. Additionally, we use a new criterion of eutacticity that provides a measure of the degree of eutacticity of a star which is not strictly eutactic. With these tools we study the variability of eutacticity during geological time and to analyze pentamery variability during the evolution of sea urchins.
Sea urchins are pentameric organisms with an apical structure, called the apical disc (Melville and Durham, 1966). This structure includes five ocular plates (OP) that can fold the vector star associate with each sea urchin species (see Fig. 1 and Section 3 for a detailed description). In this work, we show that OP can be useful even in ovoid echinoids, such as Spatangoids, since the OP are almost tangential to the aboral surface (opposite to oral surface). Using the OP to define the star of vectors, we analyze the regularity and changes in a macroevolutive and taxonomic level in a collection of 157 extinct and extant sea urchins. We conclude that evolution has preserved a high degree of regularity and, consequently, that the apical disk is a homogeneous and geometrically stable structure through the geological time. Low values of regularity were recorded in some specific families and its biological consequences are discussed. This paper is organized as follows. In Section 2 a mathematical introduction to the concept of eutactic star is presented. Section 3 describes the structure of the apical disc and its biological importance, making it the obvious choice to define a vector star which characterizes each specimen. Experimental methods and results are devoted to Section 4 and, finally, discussion and conclusions are presented in Section 5.
Our main hypothesis is that the concept of regularity of a biological form may play an important role in the study of phenotipic variation in evolution.
For this goal, one must first be able to establish a formal criterion defining regularity of a geometrical form, including a measure of how regular a form is.
Mathematically, this property has not been defined and here, as a first step along this direction, we adopt the concept of eutacticity that, as we shall show, is closely related to regularity.
We shall deal with a set of N vectors {a 1 , a 2 , . . . , a N } in R n , with a common origin, called star. In this case N > n so the set of vector con not be linearly independent. The star is called eutactic if its vectors are orthogonal projections of N orthogonal vectors in R N , that is, there exist N orthogonal vectors {u 1 , u 2 , . . . , u N }, in R N , and an orthogonal projector P : R N → R n such that
The notion of eutacticity (from the Greek eu=good and taxy=arrangement) was firstly introduced by the Swiss mathematician L. Schläfli (about 1858) in the context of regular polytopes. Later, Hadwiger (1940) noticed that the vectors of an eutactic star are projections from an orthogonal basis in higher dimensional spaces and proved that the star associated to a regular polytope is eutactic. Thus, eutacticity is associated with regularity and the remarkable properties of eutactic stars have been useful in different realms such as quantum mechanics, sphere packings, quasicrystals, graph and frame theory and crystal faceting (see Aragon et al. (2005) and references therein).
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