Terminology

Terminology follows Spencer and Wright (Reference Spencer, Wright and Moore1966) and Blake (Reference Blake2018).

Caveats

Falloaster anquiroisitus is known from a single small specimen, the disk largely missing and the arm remnants disrupted and variably weathered. In one sense, only a single surface is exposed, but crucial to evaluation, aspects of both dorsal and ventral features are observable because weathering at the outcrop differentially removed overlying sediment to reveal some ossicles essentially in dorsal aspect (Fig. 1.4, 1.6) while others were being eroded to a submedial interval to expose aspects of morphology nearly as would be seen in ventral view (Fig. 1.5). Still other ambulacrals were completely lost, leaving only ossicular impressions on the underlying sediment (Fig. 2.6, to left). Because the specimen was eroded down from above, as is documented by exposure of intact, complex dorsal surfaces of the ambulacrals (Fig. 1.4), the false ventral exposures are placed in quotes for convenient reference, as ‘ventral.’

Given the rarity of articulated echinoderms in the Garden City Formation, discovery of addition specimens in the near future is unlikely, and therefore F. anquiroisitus is documented here despite availability of only a single specimen.

The core significance of F. anquiroisitus is not its morphologic novelty, striking though it is, but rather the additional evidence it provides for early asterozoan diversification that has sometimes only been implicitly recognized by earlier authors (e.g., Schöndorf, Reference Schöndorf1910; Spencer Reference Spencer1914–1940; Schuchert, Reference Schuchert1915; Blake, Reference Blake1995, Reference Blake2013, Reference Blake2014; Mooi and David, Reference Mooi and David2000; Blake and Guensburg, Reference Blake and Guensburg2015).

Remarks

Although Falloaster is judged to lie outside of asterozoan class-level taxa as formulated by Blake (Reference Blake2013, Reference Blake2018), the new genus is recognized only at the family level and below because the state of knowledge is insufficient to justify new higher-level terminology.

Type and only-known genus

Falloaster n. gen.

Diagnosis

As for the type species, by monotypy.

Type species

Falloaster anquiroisitus n. gen. n. sp., by monotypy.

Diagnosis

As for the type species, by monotypy.

Etymology

Latin, fallo, deceive (at casual inspection, the only known specimen of Falloaster n. gen. appears to be a peculiar asteroid, but perhaps so only because poorly preserved) and Latin, aster, star.

Remarks

As for the type species, by monotypy.

Holotype

IMNH IP-276/849.

Diagnosis

Asterozoan with ambulacral dorsal surfaces traversed by two prominent lateral ridges separated by medial depressed area, together yielding bilateral ossicular shape. Lateral ridges together with upturned abradial and adradial margins of successive ambulacrals form elliptical surface leading to enlarged podial pore; pore not partially roofed by ledge. Ambulacral in ‘ventral’ aspect of ‘hourglass’ shape; pore where best-preserved edged by continuous circular rim. Abradial ambulacral margin sharply upturned to rim and support abactinal ossicles. No adambulacral or other virgal-series derivatives present. No ambital framework series, represented by marginal ossicles among asteroids, present. Dorsal arm surfaces on each side of arm midline formed by single series of enlarged, oblique rectangular, plate-like abactinals abutted at arm midline; configuration suggests carinal series absent. Skeleton of central portion of disk unknown. Remaining disk-edge ossicles consisting of two series, differing appearances suggesting possible partial exposure of both dorsal and ventral ossicles. Remaining arm-base ossicles similar to enlarged arm abactinals. Enlarged, semicircular madreporite(?) at disk edge.

Occurrence

Garden City Formation, Early Ordovician (Floian/Blackhillsian), Strigigenalis plicolabeona Zone (Adrain et al., Reference Adrain, McAdams and Westrop2009), Bear River Range, Bear Lake County, Idaho. Further locality data available from the Idaho Museum of Natural History.

Description

Disk large relative to arm length, although specimen distended with compaction, partially obscuring shape. Disk apparently domal in life and arms subcylindrical, on the basis of probable loss of dorsal disk ossicles with retention of some interbrachial-edge ossicles and overall arm configuration. Interbrachial arcs probably narrowly rounded in life. Arm outline weakly rounded to nearly straight-sided, outline curving toward more abruptly rounded arm tips; overall outlines of arms suggest flexibility in life.

Dorsal skeletal morphology of entire arm length unavailable. Single series of enlarged, oblique rectangular, plate-like ossicles extends from arm midline to arm margin. Two arm-to-disk junctions retain similar plate-like, vaulted ossicles; abutment line aligned with arm midline, pair vaulted and aligned at arm midline, overlapped laterally by two or three similar ossicles (Fig. 2.4, ‘v’). Positioning of plate-like ossicle at arm midline supports inferred absence of carinal series.

As preserved, ossicles of successive interbrachial arcs differ: three interbrachia edged by robust, approximately rectangular ossicle with triangular reentrant opening toward disk interior (Fig. 2.1, ‘n’). Possible madreporite (‘md’) enlarged, irregular, semicircular, distal to and abutting adjacent interbrachial ossicle. One interbrachium exposes double, separated rows or cycles of multiple? robust rectangular ossicles of slightly irregular outline; one interbrachium appears to retain portion of cycle toward disk center (Fig. 1.3). Disparity of interbrachial ossicles arguably represents both dorsal and ‘ventral’ disk margins, but identification of surfaces is unclear because vaulted arm base abactinals abut both interbrachial ossicular types (Fig. 1.3, ‘southeast’ interbrachium).

Ambulacral ossicles vaulted; ossicles on opposite sides of arm midline slightly offset; cross-furrow articulation surfaces at most finely pitted, suggesting soft tissue linkage rather than interlocking skeletal tooth-and-socket arrangement as typical of asteroids. Ambulacrals robust, hourglass-shaped in overall outline; shape defined by enlarged podial pores. Sequential ambulacrals appear to have abutted along upright edges rather than overlapped. Dorsal surfaces of ambulacrals bordered laterally by transverse ridges that merge toward arm midline; abradial margins of ambulacrals upturned; depressions between transverse ridges of each ossicle floored by ossicular material. In ‘ventral’ aspect, remaining ossicular material largely leached, but thickened abradial edge suggests denser skeletal material at ossicular margin (Fig. 2.2, 2.4, 2.5, ‘r’); pores bordered by semicircular rim that together with transverse ridges and upturned edges of dorsal surfaces likely supported podial tissues. In ‘ventral’ aspect, margins of podial pore forming walls of large water vascular radial channel; degree of ‘ventral’ closure of channel in life unknown due to sediment compaction, splaying of channel, and outcrop weathering. No indication of transverse channel to podium, as in Asteroidea, remains. No adambulacral or other virgal series ossicles or ambital framework ossicles developed. No accessory ossicular material (e.g., granules, spinelets) preserved.

Etymology

Latin, anquiroisitus, seek on all sides, inquire into; in reference to the new species anthropomorphically ‘seeking’ an asteroid organization that it did not attain.

Remarks

Among classes, morphology of F. anquiroisitus is deemed most suggestive of that of the Asteroidea, although the evolutionary significance of the similarities, whether plesiomorphic or homoplastic, is not known. It is also unknown whether similarities might have emerged before or after evolution of robust, readily preserved skeletons within the asterozoan complex (Blake and Guensburg, Reference Blake and Guensburg2015, p. 484). Exemplifying morphologic complexities and uncertainties, absence of virgal-series derivatives and marginals is emphasized for Falloaster, although virgal derivatives were not recognized in problematic Catervaparmaster Blake, Reference Blake2000 (see also Blake and Guensburg, Reference Blake and Guensburg2015), and among Paleozoic asteroids, marginals are not recognized in Illusioluidia Blake and Guensburg, Reference Blake and Guensburg1989, although because of the proportionately small, uniform ossicular sizes, final determination is difficult for that genus.

Falloaster anquiroisitus is diagrammatically compared with the order Euaxosida sensu Blake (Reference Blake2018) (Fig. 3). Unless genuinely absent in life, central disk ossicles were lost from the F. anquiroisitus specimen, and evaluation of ossicles of the interbrachial rim is problematic. The two arms of Figure 1.1, 1.2, pointed ‘southeast’ and ‘southwest,’ appear contradictory; however, both ‘ventral’ and dorsal ossicular morphologies are exposed, as noted, the appearances resulting from specimen undulation at the time of burial, sediment compaction, and differential weathering, the differences in horizon in the living individual perhaps as little as a millimeter (see figure scales). The arm pointing ‘southeast’ is weathered deeply enough to expose the splayed water vascular radial channel as well as the hourglass-shaped ambulacrals, which are to the ‘southwest’ and ‘northwest’ of the dorsal surfaces of the ambulacrals, whereas most ossicles were lost from the remaining two arms. The areas both within the specimen periphery and beyond expose sedimentary matrix.

In life, the dorsal disk of many asteroids is a granular mosaic of many ossicles that in fossilization can yield a texture at least superficially suggestive of that of the F. anquiroisitus specimen; however, the pitted appearance of the matrix beyond the specimen is similar to that of the interior, indicating that the pitted surface is a product of weathering rather than skeletal remnants or impressions of remnants. Although the disk edge of the F. anquiroisitus specimen retained some ossicles, any central disk ossicles were lost. Many asterozoans are domal in life, as probably was F. anquiroisitus, which could have led to taphonomic loss of dorsal ossicles. Alternatively, the disk might have collapsed toward the mouth frame, where ossicles might remain hidden by sediment. This interpretation is deemed less probable because at the time of death, the sediment was firm enough to retain ambulacrals in a vaulted position. A third possibility is that the dorsal disk was at most only weakly calcified, but this interpretation is not favored because of the overall robust nature of the remaining skeleton.

Because of preservational limitations, interpretation of F. anquiroisitus ultimately must emphasize the two best-preserved arms, and absence of both virgal-series derivatives (e.g., adambulacrals) and of ambital framework series in the arms is important. Although much has been lost, much remains, with neither arm configuration nor skeletal fragments providing suggestion of either adambulacrals or marginals. Configuration of the arm margins provides positive data. There are no suggestions of flattening or faceting of the recurved abradial surfaces of ambulacral ossicles that would argue the presence of a marginal series. In addition, among asteroids, articulation structures between ambulacrals and adambulacrals are always differentiated, but the narrow, resistant (denser stereom?) ‘ventral’ abradial ambulacral edges here provide no such suggestion. Podial openings are readily recognized in ‘ventral’ aspect (Figs. 1.5, 2.3) although in dorsal aspect (Figs. 1.6, 2.4), sediment fill is similar to that found in the floored spaces between the transverse ridges of the ambulacral, thereby obscuring the pores in this view. Overall configuration of both ambulacral ossicles and what remains of the disk edge is not found elsewhere among Asterozoa.

Although ambulacral ossicles are paired across the arm midline in later asteroids, pairing is weak in somasteroids, in earlier asteroids, and in Falloaster.

All known asterozoans share a central ventral mouth frame. Although the disk skeleton of the F. anquiroisitus is incomplete, there is no reason to argue any departure from the norm. The abradial edges of ambulacral ossicles appear to closely abut the disk-margin ossicles in at least two places (Figs. 1.3, 2.2–2.6), thereby arguing continuity in life; however, such continuity would imply a construction otherwise unknown among asterozoans, requiring assignment of ambulacral and body wall ossicles to a single ossicular series. Thus, taphonomic displacement is the likely cause of observed ossicular positioning.

Ambiguity also surrounds interpretation of the plate-like ossicles at the disk edges of two of the arms (Figs. 1.3, 1.4, 1.6, 2.1, 2.4, 2.5). The ossicles are similar in form and orientation to the single ossicle that remains in place more distally on one arm (Fig. 2.4, ‘d’), and two or three similar ossicles overlap the two at the disk edge (Fig. 2.4, the paired ‘v’). Nevertheless, as preserved, they are also overlapped by the edges of the adjacent ambulacral ossicles, the abactinal ossicular interpretation demanding their deflection beneath the ambulacrals, which is judged reasonable given specimen flattening with sediment compaction. Ossicles of the adjacent arm, although less complete, are similar, their position more in accord with interpretation as part of the dorsal skeleton. An alternative explanation, that the pair are mouth-angle ossicles, is rejected in part because of the arguments presented and because the pair are at the disk/arm juncture, not at the asterozoan mouth-frame site medially on the disk. Further, the two ossicles are enlarged, whereas mouth-angle ossicles of asteroids typically are proportionately small. Vaulting orientation is incorrect for a mouth-angle ossicle pair, and there is no suggestion of the mouth-edge podia found in many asteroids.

The preceding treatment has stressed direct and comparatively narrow comparison of ossicular appearances, but a second approach to two problematic aspects of the morphology of Falloaster begins with the echinoderm skeletal analyses developed by Mooi and David (Reference Mooi and David1998, Reference Mooi and David2000, Reference Mooi and David2008) in which the axial skeleton, consisting of the ambulacral sequence, is fundamentally distinct from the remainder, the various extraxial elements.

At the interbrachial arcs of the Falloaster, ambulacrals now physically abut the interbrachial arc ossicles (Fig. 2.2, 2.5), suggesting a single ossicular series, which in turn would argue that the arc ossicles are a part of the axial series. Because disk ossicles of the Falloaster are so incompletely known, and Falloaster is argued to be a representative of a clade aside from the four recognized classes, such a hypothesis cannot be simply or wholly dismissed. Nevertheless, treatment of the occurrences as products of taphonomic alteration is both reasonable and in accord with axial-extraxial separation. (However, ossicular juxtaposition raises unresolved questions concerning Falloaster skeletal and body configuration in life: What was overall life configuration such that the ambulacral ossicles were preserved aligned with disk edge ossicles?)

Second, the vaulted plates at the disk edge (Fig. 2.4, ‘v’) are similar in form to the remnant dorsal arm ossicle (Fig. 24, ‘d’), and one of the group also abuts an interbrachial arc ossicle (Fig. 24, ‘b’), thereby also arguing an extraxial rather than an axial status for these ossicles.