A number of characters help distinguish the extant Arbacioida from their closest living relatives. Each is discussed in turn with comments on the character’s wider distribution amongst fossil forms.
1. Presence of a basicoronal plate
In the development of euechinoids plating in the interambulacral zones begins with a single element, the basicoronal plate (e.g. Gordon 1926). While this plate is retained in the tests of all irregular echinoids, it is generally resorbed, or at least reduced and internalized, in most regular echinoids. Only in Arbacioida, amongst regular echinoids, does the basicoronal plate remain external and visible, being especially prominent in the smaller, deep-water forms (e.g. Döderlein 1906; Agassiz and Clark 1908; Mortensen 1935). In most arbaciids, this basicoronal plate carries a single, prominent tubercle positioned centrally. In Arbacia and Tetrapygus, however, while the basicoronal plate is retained into adulthood, it is less evident, as there is no obvious median tubercle and suture lines can be difficult to trace.
Amongst fossil genera a basicoronal plate is clearly evident in Noetlingaster (Smith 1995), Codiopsis (Smith and Rader 2009), Gymnodiadema (this paper), Magnosia (this paper) and Eucosmechinus. It is present but much reduced and without a primary tubercle in Dubarechinus (this paper) and in Glypticus, where a tiny wedge-shaped element can sometimes be made out bordering the peristome in the interradius. No trace of the basicoronal plate remains in the adult tests of Goniopygus, Cottaldia, Glyphopneustes, Mimiosalenia and non-arbacioids such as Psephechinus and Hemipedina.
2. Sphaeridia positioned in pits at the perradius
Sphaeridia are small, club-like appendages that function as balance receptors and are developed only in euechinoids. They can be positioned either along the mid-line of the ambulacra, between the two columns of primary tubercles, as in all extant Arbaciidae or sandwiched between the pore zone and the perradial band of tubercles, as in most other regular euechinoids (see Kroh and Smith 2010). Furthermore, sphaeridia are situated in obvious pits in the extant Arbaciidae, cassiduloids, clypeasteroids (where they are often fully enclosed within the plates), and in some extinct Salenioida and Aspidodiadematoida, whereas they attach to superficial granules, sometimes with just the slightest trace of a dimple, in all other taxa. In all extant Arbaciidae, with the exception of Coelopleurus, there is but a single sphaeridial pit in each ambulacrum, which is found centrally near the peristome edge. Coelopleurus differs in having a short, slightly offset row of 4–6 pits lying along its perradial suture close to the peristome.
In fossil material, the presence of sphaeridia is usually only demonstrable when they are housed in pits. Sphaeridial pits are present in Noetlingaster, where a short adoral row of pits is found along the perradius (Smith 1995). A similar pattern is evident in many fossil species of Coelopleurus. Codiopsis has a single median tubercle in each ambulacrum positioned close to the peristome edge and accompanied by a slight depression on its adapical side. This is likely to be the sphaeridial attachment site. Gymnodiadema has no sphaeridial pits, but it does have a series of small distinct granules that is developed down the perradius in the obvious naked zone: these might be sphaeridial tubercles. Glyphopneustes has a double series of shallow sphaeridial pits, but these form two adradial columns lying between the two bands of pore-pairs and the median zone of perradial tubercles. This is the position sphaeridia occupy in non-arbacioid taxa. No trace of sphaeridial pits can be found in Magnosia, Eucosmechinus, Glypicus, Goniopygus, Acropeltis, Dubarechinus or Cottaldia.
3. Aboral tubercles highly reduced or absent
Although not all modern arbaciids have a strong oral–aboral differentiation in tubercle pattern, this is a distinctive feature of many. Pygmaeocidaris, Dialithocidaris, Habrocidaris, Podocidaris, Arbaciella and some modern species of Coelopleurus all have tubercles confined to the oral surface—their upper surface is devoid of all tubercles and spines. Tubercles in other Coelopleurus species and in Arbacia and Tetrapygus continue aborally, but there is often a wide, median, naked zone and the tubercle pattern differs strongly between the oral and aboral surfaces in Coelopleurus. No other extant regular echinoid group ever develops such a clear oral-aboral demarcation in tuberculation pattern.
Of the fossil members only Gymnodiadema, Codiopsis and possibly Dubarechinus entirely lack aboral tubercles. Glypticus, Pleiocyphus and Brochechinus, however, show a sharp change in tuberculation pattern, with well-developed tubercles below the ambitus and only very much reduced primary tubercles continuing to the apex. There is also an obvious, but less drastic change in tuberculation pattern at the ambitus in Magnosia and Eucosmechinus. In Acropeltis, Goniopygus, Mimiosalenia and Glyphopneustes tubercles show no clear oral–aboral differentiation being well developed on all plates: tubercles are largest at the ambitus and decrease in size gradually both adorally and adapically.
4. Presence of well-developed epistroma covering the surface of plates
Epistroma is the term given to dense wart-like projections developed on the outer surface of plates. In extant arbaciids, this takes two forms, as glassy tubercles or papillae composed of imperforate calcite that project from the test-like stumpy spines, and as lower, more irregular series of bumps and nodes termed epistroma which generally retain stereom perforations. Glassy papillae occur in some irregular echinoids (echinoneioids, clypeasteroids) but are otherwise restricted to arbaciids amongst the modern fauna. They are developed aborally in all the modern deep-sea forms, where they are either organized into well-defined horizontal rows (Pygmaeocidaris, Dialithocidaris, Podocidaris, Arbaciella) or lie irregularly scattered over the plates (Habrocidaris). Glassy papillae are also found in some Coelopleurus where they are often arranged as two columns, one on either side of the broad naked interradial zone that is developed adapically. The test surface in Coelopleurus, Arbacia and Tetrapygus has a fine ornament of epistroma wherever there are no tubercles.
Glassy papillae are well developed in Codiopsis and Gymnodiadema where they lie scattered over the plate surface, as in Habrocidaris. Poor preservation makes it impossible to tell whether Dubarechinus has these also, but they are definitely absent from all other genera. A very coarse epistroma is developed over the entire aboral surface in Glypticus, Pleiocyphus and Brochechinus giving these taxa a very distinctive appearance. A fine epistroma is present in Noetlingaster giving the plates a distinctive reticulate ornamentation (Smith 1995). Epistroma is rarely present along the interradial suture of supra-ambital plates in Magnosia and is completely absent from Acropeltis, Goniopygus, Mimiosalenia, Atopechinus and Glyphopneustes.
5. Apical disc dicyclic and free of tubercles
In extant arbaciids, the apical disc is always firmly bound to the corona and plating is mostly dicyclic (it becomes hemicyclic in adult Tetrapygus and rarely in Arbacia). Furthermore, the disc is distinctive in being completely tubercle free, which is in marked contrast with those of most other regular echinoids.
Noetlingaster, Codiopsis, Gymnodiadema, Magnosia, Eucosmechinus, Glypticus, Pleiocyphus and Brochechinus also have dicyclic apical discs devoid of tubercles. In contrast, while the discs of Acropeltis, Goniopygus, Mimiosalenia and Glyphopneustes are dicyclic and firmly bound to the corona, these all bear tubercles. Acropeltis has a single primary tubercle central on each genital plate while the others have perianal tubercles usually situated in pits around the rim of the periproct. Arbia and Cottaldia discs are unusual in having scattered secondary tubercles.
Another aspect of the apical disc is the presence of sutural pits. These are absent from all extant arbacioids, but are developed in Goniopygus, Mimiosalenia and Glyphopneustes. Superficially, the disc of Glypticus, Pleiocyphus and Brochechinus also appears pitted, but this ‘pitting’ is created by the thick epistromal overgrowth, not by excavation into the plate surface.
6. Anal opening covered by 4–6 valve-like plates
This is a feature seen in all extant arbaciids, but unfortunately is rarely preserved and remains unreported from any fossil arbacioid taxa.
7. Tubercles in ambulacral and interambulacral zones similar in size; interambulacral zones with rows of subequal tubercles
The arrangement of tubercles in extant arbaciids is rather consistent. Those in ambulacral and interambulacral zones are developed to the same extent and those on interambulacral plates are set into rows. This is the case in Tetrapygus and Arbacia where there are many tubercles to a row. In the deep-sea forms, there are typically just two subequal tubercles on adoral plates. Coelopleurus can have just a single primary tubercle on adoral plates, but usually has two.
Well-developed rows of subequal tubercles are developed in Noetlingaster, Codiopsis, Arbia, Magnosia, Eucosmechinus and Gymnodiadema. However, Dubarechinus, Glypticus, Acropeltis, Brochechinus, Pleiocyphus, Goniopygus, Mimiosalenia and Glyphopneustes only ever have a single primary tubercle on interambulacral plates. Furthermore, in Goniopygus, Mimiosalenia and Glyphopneustes, the ambulacral tubercles are always considerably smaller than those in a similar position in interambulacral zones. The fossil Murravechinus (Philip 1965), a subgenus of Coelopleurus, is unusual in having ambulacral tubercles very much larger than interambulacral tubercles.
8. Tubercles imperforate and non-crenulate
Although by no means unique to arbaciids, all extant members have imperforate and non-crenulate tubercles. The same is true for most of the fossil genera under discussion with the exception of Gymnodiadema, Dubarechinus and Cottaldia. Gymnodiadema has perforate tubercles with a clear non-crenulate parapet surround the mamelon (Fig. 4f), while tubercles of Dubarechinus are both perforate and crenulate. The tubercles in Cottaldia are, for the most part, imperforate and non-crenulate, but the oral tubercles in large individuals are definitely perforate (Smith and Wright 1996, text-fig. 113).
9. Oral and aboral tube-feet are strongly differentiated
Unlike camarodonts, arbacioids often show a strong differentiation between oral and aboral tube feet that is also matched in the morphology of their pore-pairs. In Arbacia, the aboral tube-feet are broad and morphologically specialized for gaseous exchange, whereas their oral tube-feet are highly muscular and suckered (Smith 1978). These differences are reflected in the morphology of the pore-pairs, with aboral pore-pairs being broad with a wide interporal partition, and oral pore-pairs being small, circular and with a wide periporal muscle attachment area. Large phyllodes (expanded adoral zones of tube-feet) are developed in Arbacia and Tetrapygus but not in Coelopleurus or any of the deep-sea forms. Only Sexpyga is reported to have weak phyllodes. Tube-feet and pore-pairs are clearly differentiated around the test in Arbacia, Tetrapygus and Coelopleurus. However, in the other modern deep-sea forms, all tube-feet remain small and undeveloped and no differentiation is apparent.
Amongst fossil taxa, there is a clear differentiation of pore-pairs evident in Noetlingaster, Codiopsis, Gymnodiadema, Magnosia, Glypticus, and possibly Arbia. Strong phyllodes are developed in Codiopsis, Gymnodiadema, Magnosia and Eucosmechinus, weak phyllodes are present in Dubarechinus, Glypticus, Pleiocyphus, Brochechinus and Acropeltis and are absent in Noetlingaster and Arbia.
10. Ambulacral compounding in the arbaciid style
Arbacia has trigeminate plate compounding with a large middle element and upper and lower elements reduced to small demiplates and this is what is typically taken as arbaciid-style compounding. Exactly the same pattern is evident in Coelopleurus and Tetrapygus, although in that taxon, there are four elements (three demiplates) to a compound plate. Plate compounding in deep-sea forms is less developed, possibly because of their small size, and often a simple plate alternates with a pair of elements bearing a primary tubercle (Mortensen 1935).
Ambulacral plate compounding is diverse amongst fossil representatives. Codiopsis has true arbaciid plate compounding throughout, as does Noetlingaster, although one of its demiplates often becomes entirely occluded. However, Magnosia, Goniopygus, Acropeltis and Glypticus have plate compounding in which only the lower element is a demiplate while the upper element is reduced in size but not cut off from the perradius. Gymnodiadema and Dubarechinus are unique in having entirely simple plating aborally.