Abstract
Despite surviving longer than any other blastozoans, blastoids were exceptionally conservative in their morphology and usually symmetrically pentaradiate. Astrocrinus is a rare exception that lacked a stem and some thecal plates, although differing interpretations of its morphology and taxonomy have been published. Astrocrinus had a flattened, tetralobate theca covered in minute spines, and with a plane of symmetry through the AB interray and the D ray. Four ambulacra are long, thin, and curved down deep sinuses between the thecal lobes to reach the basal surface. The D ambulacrum is short, broad and horizontal. Astrocrinus tetragonus was first described from the Carboniferous Limestone of Settle, Yorkshire. Here it is recorded for the first time from the Brigantian, near Grassington. The new specimens confirm that A. tetragonus had a single basal plate which is kite-shaped, entirely surrounded by four radials and separated from the D ray radial. Astrocrinus benniei was described from the Scottish Brigantian and its basal plate is elongate pentagonal with a short common suture with the D ray radial. Astrocrinus occurs in the Irish Asbian and Brigantian. To date, only A. tetragonus can be confirmed from Ireland.
Of all blastozoans, the blastoids survived the longest, from the Middle–Upper Ordovician boundary to the Upper Permian. Blastoids also have the most conservative thecal morphology of any blastozoan group and exhibit nearly perfect pentaradial symmetry. Almost all the different blastoid thecal shapes are constructed from identical plates arranged in the same pattern. Usually, the only variation is in the number of deltoid plates in the posterior, or CD, interambulacrum (Carpenter 1884, 1891). The posterior deltoids occupy a very small part of the theca, all the rest of which is composed of the same 17 plates; three basals, five radials, five lancets and the other four deltoids. One rare exception to this generalization is the unique Lower Carboniferous genus Astrocrinus Morris, 1843, which has a four-lobed theca, a reduced number of basal plates and no stem. Despite having been discovered very early in the history of blastoid research, and the subject of detailed studies largely because of its unusual morphology (Etheridge 1876; Wanner 1932; Macurda 1977), some details of its thecal plating are still uncertain. The recent discovery by CRCP of an Astrocrinus specimen from the Middle Limestone, near Bare House, north of Grassington, Yorkshire, provided the opportunity to improve our knowledge of this unusual blastoid. It was soon discovered that Jeremy Savill had first found Astrocrinus at exactly the same place north of Grassington in the 1980s and about a dozen specimens are known from this locality. The latest specimen was found among some fine-grained debris from a small section of demolished dry-stone wall (SE007666). The debris derived from frost shattering of the stones in the wall, which almost certainly came from immediately adjacent exposures of the Middle Limestone where it is especially rich in the blastoids Orbitremites Austin and Austin, 1842, and Codaster McCoy, 1849 (Joysey 1955). The latest specimen lacks the AB interradius and all specimens from Yorkshire are weathered. Nevertheless, this preservation emphasizes the plate sutures and cleavage, both of which help in determining the precise thecal plate arrangement, especially the presence of a single kite-shaped basal plate.
Morris (1843) and Austin (1848) recorded that the original material of Astrocrinus came from Settle, Yorkshire. Subsequently, Joysey (1955, p. 218) confirmed that the type species, A. tetragonus (Austin and Austin, 1843), had been found ‘near the disused shaft of Pikedaw Mine’, between Settle and Malham. We now know that over 80 specimens have come from this locality, some of which also confirm the single kite-shaped basal plate. The horizon is in the Upper Hawes Limestone and so significantly lower (basal Brigantian) than the blastoid-rich exposures high in the Middle Limestone at Grassington (mid Brigantian; Fig. 1). It is highly likely that the Pikedaw Mine locality is the type locality. As far as we are aware, no-one has reported Astrocrinus from nearer to Settle.
Stratigraphic occurrences of Astrocrinus Morris, 1843, in the British Isles. 1: Oyster Hall, Fenit, Co. Kerry, Ireland. 2: Slievemore, Co. Sligo, Ireland. 3: Lisdowney, Co. Kilkenny, Ireland. 4: Malham, Yorkshire, England. 5: Grassington, Yorkshire, England. 6: Midland Valley, Scotland.
Later, Etheridge (1876) described Astrocrinites? benniei from the Lower Limestone Group of the Midland Valley of Scotland. Waters and Sevastopulo (1984, fig. 2, p. 140) also recorded Astrocrinus from two horizons in Ireland, the high Asbian of Slievemore, and the Brigantian of Lisdowney and Oyster Hall (Fig. 1).
The Grassington horizon is dominated by relatively coarse-grained, poorly-sorted biosparite, in which the bioclasts are largely echinoderm plates. The rock appears to have been deposited as a well-washed sediment. One slab containing Astrocrinus has individual crinoid columnals as big as the entire cup. This seems a most unexpected environment for such a small, unattached blastoid, but much better suited to attached blastoids, such as Orbitremites and Codaster and crinoids. Both the blastoids and crinoids would not only benefit from being permanently anchored by their stems, but also elevated in the water column where food sources were probably richer. Both were probably rheophilic. Indeed, south of Bare House the richest blastoid beds cap a local elevation on the seafloor and it seems the blastoids survived best on such high points. Joysey (1955, p. 213) recorded the association of the two common blastoid genera with reef knolls.
The purposes of this paper are to describe the morphology of the new specimens and compare them with previously published accounts of Astrocrinus. It is also relevant to discuss the stratigraphic occurrences in Yorkshire and elsewhere, and the mode of life of this unusual blastoid. Specimens described herein are deposited in the Sedgwick Museum, Cambridge (CAMSM), the National Museum of Scotland, Edinburgh (NMS), and Trinity College, Dublin (TCD).
Systematic palaeontology
Eublastoidea Bather, 1899a
Order Fissiculata Jaekel, 1918
Family Astrocrinidae Austin and Austin, 1843
Genus Astrocrinus Morris, 1843
Synonyms
Astracrinites Austin and Austin, 1842, p. 110 (nom. nud.); Astrocrinites Austin and Austin, 1843, p. 206 (non Astrocrinites Conrad, 1840, p. 204, nom. nud.); Zygocrinus Bronn, 1848, p. 1381.
Type species
Astrocrinites tetragonus Austin and Austin, 1843, by original designation (Morris, 1843, p. 49), from the Carboniferous Limestone of Settle, Yorkshire.
Diagnosis
Stemless fissiculates with a spiny, tetralobate theca, with a plane of bilateral symmetry through the D radius and AB inter-radius; with the D ambulacrum short and broad and the other four long and narrow; with a single basal plate.
Remarks
Breimer and Macurda (1972, p. 29) restricted the family Astrocrinidae to the type genus, Astrocrinus. Astrocrinus is unique in its flattened, tetralobate thecal shape, in the surface ‘ornament’ of fine spines, and in possessing one basal plate. Etheridge and Carpenter (1886, pl. 20, fig. 1) illustrated the type species, A. tetragonus, showing a single, small, central, kite-shaped, basal plate entirely surrounded by four radial plates and not in contact with the D radial (Figs 2b, 3a, herein). Wanner (1932, p. 457) examined a large number of Scottish specimens of A. benniei. Although he stated that the spines obscured the plate sutures, he illustrated both a kite-shaped basal (fig. 3, p. 458) very like that of A. tetragonus, and a more common version with an elongate basal reaching the D ray radial plate (fig. 4a, b). In the treatise, Fay and Wanner reproduced Etheridge and Carpenter's figure of A. tetragonus with a kite-shaped basal (1967, fig. 254.3c) but only Wanner's (1932) diagrams of A. benniei (1967, fig. 255.1b, c, p. S416) with the elongate basal plate extending to the D ray radial (Fig. 3b, herein), thus suggesting that the two species were characterized by different shaped basals. Macurda (in Breimer and Macurda 1972, p. 29) regarded A. benniei as a junior synonym of the type species, A. tetragonus, and wrote ‘probably 3 basals, one elongate supporting small modified D radial’. They also illustrated two peels through a Scottish specimen of Astrocrinus (1972, fig. 33.15 and 16, p. 123), which clearly show a narrow basal plate between the limbs of the C and E radials. Later, Macurda (1977, pl. 32, fig. 1) provided a stereophotograph that clearly shows the basal plate reaching the D ray radial in a specimen from Carlops, Peebleshire, Scotland, as well as a camera lucida drawing of the base of a theca (Macurda 1977, fig. 1, p. 230) from Invertiel, Fife, Scotland. The drawing shows two basal plates, one of which was central aborally and the other a thin plate that extended up to meet the D ray radial plate. Both specimens were identified as A. tetragonus, but Macurda synonymized the two species. So, we think that the English and Scottish specimens differ consistently from each other in the shape and size of the single basal plate and therefore that both described species are valid. Certainly, the few specimens of A. benniei that we have seen consistently show an elongate, pentagonal basal.
Thecal plate arrangement in Astrocrinus tetragonus (Austin and Austin, 1843). (a) Oral view, showing the central mouth (M) surrounded by five lancet plates that underlie the ambulacra (A–E). The anus (A) lies in the CD interambulacrum and hydrospire clefts (black) are prominent in the other four interambulacra, which are largely composed of deltoid plates. (b) Aboral view showing five radial plates, four of which are in direct contact with the single basal plate. (c) D ray lateral view, showing the radial:deltoid sutures. Note the highly flattened thecal profile (c) and tetralobate plan view (a, b). Based on CAMSM X.50354.
Basal views of (a) Astrocrinus tetragonus (Austin and Austin, 1843) and (b) Astrocrinus benniei (Etheridge, 1876), showing different shaped single basal plate (centre, white). A–E positions of the five ambulacra (a) and the five radial plates (b), according to Carpenter's (1884, 1891) system of denoting rays. Radial plates shaded grey. (a) Redrawn from Etheridge and Carpenter (1886, pl. 20, fig. 1). (b) Modified from Macurda (1977, fig. 1, p. 230).
Astrocrinus is also recorded from three Irish localities at two distinct horizons (Waters and Sevastopulo 1984, fig. 2, p. 140). Photographs of the base of one specimen from Lisdowney, County Kilkenny (TCD 9806) taken by DBM show a single kite-shaped basal plate. So, we conclude that at least A. tetragonus occurs in Ireland.
Stratigraphic range
Asbian to Brigantian, England, Ireland and Scotland.
Description
Astrocrinus has a flattened, tetralobate theca with the longest lobe in the AB interray, the shortest and broadest centred on the D ray, and two lateral lobes centred on BC and EA interrays (Fig. 2a, b). The A, B, C and E ambulacra are long and narrow, set in deep sulci between the thecal lobes and curve down to reach the aboral surface (Fig. 2a). The D ambulacrum is short and broad, has few brachioles and is entirely confined to the oral surface. Hydrospire clefts are present adjacent to the ambulacra (Fig. 2a) and the surface of the thecal plates is covered with fine spines.
Thecal plating is unusual in that there is only a single basal plate (Figs 2b, 3, 4a), which corresponds to the small basal in the AB interray of other blastoids. The five radials are not divided into obvious radial bodies and radial limbs, and the radial:deltoid sutures run horizontally around the theca (Fig. 2c). The mouth frame is formed of five plates, four regular deltoids and a superdeltoid in the CD interray, which surrounds the anus. According to Breimer and Macurda (1972, p. 122), internally there are two unequal cryptodeltoids and a hypodeltoid. Four of the five lancets are long, narrow and curved around the theca (Fig. 2c). They support the A, B, C and E ambulacra, and are exposed centrally adorally in A. benniei, but not in A. tetragonus. The D lancet is much shorter and flatter.
Stereophotos of Astrocrinus tetragonus (Austin and Austin, 1843) uncoated, Middle Limestone, Grassington. N Yorkshire, CAMSM X.50354. (a) Oblique aboral view to highlight single, kite-shaped basal plate by cleavage reflection. Radials A and B bound the short sides (below), C and E the long sides. The deep groove (above right) is the common suture between the C and E radials. In this view the D radial is not visible. (b) Slightly oblique lateral view with the basal plate just visible (bottom centre), the deep groove between the C and E radials leading to the triangular D radial (above centre). Note the spiny ornament (right). (c) Oral view showing the mouth surrounded by five deltoid plates, very narrow food grooves, and the radial clefts of the A and B ambulacra (above). Scale bar = 2 mm.
The ambulacra are separated from the adjacent radial and deltoid plates by deep hydrospire clefts (Fig. 2a). Ambulacra are composed of small, triangular, adoral, outer side plates and larger aboral side plates, alternately on either side of the central food groove. Each brachiole arose from a small facet shared unequally between the outer side plate and side plate (Fay 1961, fig. 2, p. 21). Brachioles are not preserved on specimens described so far and no trace of a stem is preserved, not even a cicatrix for its attachment (Figs 2b, 3, 4a).
Remarks
Astrocrinus is a very distinctive genus. No other blastoid is so flattened in an oral-aboral direction, nor tetralobate in outline, nor has a single basal. The stem is absent in two other genera. Finally, the spiny surface ornament is also unique. It is asymmetrical and its orientation is consistent across each individual plate, but changes from plate to plate (Macurda 1977, pl. 31, fig 1; pl. 32, figs 1, 3). The size and shape of the single basal is of taxonomic importance in Astrocrinus (Fig. 3). As this appears to be a new observation, we have restricted the synonymies to those references where we can be fairly certain of the specific identities.
Astrocrinus tetragonus (Austin and Austin, 1843)
Stereophotos of Astrocrinus tetragonus (Austin and Austin, 1843), Middle Limestone, Grassington. North Yorkshire, CAMSM X.50354. Distal tip of ambulacrum E showing alternating side plates and smaller outer side plates (below) with deep EA hydrospire cleft above and nonfunctional DE cleft (below) so that the outer side plates are in contact with the E ray radial. Note the radial:deltoid suture passing up the EA thecal lobe to its tip (centre top) and the deep cleft for the common suture between the C and E radials (below left) leading to the D ray radial (below right). Scale bar = 1 mm. Specimen whitened with fingerprint powder.
1842 Astracrinites tetragonus Austin and Austin: p. 110, nom. nud.
1843 Astrocrinites tetragonus Austin and Austin: p. 206.
1843 Astrocrinus tetragonus (Austin); Morris: p. 49.
1848 Astrocrinus tetragonus (Austin and Austin); Austin: p. 293.
1876 Astrocrinites tetragonus Austin and Austin; Etheridge: pl. 13, fig. 26.
1886 Astrocrinus tetragonus Austin and Austin; Etheridge and Carpenter: p. 300, pl. 20, figs 1, 2.
1899b Astrocrinus tetragonus T & T Austin; Bather: p. 2.
1899b Zygocrinus tetragonus (T & T Austin); Bather: p. 70.
?1955 Zygocrinus tetragonus Austin; Joysey: p. 218.
1967 Astrocrinus tetragonus Austin and Austin; Fay and Wanner: fig. 254.3a–d, p. S415.
1984 Astrocrinus tetragonus (T & T Austin); Waters and Sevastopulo: pp. 140, 146.
Diagnosis
A species of Astrocrinus with a small, central, kite-shaped basal plate entirely separate from the D ray radial plate.
Material
The original of Etheridge and Carpenter (1886, pl. 20, fig. 1) plus about a dozen specimens from Grassington and over 80 from near Pikedaw Mine, Malham, can definitely be attributed to this species. It seems likely that all specimens from Yorkshire belong to A. tetragonus. One specimen from Lisdowney, County Kilkenny, Ireland (TCD 9806), also has a kite-shaped basal and can be attributed to A. tetragonus.
Localities and stratigraphic levels
As here understood A. tetragonus is known from the type locality, ‘Settle, Yorkshire’, near the Pikedaw mine shaft, Malham and north of Grassington, Yorkshire, and Lisdowney, County Kilkenny, Ireland. Many specimens have been found from the Upper Hawes Limestone (basal Brigantian) near the Pikedaw Mine shaft between Settle and Malham, Yorkshire. This is probably the type locality, but that is not known for certain. The Grassington level is equivalent to the lower part of the Scar Limestone of the Alston Block (Colin Waters, pers. comm. to CRCP, December, 2020) and thus about mid Brigantian. According to Waters and Sevastopulo (1984, p. 146) the Lisdowney locality is of late Brigantian age (Fig. 1).
Description
The theca has flattened oral and aboral surfaces, and is tetralobate in plan view (Figs 2a, b, 3a), reaching 4.0 mm high and 7.8 mm wide as preserved. The new specimen is missing the lobe in the AB interray and the tip of one lateral lobe (Fig. 4a, c). It is estimated to have been about 8.5 mm wide originally. The two lateral lobes are pointed, the D ray lobe broader and rounded. From other specimens the AB lobe was also rounded in outline and the longest. The oral surface shows a small, equant mouth 0.75 mm across and surrounded by five deltoid plates (Fig. 4c). The lancet plates are pointed adorally, inserted between the deltoids and reach to within 0.1 mm of the oral margin. Lancets of the A–C and E ambulacra are long thin plates, reaching 0.6 mm maximum width and with narrow food grooves 0.1 mm wide (Fig. 2a, c). They are entirely covered by side plates. The D ray lancet is wider and shorter than the other four, but damaged. It could not have exceeded 1.6 mm long and reaches only to 0.3 mm from the oral margin. Hydrospire clefts are present on either side of the ambulacra and reach a maximum width of 0.3 mm adorally (Fig. 4c). The clefts on the anal side of the C and E ambulacra are narrower (Figs 4c, 5), so the ambulacra are not central in the sinus. The side plates of the A and B ambulacra appear narrower on the side closest to the AB–D ray plane of symmetry. Ambulacra C and E are longest, but only slightly longer than ambulacra A and B. Ambulacra C and E twist slightly along their length, so that brachioles at the distal tips would have been nearly perpendicular to the AB–D ray plane when erect. The margin of the periproct within the CD deltoid plate is just visible in the damaged area of the D ray lobe. Ambulacra A–C and E (Fig. 5) curve down the theca to the aboral surface and reach 0.8 mm wide centrally. Side plates are largest at the ambitus and taper gently to the ambulacral tips (Fig. 5), where there are five in 0.75 mm length. These ambulacra originally had between 60 and 65 brachioles per ambulacrum. The theca includes a single, central, kite-shaped basal plate 1.55 mm long by 1.25 mm wide (Fig. 4a) and separated from the D radial by a deep, common suture between the C and E ray radials 1.65 mm long (Fig. 4b). The rest of the base of the theca is largely formed by the four, large radial plates of the A–C and E rays (Figs 2b, 3a, 4a); the D ray radial is smaller and almost confined to the upper side of the theca. Five deltoids make up most of the oral surface, with the lancet plates interpolated between them (Figs 2a, 4c). The surface of the plates was covered with fine spines in life, traces of which survive in the C radial of the BC lobe (Fig. 4b right).
Remarks
Astrocrinus tetragonus differs from the only other described species, Astrocrinus benniei, in the shape of the single basal plate. In A. tetragonus the basal is central in the aboral surface, directly below the mouth, kite-shaped and in contact with the A, B, C and E radial plates (Figs 2b, 3a, 4a). The more acute angle points towards the D ray radial, but in CAMSM X.50354 the tip of the basal is separated from the D radial by 1.5 mm and the basal is only 1.55 mm long by 1.2 mm wide (Fig. 4a). In A. benniei the single basal is elongate, five sided and extends all the way up to the D ray radial, with which it has a short suture (Fig. 3b). The specimen illustrated by Macurda (1977, pl. 32, figs 7, 9) has a basal 1.92 mm long by 0.71 mm wide and with a common suture with the D radial 0.29 mm wide.
Astrocrinus benniei (Etheridge, 1876)
(Fig. 3b)
1876 Astrocrinites? benniei Etheridge: p. 103, pl. 12, figs 1–11, pl. 13, figs 12–22.
1932 Zygocrinus benniei (Etheridge); Wanner: p. 456, fig. 1, p. 458, figs 2–4.
1967 Astrocrinus benniei (Etheridge); Fay and Wanner: fig. 255.1, p. S416.
1977 Astrocrinus tetragonus (Austin and Austin); Macurda : p. 228, text-fig. 1, pl. 32, figs 1, 7, 9.
Diagnosis
A species of Astrocrinus with an elongate pentagonal basal plate that has a short common suture with the D ray radial plate.
Material
Description based largely on NMS 1958.1.2361 and 1958.1.2376. Astrocrinus benniei is apparently more common than A. tetragonus. Bather (1899b, p. 69–70) listed 50 specimens of A. benniei, but only two of A. tetragonus in the Natural History Museum. The Geological Survey online database lists 19 registration numbers for A. benniei, some of which may represent more than one specimen, but no specimens of A. tetragonus. All specimens of A. benniei were received from the Edinburgh office. Macurda (1977, p. 228) listed eleven specimens of A. benniei in the Scottish National Museum. There are 36 examples in the Sedgwick Museum, compared with eight specimens of A. tetragonus from Yorkshire and two more without locality. Wanner (1932, p. 456) recorded that he examined 190 specimens of A. benniei loaned by James Wright.
Localities and stratigraphic levels
Both specimens on which the description is based are from the Lower Limestone Group of Scotland; the former from Carlops, Peebleshire, the latter from Invertiel, Fife. All Scottish localities are from the topmost Brigantian (Browne et al. 1999; Fig. 1).
Description
(Based largely on Macurda 1977, p. 228 et seq.) Theca very similar to A. tetragonus, but smaller (4.5 mm AB–D ray) and less lobate (Fig. 3b). How much the difference in thecal outline is due to the smaller size is unknown. The C and E ambulacra are longest and obvious in aboral view. The A and B ambulacra are next longest, but only traces of their tips are visible aborally. They had about 40 brachioles each. The D ambulacrum is shortest, confined to the oral surface, and with less than 16 or 18 brachioles in total. All the lancet plates become slightly exposed centrally near the mouth. The theca is covered with asymmetrical spines that are parallel within individual plates (see Macurda 1977, pl. 32, fig. 1). The most obvious difference from A. tetragonus is in the shape of the single basal plate, which extends from the centre of the aboral surface up to meet the D radial (Macurda 1977, pl. 32, fig. 1; Fig. 3b herein).
Remarks
The different shape of the single basal plate is an unequivocal character on which to base the two known species. Although requiring further confirmation, it appears that the English specimens from Yorkshire are likely all to be A. tetragonus, although McKenney Hughes (1908, p. 309) recorded A. benniei from Settle, Yorkshire. In contrast, the Scottish specimens are all likely to be A. benniei. On that assumption, A. benniei is the more common species (see under material above).
Mode of life
Astrocrinus lacks a stem. An excellently preserved example of A. benniei illustrated by Macurda (1977, pl. 32, fig. 7) with an undamaged aboral surface shows no sign of a stem attachment. The aboral surface is covered with the minute spines that characterize the genus. All researchers have assumed that Astrocrinus was free living. Its occurrence in the shales above the top two limestones within the Lower Limestone Group of Scotland is not surprising. Presumably the shales accumulated in relatively quiet waters where Astrocrinus would have been able to maintain its position on the seafloor and feed without being swept away by vigorous currents. The environment at Grassington is quite different. Although most specimens were discovered loose, virtually all the limestones in the immediate vicinity are biosparites with the bioclasts predominantly pelmatozoan plates (Fig. 6). Fine-grained sediment was not accumulating in this vicinity at the time and the other blastoids, Codaster and Orbitremites are typically found in association with local highs. Furthermore, Joysey described the blastoids as occurring in thin, discrete beds, separated by sharp boundaries from blastoid-free crinoidal limestone. He also described the crinoid debris in the blastoid beds as being ‘ungraded and unsorted, and containing occasional complete calices, and minute fragments of arms and pinnules’ (Joysey 1955, p. 213). Both observations and the association of blastoids with ‘reef knolls’ all suggest rapid deposition in a high-energy environment in which a small, free-living blastoid such as Astrocrinus would have had difficulty in avoiding either being swept away or buried. The other pelmatozoans are adapted to a rheophilic environment. Even if the environment suffered from sediment starvation, it still experienced high energy currents.
Surface of blastoid horizon from the Middle Limestone, near Bare House, Grassington, Yorks, showing many Orbitremites calices and other echinoderm debris. Coin is 23.5 mm across.
The most likely explanation is that occasional examples of Astrocrinus could become securely wedged aided by their spiny surfaces. If this occurred at the base of the stems of a ‘forest’ of crinoids, the energy of the immediately surrounding environment might well have been largely baffled, as suggested by Breimer and Macurda (1972, p. 300) for small blastoids generally. Traditionally, filter feeders benefit from being elevated in the water column, thus gaining first access to suspended food particles. However, Astrocrinus has very narrow ambulacra, with concomitantly narrow food grooves (about 0.1 mm across at maximum). It probably exploited minute food particles that would not be filtered out by the crinoids above. Presumably, occasional examples were able to find a protected niche in which they could survive. The stellate thecal outline and low profile would have helped stabilize the small theca (Koverman and LaBarbera 2004).
Breimer and Macurda (1972, p. 306) assigned Astrocrinus to their life style type IV, in which blastoids lived with the mouth directed upwards (assuming a horizontal surface). In this orientation, when the D ray brachioles were elevated they would have interacted with any currents, acting like a sea anchor or weathervane. The unattached theca could have rotated, which would bring the AB interray to face the currents. Such an orientation also brings the other four ambulacra so the plane of their curvature is more nearly perpendicular to the supposed current direction. The C and E ambulacra lie at the back of their radial sinuses with respect to the supposed current direction. They twist along their length so that more distal brachioles would be perpendicular to the supposed current. All three features would have enhanced feeding efficiency.
The spines are asymmetrical and consistently orientated within each plate. Those on the basal surface would have facilitated movement parallel to the spine elongation in one direction, but strongly resisted the opposite motion. In A. benniei the basal spines on the A and B radials are aligned at about 30° to the AB–D ray symmetry plane. They would have allowed movement towards the D ray, but resisted motion in the opposite direction. Similarly, the basal has spines parallel to the symmetry plane that would have allowed motion towards the D radial, but not the other way. The C and E radials have spines orientated at right angles to the symmetry plane, but the very long C and E ambulacra mean these radials are slightly concave and would have had minimal contact with the underlying substrate. If the D ray brachioles did act as a sea anchor and tended to drag the theca backwards in a strong current, the basal spines are orientated in such a way as both to allow the movement and bring the symmetry plane into line with the current direction. Thus, it seems the development of the spinose ‘ornament’ and the loss of the stem are related and may both have enhanced the feeding efficiency of such a small blastoid in the orientation and environment that Breimer and Macurda envisioned for it.
Acknowledgements
CRCP is grateful to M. Riley, Sedgwick Museum, Cambridge and T. Ewin, The Natural History Museum, London, for answering queries about specimens in their care and to C. Waters, University of Leicester, for information on Carboniferous stratigraphy in Yorkshire. G. Sevastopulo, Trinity College, Dublin, Ireland, provided useful discussions and information on Irish Astrocrinus. M Simms and S. K. Donovan provided helpful reviews of the original manuscript.
Author contributions
CRCP: conceptualization (lead), investigation (lead), methodology (lead), writing – original draft (lead); DBM: validation (lead), writing – review & editing (supporting); JJS: data curation (equal), validation (equal), writing – review & editing (supporting).
Funding
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.
Data availability
Data sharing is not applicable to this article as no datasets were generated or analysed during the current study.
Scientific editing by Paul Wignall
- © 2021 The Author(s). Published by The Geological Society of London for the Yorkshire Geological Society. All rights reserved
The lower Carboniferous blastoid Astrocrinus tetragonus (Austin and Austin), from Grassington, Yorkshire, and the validity of Astrocrinus benniei (Etheridge)
