Arbacia ballenensis Courville & Mooi & Poulin & Saucede 2023, n. sp.
Creators: name: Courville, Erwan
name: Mooi, Rich
name: Poulin, Elie
name: Saucede, Thomas
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Animalia
Echinodermata
Echinoidea
Arbacioida
Arbaciidae
Arbacia
Arbacia ballenensis

description: Arbacia ballenensis n. sp.
Figures 2–4, table 1
Diagnosis. Arbacia with low, numerous interambulacral plates (N = 19 at TD = 20 mm); up to five subequal tubercles in interambulacral plates at the ambitus (TD = 20 mm); inner interambulacral tubercles close to the adoral plate margin; naked interambulacral areas elevated above adjacent ambulacra.
Type material. Holotype specimen CASG 103360, housed in the Geology collections at the CAS, San Francisco, California, USA (Figs. 2A–D). The remaining specimens in lot CASG 91189, which includes 49 complete or partial tests, are designated as paratypes.
Referred material. Sixty-two complete tests lacking articulated spines ranging from 6 to 20 mm TD, plus seven test fragments. In addition, there are several isolated spines in the poorly consolidated sandy sediments, some of which are also preserved within the peristome.
Etymology. The name is derived from the type locality on Punta Ballena, as this species is so far known only from the Pliocene—Pleistocene of this point along Bahía San Rafael, close to San Francisquito (Baja California Norte, México).
Stratigraphy. Late Pliocene to Early Pleistocene. Unnamed beds, middle part of the Punta Ballena fan (Johnson et al. 2017).
Taphonomy. Specimens with complete tests (except for seven fragments), locally abundant in one specific layer, sometimes with attached balanomorph barnacles. It would appear from this, and the excellent condition of the surface details on the specimens that they were dead when buried, with little or no post-mortem transport (see below).
Description. Test small, largest specimen barely reaching 20 mm TD (Fig. 2), test relatively low (height to diameter ratio <0.5; Fig. 2C), flattened orally, test outline circular in apical view (Figs. 2A, 3A, C).
Apical system —Approximately one third test diameter (Figs. 2A, 3A, C, D), ratio of apical system diameter to TD decreasing with size from 35% TD (TD = 6 mm) to 29% TD (TD = 20 mm), apical system dicyclic, all ocular plates always exsert (Figs. 2A, 3A, C, D, F). Periproct elongated in direction 1-IV. Gonopores usually large, slightly elongated, positioned close to adoral edge of genital plates (Figs. 2A, 3A, D, F). Gonopores already developed in specimens between 8 and 10 mm TD (Figs. 3A–D). Ocular plates with large granule (possibly a small tubercle) close to adoral margin. Genital plates free of tubercles.
Peristome —Usually less than 50% of test diameter (mean ratio PD/TD = 46%; Figs. 2B, 3B). Buccal notches shallow. Tags short (length <width, spanning <1 tubercle). Perignathic girdle thin, auricles separate, not in contact over interior of perradial suture (Fig. 3G). Epistroma well-developed, forming homogeneous granulation on ocular and genital plates (Fig. 3A).
Ambulacra —Wider than half width of interambulacra. At ambitus, each ambulacral plate with one small, imperforate, non-crenulate tubercle positioned adradially (Figs. 2A, 3F). Ambulacral plates small and numerous. Ambulacral tubercles same size as interambulacral tubercles at ambitus, slightly decreasing in size adapically (i.e. above ambitus). Tubercles small and well-developed aborally, forming two rows of tubercles on aboral surface, almost reaching apical system (Figs. 2A, 3A, D). Presence of well-developed granules in between tubercles perradially. Ambulacral plates trigeminate, of arbaciid type (Fig. 2D), forming more complex compound plates adorally, slightly widening to form short, narrow phyllodes. Pores rounded, spaced, strongly conjugate aborally with large, prominent ridge between pores (Fig. 2D). On oral side, one perradial sphaeridial pit (usually well preserved) near peristomial edge (Figs. 2B, 3B, E).
Interambulacra —Plates wider than their height at ambitus (height/width ratio <0.2), with up to five (TD =
20 mm), small, imperforate, non-crenulate primary tubercles (Fig. 2C), slightly decreasing in diameter toward interradius, on adoral margin of plates. Maximum number of interambulacral tubercles at ambitus decreases to four (TD = 17 mm), three (TD = 15– 8 mm) and commonly only two in smallest specimens (TD = 10– 8 mm). Number of interambulacral plates usually between 17 and 18 (TD> 17 mm), decreasing in number slightly in small specimens to 12 or 13 plates in each interambulacrum (TD = 8 mm). Plates lowest above ambitus, height of plate constant on aboral surface (<1 mm, TD = 20 mm; Fig. 2A). Tubercles in outer row slightly reduced in size above ambitus (Fig. 2A). Number of interambulacral tubercles reduced abruptly from five at ambitus to two or three plates above ambitus (TD = 20 mm). Inner tubercles slightly smaller than outer ones (Fig. 2C), abutting adoral margin of plates, close to outer row of primary tubercles. Second inner tubercle well developed along aboral surface, usually forming columns adapically to middle part of aboral surface (Figs. 2A, 3A, D). Outer row of tubercles same size from ambitus to apical system (Fig. 2A). Additional column of interambulacral tubercles between first column and pore area developed in largest specimen (TD = 20 mm; Fig. 2A) but restricted to ambitus and lowermost part of aboral surface (Fig. 2D). Large tubercles surrounded by several tiny granules. Areoles of tubercles poorly developed above ambitus, barely larger than mamelon. Interambulacral naked area wide, extended adorally to ambitus (Figs. 2A, 3A, C–D). Epistroma consisting of conspicuous, randomly distributed punctation (Fig. 3A). On aboral side, members of second perradial column of tubercles progressively reduced, conjoined adapically to form continuous ridge between adradial column of tubercles and perradial naked area of ambulacrum (Fig. 3A).
description: Published as part of Courville, Erwan, Mooi, Rich, Poulin, Elie & Saucede, Thomas, 2023, Arbacia ballenensis sp. nov. (Echinoidea, Arbacioida): A new species reveals diversification of the genus in Central America, pp. 555-566 in Zootaxa 5336 (4) on pages 558-560, DOI: 10.11646/zootaxa.5336.4.6, http://zenodo.org/record/8282178