Maretia sp.

Holmes, Francis C., 2014, A late Miocene record of the echinoid Maretia (Echinoidea, Spatangoida) from Victoria, Australia., Memoirs of Museum Victoria 72, pp. 63-72 : 64-67

Maretia sp. aff. planulata ( Lamarck, 1816)

Material. Specimens NMV P324331–P324338 from the stratotype locality of the late Miocene Tambo River Formation (Mitchellian, Tortonian-Messinian) at Swan Reach, Victoria [ NMV locality PL 3110]. A quantity of disarticulated plates together with a sample of matrix found with these specimens are numbered NMV P322439.

Description. Test ovate to sub-pentagonal tapering to a semitruncated posterior margin, slightly flattened adjacent to anterior depression, and weakly arched adapically with apex approximately central; margins rounded. Interambulacrum 5 mildly raised on the aboral surface at the interradial suture, and on the adoral surface swollen to form a posterior keel. Length of specimens range from 50-55 mm with width varying from 78-88%TL. Apical system showing 4 gonopores partially preserved on only one specimen (fig.3A); no detail of plate structure or hydropores can be discerned. Centre of apical disk 40-43.5%TL from anterior ambitus.

Anterior ambulacrum III, narrow and flush aborally, slightly depressed at ambitus and on adoral surface; no detail of pores. Anterior paired petals quite indistinct and possibly rudimentary but, based on ambulacrual plate suture, appear flush, straight sided, narrow and with maximum width only 3/5 that of posterior pair; detail of pores and tuberculation too poorly preserved to describe. Posterior paired petals, lanceolate, closing distally, wider and longer than anterior pair, pores eye shaped, outer and inner appear to be similar in size, not strictly conjugate but each side of plate sutures between pairs angularly sunken. Interporiferous zone covered with randomly placed small tubercles and numerous miliary granules, the latter extending across the pore zones (fig. 2F).

Periproct longer than wide situated on steep undercut truncate face, exact shape indistinct. Peristome also poorly preserved in all specimens but clearly wider than long with convex anterior lip of labrum overhanging posterior side. Phyllodes and groups of small tubercules at termination of interambulacra with peristome partially visible on some specimens (fig. 3B).

Labrum narrow and very elongate, just contacting sternal plates adjacent posterior end of adjoining third ambulacral plates, and with a number of small tubercles on anterior lip, similar to those at end of adoral interambulcra (fig. 3D,E). Sternal plates paired, long, narrow and triangular, extending to posterior end of adjoining fifth ambulacral plates. The posterior ends of the plates are covered with small tubercles for 40-45% of their length and possess a ventral apex at the centre just anterior of the posterior sutures (fig. 2E). Episternal plates, covered with small tubercles, are probably triangular and undercut by re-entrant sixth ambulacral plates; junction with sub-anal plates indeterminate. Due to extensive posterior damage, the subanal fasciole is partially visible on only two specimens; a small indistinct section crossing the episternal plates (fig. 3G), and a re-entrant section adjoining the periproct (fig. 3H).

Heterogeneous, moderately spaced, perforate and crenulate, primary tubercles with sunken areoles as well as occasional small tubercles occur on the aboral surface of interambulacra 1-4 (fig. 3C). Several small tubercles and a few primaries, the latter generally towards the posterior ambitus, occur on interambulacrum 5. Small closely spaced tubercules immediately below the ambitus in ambulacra 1-4 increase in size but reduce in quantity adorally to form distinct radiating rows of primary tubercles. Plates surrounding the periproct are also covered with small tubercles.

Remarks. All specimens are to some degree deformed, often incomplete, with some of the plates cracked and their sutures opened up. Nevertheless it is possible to compare individual features of the specimens with those of the three extant species of Maretia : M. planulata , M. carinata , and M.? cordata ( Table 1).

Apart from primary generic characteristics present in all four species compared in the table, approximately 70 percent of the listed diagnostic features are common to both M. sp. aff. planulata and the type species M. planulata . Comparison with M. carinata is more difficult to summarise as two important diagnostic features that distinguish it from M. planulata , the number of pore pairs in paired petals and the development of the phyllodes, are not preserved in the fossil specimens. Excluding the generic characteristics and these latter features, M. sp. aff. planulata has only about 30 percent of its diagnostic characteristics in common with M. carinata . Similarity between M. sp. aff. planulata and M.? cordata is quite minimal.

In descriptions of Maretia species, the density of tubercles on parts of the test has been used as a diagnostic feature; particularly the density of primary tubercles on aboral interambulacra 1-4. However, based on the extant specimen from the Philippines (fig. 2G) and numerous published photographs of M. planulata (e.g. Mortensen, 1951; Fisher, 1966; Schultz, 2005; Kroh, 2007; Smith and Kroh, 2011), there appears to be considerable variation in density and arrangement of these tubercles, making comparison of this feature between species of the genus, somewhat tenuous.