Arbacia ballenensis, Courville & Mooi & Poulin & Saucede, 2023

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, Zootaxa 5336 (4), pp. 555-566 : 558-560

publication ID	https://doi.org/ 10.11646/zootaxa.5336.4.6
publication LSID	lsid:zoobank.org:pub:4C9E1AC9-5203-48A9-9C4E-7C0F3209ADF1
DOI	https://doi.org/10.5281/zenodo.8284519
persistent identifier	https://treatment.plazi.org/id/03A00A15-5758-B36D-FF64-DE3A3D2289E4
treatment provided by	Plazi
scientific name	Arbacia ballenensis
status	sp. nov.

Treatment

Arbacia ballenensis n. sp.

Figures 2–4 View FIGURE 2 View FIGURE 3 View FIGURE 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 View FIGURE 2 ). 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 View FIGURE 2 ), test relatively low (height to diameter ratio <0.5; Fig. 2C View FIGURE 2 ), flattened orally, test outline circular in apical view ( Figs. 2A View FIGURE 2 , 3A, C View FIGURE 3 ).

Apical system —Approximately one third test diameter ( Figs. 2A View FIGURE 2 , 3A, C, D View FIGURE 3 ), 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 View FIGURE 2 , 3A, C, D, F View FIGURE 3 ). Periproct elongated in direction 1-IV. Gonopores usually large, slightly elongated, positioned close to adoral edge of genital plates ( Figs. 2A View FIGURE 2 , 3A, D, F View FIGURE 3 ). Gonopores already developed in specimens between 8 and 10 mm TD ( Figs. 3A–D View FIGURE 3 ). 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 View FIGURE 2 , 3B View FIGURE 3 ). 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 View FIGURE 3 ). Epistroma well-developed, forming homogeneous granulation on ocular and genital plates ( Fig. 3A View FIGURE 3 ).

Ambulacra —Wider than half width of interambulacra. At ambitus, each ambulacral plate with one small, imperforate, non-crenulate tubercle positioned adradially ( Figs. 2A View FIGURE 2 , 3F View FIGURE 3 ). 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 View FIGURE 2 , 3A, D View FIGURE 3 ). Presence of well-developed granules in between tubercles perradially. Ambulacral plates trigeminate, of arbaciid type ( Fig. 2D View FIGURE 2 ), 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 View FIGURE 2 ). On oral side, one perradial sphaeridial pit (usually well preserved) near peristomial edge ( Figs. 2B View FIGURE 2 , 3B, E View FIGURE 3 ).

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 View FIGURE 2 ), 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 View FIGURE 2 ). Tubercles in outer row slightly reduced in size above ambitus ( Fig. 2A View FIGURE 2 ). 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 View FIGURE 2 ), 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 View FIGURE 2 , 3A, D View FIGURE 3 ). Outer row of tubercles same size from ambitus to apical system ( Fig. 2A View FIGURE 2 ). Additional column of interambulacral tubercles between first column and pore area developed in largest specimen (TD = 20 mm; Fig. 2A View FIGURE 2 ) but restricted to ambitus and lowermost part of aboral surface ( Fig. 2D View FIGURE 2 ). 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 View FIGURE 2 , 3A, C–D View FIGURE 3 ). Epistroma consisting of conspicuous, randomly distributed punctation ( Fig. 3A View FIGURE 3 ). 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 View FIGURE 3 ).