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Revista de Biología Tropical, ISSN electrónico: 2215-2075 Vol. 69(S1): 404-422, March 2021 (Published Mar. 10, 2021)

Morphology of endoskeleton and spination in the sea star

Midgardia xandaros (Brisingida: Brisingidae) from the Gulf of Mexico

Brenda Lizbeth Esteban-Vázquez

Magdalena De los Palos-Peña

Francisco Alonso Solís-Marín

Alfredo Laguarda-Figueras

1. Facultad de Ciencias, Universidad Nacional Autónoma de México, Ciudad de México, México; brendaestebanv@

gmail.com (*Correspondence).

2. Posgrado en Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México, Ciudad de México, México;

mdelospalos@ciencias.unam.mx

3. Colección Nacional de Equinodermos “Dra. Ma. E. Caso Muñoz”, Laboratorio de Sistemática y Ecología de

Equinodermos, Instituto de Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México, Ciudad de

México, México; fasolis@cmarl.unam.mx, laguarda@cmarl.unam.mx

Received 15-VII-2020. Corrected 10-IX-2020. Accepted 26-X-2020.

ABSTRACT

Introduction. The deep-sea asteroid species of Brisingida have a nearly global distribution but have remained

poorly understood due to their deep bathymetric distributions and fragile skeletons. Objective. To describe the

external and internal morphology of Midgardia xandaros including the skeletal arrangement, through multifocal

and SEM techniques. Methods. We examined a total of 21 specimens, including 27 arm fragments, from the

Gulf of Mexico and Honduras. Two specimens were dissected. Results. Detailed descriptions of pedicellariae,

abactinal, intercostal, inferomarginal, adambulacral, ambulacral, odontophore, and oral ossicles, and their spines

are provided, emphasizing the articulations and muscle attachments. C-shaped valves pedicellariae and small

pedicellariae valves with shorter denticulation areas were recognized. Conclusions. The morphological descrip-

tion of M. xandaros is expanded, providing the most extensive description of abactinal, first adambulacral, first

and subsequent inferomarginal ossicles, abactinal spines, and C-shaped, crossed pedicellariae, as well as the

distal arm plates, for a brisingid species using SEM to date.

Key words: ossicles; oral frame; pedicellariae; spines; deep-sea.

Esteban-Vázquez, B.L., De los Palos-Peña, M., Solís-

Marín, F.A., & Laguarda-Figueras, A. (2021).

Morphology of endoskeleton and spination in the sea

star Midgardia xandaros (Brisingida: Brisingidae)

from the Gulf of Mexico. Revista de Biología

Tropical, 69(S1), 404-422. DOI 10.15517/rbt.

v69iSuppl.1.46381

Brisingids are deep-sea asteroids that have

a body shape similar to ophiuroids due to their

small, circular disk, which is clearly differenti-

ated from their six to twenty, long and slender

arms. They also resemble crinoids when they

raise their long, spined arms into the water col-

umn for suspension feeding (Clark & Downey,

1992; Mah & Blake, 2012; Gale, Mah, Hamel

& Mercier, 2014).

The species of Brisingida Fisher, 1928

have a nearly global distribution but have

remained poorly understood due to their deep

bathymetric distributions and fragile skeletons.

They were initially considered as one family,

but after comparison of all the genera from

the Atlantic waters, two well-defined families

were designated: Brisingidae Sars, 1875 and

Freyellidae Downey, 1986 (Downey, 1986;

Clark & Mah, 2001; Gale et al., 2014).

The family Brisingidae is defined by lack-

ing bare interradial plates on the disk, and

having arms constricted at their base whose

DOI 10.15517/rbt.v69iSuppl.1.46381

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abactinal plates form costae, and includes the

genus Midgardia Downey, 1972 (Downey,

1986; Zhang, Wang, Zhou, & Zhang, 2019).

This genus is distinguished by having 11 to 13

deciduous arms, constricted at their base, and

acute interradial arcs; bare interradial plates

absent on the disk and numerous, remarkable

costae made up of imbricate plates (Downey,

1986). It includes only one species, Midgardia

xandaros Downey, 1972, whose type locality

is Veracruz, Mexico (19°2’36” N & 95°27’30”

W, 457.2 m), and has been reported for the

Gulf of Mexico and Honduras (366 to 475

m) (Downey, 1972; Pequegnat, Gallaway &

Pequegnat, 1990; Clark & Downey, 1992;

Durán-González, Laguarda-Figueras, Solís-

Marín, Buitrón-Sánchez & Torres-Vega, 2005;

Solís-Marín et al., 2013).

In the 19th century, asteroid ossicles start-

ed to be described as part of taxonomic studies

(e.g. Agassiz, 1877; Viguier, 1879; Ludwig,

1897), providing the foundations and terminol-

ogy for future skeleton morphological works.

In particular, Sars (1875) described Hymeno-

discus coronata (= Brisinga coronata), giv-

ing details and illustrations of the skeleton,

pedicellariae, spines, muscles, nervous and

digestive systems, as well as notes about its

regenerative capacity and ontogeny. Until now,

Sars (1875) work has been the most extensive

description for a brisingid species.

Subsequent taxonomic studies about bris-

ingids have largely been based on external

morphology (e. g. Fisher, 1917; Fisher, 1918;

Fisher, 1928; Baranova, 1957; Tortonese, 1958;

Downey, 1972; Downey, 1973; Downey, 1986;

Clark & Downey, 1992; McKnight, 2006; Bena-

vides-Serrato, Borrero-Pérez, & Díaz-Sánchez,

2011; Mah, 2016; Zhang et al., 2019; Zhang,

Zhou, Xiao, & Wang, 2020). Meanwhile, Spen-

cer and Wright (1966), Blake (1987) and Gale

(1987) mainly provided insights into the skele-

tal morphology of brisingids at an ordinal level,

emphasizing the arrangement of the ossicles of

the oral frame.

Regarding pedicellariae morpholo-

gy, Emson and Young (1994) provided the

first Scanning Electron Microscopy (SEM)

photographs of brisingid pedicellariae, describ-

ing them in great detail and making morpho-

logical comparisons with Labidiaster annulatus

Sladen, 1889 and Stylaterias forreri (deLo-

riol, 1887). Also, Mah (1999) in his taxonom-

ic work of Brisingaster robillardi (deLoriol,

1883), showed SEM photographs of pedicel-

lariae for this species and Novodinia helenae

Rowe (1989). Finally, Vickery and McClintock

(2000) described the tube-feet of brisingids as

semi-flat-tipped and non-suckered, proposing

their morphology as a taxonomic character at

an ordinal level.

Recently, the phylogenetic studies of Gale

(2011) and Fau and Villier (2019) provided

well-described ossicles of brisingids, using

SEM techniques, and updating the skeletal

terminology for asteroids in general. Likewise,

Fau and Villier (2018) described the first ambu-

lacral ossicles of B. robillardi. However, they

showed isolated ossicles from different brisin-

gid species; therefore, the complete description

of the skeleton of a brisingid species is still

needed. Here, we describe the external and

internal morphology of Midgardia xandaros

Downey, 1972, including the skeletal arrange-

ment, through multifocal and SEM techniques,

also providing additional notes about the gen-

eral morphology of a brisingid.

MATERIALS AND METHODS

Study area: The Gulf of Mexico is a

semi-enclosed ocean basin located between

Cape Sable, Florida, United States and Cabo

Catoche, Quintana Roo, Mexico. It has a

shoreline extension of 5 696 km and a mean

water depth of 1 615 m (Fautin et al., 2010;

Turner & Rabalais, 2019). It also receives a

vast sediment discharge from the Mississippi

Delta, and other rivers such as Coatzacoalcos

and Papaloapan, making the deep-sea floor ter-

rigenous and muddy. In contrast, the platforms

of Florida and Yucatan Peninsula have carbon-

ate sediments (Armstrong-Altrin et al., 2015;

Ramos-Vázquez, Armstrong-Altrin, Rosales-

Hoz, Machain-Castillo & Carranza-Edwards,

2017; Ward & Tunnell, 2017). The deepest

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area of the Gulf of Mexico is the Sigsbee Abys-

sal Plain which ranges from 3 750 to 4 400 m

(Fautin et al., 2010; Turner & Rabalais, 2019).

The water circulation is mainly given by

the Loop Current that originates at the Yucatan

Channel, which connects the Gulf of Mexico

with the Caribbean Sea and is a semi-enclosed

basin located between 9° N and 22° N latitude

and 60° W and 89° W longitude. The Caribbean

deep-sea has many small canyons, seamounts,

and table mounts, such as the Explorer Bank,

located off Honduras (Miloslavich et al., 2010;

Palanisamy, Becker, Meyssignac, Henry, &

Cazenave, 2012; Ward & Tunnell, 2017).

Specimen examination: A total of 21

specimens and 27 arm fragments deposited at

the National Echinoderm Collection “Dra. Ma.

Elena Caso Muñoz” (ICML-UNAM) and the

National Museum of Natural History, Smith-

sonian Institution (USNM), and previously

collected in the Gulf of Mexico and Honduras,

were examined through a stereoscopic micro-

scope (Olympus SZX7).

The following measurements were taken

for each specimen: minor radius (r), disk diam-

eter (D

) and disk height (D

), if possible,

using a digital caliper (TRUPER Caldi-6MP).

Photographs of the main morphological struc-

tures and spination were taken with a multifo-

cal microscope (Leica Z16 APO A; Olympus

SZX-12 MDU) at Laboratorio de Microscopía

y Fotografía de la Biodiversidad II, Insti-

tuto de Biología (IB), UNAM, and Imaging

Scanning Electron Microscopy Laboratory,

Smithsonian Institution.

SEM samples: Two specimens were dis-

sected according to the method described by

Fau and Villier (2018). Fragments of arms, disk

and oral frame, as well as spines and pedicel-

lariae, were immersed in a dilute solution of

household bleach (20-40 %) followed by sev-

eral rinses in sterile water and alcohol (70 %),

dried, mounted and gold-coated on aluminum

stubs. The samples were visualized on a Scan-

ning Electron Microscope (HITACHI SU1510;

JEOL JSM6360LV) at the Laboratorio de

Microscopía y Fotografía de la Biodiversidad

I, IB, UNAM, and the SEM Academic Service,

ICML, UNAM.

The description of ossicles, spines and

pedicellariae follows the terminology proposed

by Chia and Amerongen (1975), Clark and

Downey (1992), Emson and Young (1994),

Gale (2011), Fau and Villier (2018), and Fau

and Villier (2019) (Appendix 1).

RESULTS

Systematics

Midgardia xandaros Downey, 1972

Midgardia xandaros Downey, 1972: 422-425;

Downey, 1973: 99; Downey, 1986: 19-20;

Clark & Downey, 1992: 470-471.

Description: The 21 specimens and 27

arm fragments have the following range in

dimensions: r = 13-19.3 mm, D

= 29-38.2

mm, and D

= 5-8.9 mm. Due to the lack of

complete specimens, R was not measured,

except for the holotype (R = 680 mm).

Disk small, round, flat. The abactinal

membrane of the disk is thin and delicate, so

stomach fragments may be visible abactinally

(Fig. 1A, Fig. 1B, Fig. 1C, Fig. 1D). Abactinal

plates are small, irregularly oval-shaped, and

irregularly scattered over the membrane, with

a central, elevated boss which holds a tiny,

hyaline, round based, acute spinelet whose tra-

beculae are not well-defined (Fig. 1E, Fig. 1F,

Fig. 1G). Occasionally, there are two spinelets

per abactinal plate. Madreporite in interradial

position, small respect to the disk, hemispheri-

cal, tumid, raised. Two round papulae occur

at the base of each arm (Fig. 1H). The anus is

inconspicuous, surrounded by longer abactinal

spinelets (Fig. 1I, Fig. 1J).

Arms 11 to 13 are long, thin, and fragile,

except in the robust gonadal region. Two paired

gonads with up to five saccules. This region

has numerous irregular, narrow, well-marked

costae; some rows may be incomplete (Fig.

2A, Fig. 2B). Each costae row is made up of

irregular, elongated, abactinal plates imbricated

by two costcost articulations. Each abactinal

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Fig. 1. Midgardia xandaros Downey, 1972. Body morphology and disk. A. Holotype (abactinal view). B. Holotype (actinal

view). C. Abactinal surface of the disk. D. Actinal surface of the disk. E. Abactinal ossicles of the disk. F. Abactinal ossicle

of the disk. G. Abactinal spinelet of the disk. H. Madreporite and papulae. I. Anus. J. Anus surrounded by spinelets. (USNM

11420: A-B; ICML-UNAM 2.196.2: C-I; ICML-UNAM 2.196.0: J).

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Fig. 2. Midgardia xandaros Downey, 1972. ICML-UNAM 2.196.2. Ossicles and spines from gonadal region. A. Costae

rows. B. Gonads. C. Costae (abactinal view). D. Costae spinelet. E. Intercostal (abactinal view). F. Inferomarginal spines.

G. Arm bare (abactinal view). H. Inferomarginal (abactinal view). I. Inferomarginal (actinal view). J. Inferomarginal

tubercle-shaped (abactinal view). K. Inferomarginal spine. L. Ambulacral (abactinal view). M.: Ambulacral (actinal view).

N. Ambulacral wings (actinal view). O. Arm (actinal view). P. Arm bare (actinal view). Q. Adambulacral (actinal view).

R. Adambulacral (abactinal view). S. Adambulacral spine. T. Subambulacral spine. In aqua (dashed): actam; blue: padam;

green: imabt; green (dashed): lim; green (solid): ambulacral; orange: pada; pink: imim; pink (dashed): interadam; purple:

abtam; red: dadam; white: dada; yellow: amim; yellow (dashed): lia; yellow (solid): inferomarginal.

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plate has a low boss and an acute spinelet with

a thick base that is shorter than the spinelets on

the disk (Fig. 2C, Fig. 2D). The robust, tumid

costae plates are joined to inferomarginal plates

by imabt. Between costae rows, there are small,

asymmetrical, flat, fragile intercostal plates,

and tiny, round granules embedded in the abac-

tinal membrane (Fig. 2E).

Inferomarginal plates constitute the margin

along the arms; proximally, they are wider than

long and have a large, downwards boss with an

oval spine pit; beyond the gonadal region, they

are shorter like a tubercle. They are joined with

the following inferomarginal plate through

imim, and the ambulacral plates by amim (Fig.

2F, Fig. 2G, Fig. 2H, Fig. 2I, Fig. 2J). One

inferomarginal spine very long, prominent,

acute, hyaline, laterally adpressed and fragile,

so frequently it is broken (Fig. 2K). Beyond the

gonadal region, instead of costae rows, there

are transversal bands of diminutive pedicel-

lariae, and the abactinal membrane is thinner,

making the adambulacral and ambulacral plates

more visible.

Ambulacral plates are longer than wide,

have a round top, with a shaft slightly waisted

whose stereom is labyrinthic. The base has

a semitriangular amim articulation and two

very short, curved wings (Fig. 2L). Actinally,

the plates possess two areas of short teeth and

moderately broad abtam surface at the head, a

shallow furrow and small, oval-shaped actam

area. The wings are in contact with adam-

bulacral plates through dadam and padam.

Laterally, two ambulacrals of the same column

are joined by lim and its lia articulation (Fig.

2M, Fig. 2N). Superambulacral plates absent.

Ambulacral feet biseriate, long, robust, with a

small sucker (Fig. 2O).

Adambulacral plates small, high, slightly

longer than wide (Fig. 2P). Abactinally, they

have a curved top, a conspicuous, diagonal

crest which separates padam and dadam mus-

cles, as well as dada and pada articulations,

allowing them the attachment with ambulacral

plates. The adradial extension is quite short and

angular. In actinal view, two adjacent adambu-

lacral plates of the same row along the arm are

joined laterally by interadam. Their ambulacral

margin is slightly acute and has three or four

small, shallow bosses for small, elongate, thin,

tubercle base, acicular adambulacral spines in

both proximal and distal edges (Fig. 2Q, Fig.

2R, Fig. 2S). One subambulacral spine long,

acicular and more robust than adambulacral

ones, with well-defined, straight trabeculae

(Fig. 2T). The ambulacral groove is moderately

broad. Actinal plates absent.

The most distal ambulacral, adambulacral

and inferomarginal plates are smaller and their

shape is modified (Fig. 3A). The ambulacral

plates have a well-marked shaft, making an

apex longer than wide, almost triangular, and

the base has short, angular but more noticeable

wings. In actinal view, the ambulacral plates do

not show remarkable teeth (Fig. 3B, Fig. 3C).

In respect to the adambulacral plates, they are

longer than wide, with an inconspicuous inter-

radial extension, less extensive interadam and

more curved adambulacral furrow, having only

a subambulacral boss and poorly developed

crest (Fig. 3D, Fig. 3E). In contrast, the infero-

marginal plates continue decreasing in size and

the boss occupies almost all its abactinal sur-

face, having a well-marked spine pit (Fig. 3F).

The rigid oral frame is composed of the

two first ambulacral plates, and two oral plates

joined by an odontophore, forming the contour

of the disk (Fig. 3G, Fig. 3H, Fig. 3I, Fig. 3J,

Fig. 3K, Fig. 3L).

First ambulacral plate long, high, narrow

with a curved apex, and base without wings

since there is no attachment with adambulacral

plates (Fig. 3M). Two adjacent first ambulacral

plates are joined through their radial surface

which has the lim and its respective lia on a

short process. In contrast to the ambulacral

plates from the gonadal region, the abtam and

teeth area are wider, and the actam is shallower

(Fig. 3N, Fig. 3O). Interradially, they have an

expansive, flat area for procoam and procoa,

a diagonal bridge, rugous dicoa and dicoam

areas, and smaller doda (Fig. 3P).

The oral plate is longer than the first ambu-

lacral plate, with a curved, laterally flattened

ramus, and contracted in the middle of the

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Fig. 3. Midgardia xandaros Downey, 1972. Distal arm ossicles: A-F. Oral frame ossicles and spines: G-T. A. Ambulacral

furrow. B. Ambulacral (abactinal view). C. Ambulacral (actinal view). D. Adambulacral (actinal view). E. Adambulacral

(abactinal view). F. Inferomarginal (abactinal view). G. Arm base (abactinal view). H. Odontophore insertion. I. Fragment

of the oral frame (abactinal view). J. Arm base (actinal view). K. Fragment of the oral frame (actinal view). L. Oral and

first ambulacral (actinal view). M. First ambulacral (actinal view). N-O: First ambulacral (radial view). P. First ambulacral

(interradial view). Q. Oral (actinal view). R. Oral (abactinal view). S. Oral (radial view). T. Oral (interradial view). In

aqua (dashed): actam; blue: procoa; blue (solid): oral; brown: odontophore; green: dicoam; green (dashed): lim; green

(solid): second ambulacral; orange: procoam; orange (dashed): abiim; pink: dicoa; pink (solid): first ambulacral; purple:

abtam; purple (dashed): iioa; red: poda; red (solid): first ambulacral; white: aciim; white (dashed): riom; yellow: doda;

yellow (dashed): lia; yellow (solid): first inferomarginal. (ICML-UNAM 2.196.2: A-G, J, L-N, P-T; ICML-UNAM

2.196.0: H, K, O).

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plate; the plateau is longer than wide, exten-

sive and has three shallow furrows for rng and

rvg, and the body is short, quite concave inter-

radially, oval (Fig. 3Q). In abactinal view, the

ramus has a curved top and a prominent, raised

platform; the plateau resembles wings because

of their lateral extension, and the body has a

narrow projection raised from the midradial

oral line (Fig. 3R). In radial view, the ramus

is very prominent, having an extensive area

for procoa and procoam, and the body has

quite a deep furrow; laterally, there is a narrow

surface for dicoa and dicoam, and the elevated

process which holds the riom (Fig. 3S). Inter-

radially, oral plates are joined by abiim, iioa

and aciim, showing numerous gyri; they also

have a depth orifice of odontophore attachment

(odom) whose poda is quite flat, with shallow

rng and rvg furrows (Fig. 3T).

The odontophore is longer than wide, with

an apical, triangular-shaped shelf, and con-

tracted in the middle of the plate. It articulates

with oral plates through odom and poda and

first ambulacral plates by only doda; it also

has a rectangular base where odomim joins

with inferomarginal plates. First inferomarginal

plates semitriangular, quite concave (Fig. 4A,

Fig. 4B, Fig. 4C, Fig. 4D, Fig. 4E).

Three or four long, thin, blunt, oral spines

with a round base are present which are insert-

ed both in the middle and distal edge of the oral

plate; their trabeculae are long, delicate (Fig.

4F, Fig. 4G). One suboral spine is longer, thin,

acicular, with well-marked, moderately wide

trabeculae and more robust than the suboral

ones (Fig. 4H).

The second ambulacral plates are narrower

than the first ambulacral ones and have very

short wings, almost inconspicuous (Fig. 4I,

Fig. 4J, Fig. 4K). Meanwhile, the first adam-

bulacral plates are modified into an angular,

elevated ossicle, having an apical, conspicu-

ous boss for subambulacral spines (Fig. 4L,

Fig. 4M, Fig. 4N).

Large crossed pedicellariae are scattered

over the abactinal surface of the disk and

ambulacral furrow (Fig. 4O, Fig. 4P, Fig. 4Q).

Their valves are C-shaped with three to four

canines at the top, an oval, deep diastema, up

to 25 diminutive teeth in the medial projection,

and long, spatulate base. The canines of both

valves correspond to each other. Small abd and

concave add surfaces (Fig. 4R, Fig. 4S, Fig.

5A, Fig. 5B).

Tiny crossed pedicellariae cover marginal,

oral, suboral, adambulacral and subambulacral

spines and some costae rows. They also consti-

tute the transverse bands along the arms abac-

tinally. These pedicellariae have two small,

quite curved valves with five to seven lateral

canines, a double row of median, central, short

teeth and little-marked diastema; their medial

projection has a double row of up to 13 diminu-

tive teeth, and the base is slightly spatulate and

round (Fig. 5C, Fig. 5D, Fig. 5E). Their basal

plate is longer than wide, with two short, blunt

terminal pegs and a curved articular ridge. On

the external surface of the valves, there is a

group of holes at the middle and base of the

valves where abd and add inserts (Fig. 5F, Fig.

5G, Fig. 5H).

Color in life: Bright red (Downey, 1972).

Geographic and bathymetric distribu-

tion: North of Cay Sal Bank, Florida, United

States to Honduras (366-1 280 m) (Fig. 6).

Material examined: ICML-UNAM

2.196.0 (Laguna Madre, Tamaulipas, Mexico;

24°56’3” N & 96°29’10.8” W; 760 m; N =

2; D

= 31.9 mm, dissected). ICML-UNAM

2.196.1 (Soto la Marina, Tamaulipas, Mexi-

co; 23°37’36.78” N & 97°14’30.24” W; 406

m; N = 2). ICML-UNAM 2.196.2 (Coat-

zacoalcos, Veracruz, Mexico; 18°57’107” N

& 94°20’027” W; 671.9 m; N = 2; D

= 29.4

mm, dissected). ICML-UNAM 2.196.3 (Lagu-

na Mecoacan, Tabasco, Mexico; 19°32’30”

N & 92°49’41.22” W; 561 m; N= 1). ICML-

UNAM 11525 (Laguna Madre, Tamaulipas,

Mexico; 25°3’24.06” N & 96°16’34.38” W;

995 m; N=1). ICML-UNAM 11566 (Laguna

Madre, Tamaulipas, Mexico; 24°53’50.28” N

& 96°33’34.98” W; 584 m; N=1). ICML-

UNAM 11664 (Barra Chiltepec, Tabasco,

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Fig. 4. Midgardia xandaros Downey, 1972. Oral frame and arm base ossicles: A-N. Pedicellariae: O-S. A, C: Odontophore

(actinal view). B, D: Odontophore (actinal view); E: Odontophore (interradial view); F-G: Oral spines. H: Suboral spine.

I: Fragment of oral frame (actinal view). J: Second ambulacral plate (abactinal view). K: Second ambulacral plate (radial

view). L-M: First adambulacral plates (actinal view). N: First adambulacral plate (actinal view). O: Pedicellariae of the

disk. P-Q: Large C-shaped crossed pedicellariae. R-S: C-shaped valves. In blue: odom; green (dashed): lim; green (solid):

second ambulacral; pink: odomim; pink (solid): first adambulacral; red: poda; yellow: doda. (ICML-UNAM 2.196.0: A-B,

I; ICML-UNAM 2.196.2: C-H; J-R. USNM E 11420: S).

Mexico; 19°33’16.26” N & 92°49’5.76” W;

593 m; N=3). ICML-UNAM 11748 (Tona-

la, Tabasco, Mexico; 18°58’3.48” N &

94°7’34.62” W; 710 m; N=2). ICML-UNAM

12647 (Sanchez-Magallanes, Tabasco, Mexico;

19°13’40.5” N & 93°54’51.42” W; 746 m;

N=13 arm fragments). ICML-UNAM 12667

(Frontera, Tabasco, Mexico; 19°32’49.56” N

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Fig. 6. Geographic distribution of M. xandaros. In purple: type locality (Veracruz, Mexico).

Fig. 5. Midgardia xandaros Downey, 1972. Pedicellariae. A-B: C-shaped valves. C-F: Tiny spatulate-shaped valves. G:

Tiny spatulate-shaped pedicellariae. H: Basal plate of tiny spatulate-shaped pedicellariae. In blue: abd; orange: add. (ICML-

UNAM 2.196.0: A, C, E, G; USNM E 11420: B, D, F, H).

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Revista de Biología Tropical, ISSN electrónico: 2215-2075 Vol. 69(S1): 404-422, March 2021 (Published Mar. 10, 2021)

& 92°49’18.48” W; 567 m; N=2). ICML-

UNAM 12679 (Laguna Mecoacan, Tabasco,

Mexico; 19°32’31.56” N & 93°1’29.82” W; 714

m; N=3 arm fragments). ICML-UNAM 12704

(Frontera, Tabasco, Mexico; 19°40’7.32” N

& 92°46’14.76” W; 812 m; N=11 arm frag-

ments). USNM E 11420 (Veracruz, Mexi-

co;19°2’35.442” N & 95°27’24.743” W; 475

m; N=1). USNM E 11421 (Veracruz, Mexico;

19°2’35.442” N & 95°27’24.743” W; 475

m; N=1). USNM E 20951 (North of Cay Sal

Bank, Florida, United States; 24°28’0.12” N

& 80°16’0.12” W; 802-805 m; N=1). USNM

E 20984 (Explorer Bank, Honduras; 16°40’12”

N & 82°49’47.9” W; 366 m; N=1). USNM E

34898 (South to Phleger Basin, Texas, Unit-

ed States; 27°26’24” N & 94°7’36.11” W;

1 280 m; N=1).

DISCUSSION

Midgardia xandaros is considered the star-

fish with the longest arms, being described

from a specimen with R = 680 mm by Downey

(1972). Since its designation as a new spe-

cies, only Downey (1973), Downey (1986)

and Clark and Downey (1992) added notes on

its taxonomy and phylogenetic position with-

in Brisingida from three specimens (USNM

E11420, holotype; USNM E11421, paratype;

USNM E20984, Honduras). These descriptions

were mainly based on external taxonomic char-

acters such as the shape of madreporite, arms,

abactinal, marginal, terminal, oral, ambulacral

and adambulacral plates and their respective

spines. However, detailed descriptions or illus-

trations about ossicle arrangement, spine and

pedicellariae disposition, and their structures

were not provided.

After the examination and dissection of its

skeleton, we found that the papilliform appear-

ance of abactinal plates of the disk described

by Downey (1986) and Clark and Downey

(1992) is due to the presence of a high boss and

acute abactinal spinelet. In addition, the costae

plates previously described as “bar-shaped”

are actually irregularly cruciform because

of their slightly distal and proximal costae

articulations. The numerous, irregularly round,

flattened, fenestrated, thin plates dispersed

between costae rows, as described by Downey

(1972), Downey (1973), Downey (1986) and

Clark and Downey (1992), are referred to here

as intercostal plates; their weak stereom make

them fragile and irregularly shaped.

Downey (1972), Downey (1973), Downey

(1986) mentioned that terminal plates are wider

than long, curved downwards and have cat’s

claw-like spinelets at their top. In this study,

we did not include comments related to the

terminal plates because the only specimens that

present these plates are the ones reviewed by

Downey. She also stated that marginal plates

are triangular-shaped at the proximal part of the

arms, and distally they are reduced to tubercles;

however, the proximal ones are rectangular

with short articulating areas and a robust,

downwards-pointing boss. Regarding the pres-

ence of inferomarginal spines, we also note

their absence at the basal region of the arms,

but along and beyond it, there is not a defined

pattern of attachment. They may be present in

two or three consecutive inferomarginal plates

and absent in the following one or two plates or

have an alternate presence.

Both Downey (1972), Downey (1973),

Downey (1986) and Clark and Downey (1992)

described the adambulacral plates as vertebra-

like. This appearance might be due to their

robustness, length, height and short adradial

extensions. Moreover, they considered the

ambulacral plates as T-shaped, with a narrow

waist and short, thick apex. This apex is actu-

ally the base, and even though their wings

seem to be large, SEM photos reveal their poor

development, being so short; the true apex is

high, round and the base is quite angular due

to their attachment with inferomarginal plates.

While dissociating an oral frame frag-

ment, we recognized two prominent, longer

than wide oral plates, narrower first ambulacral

plates, and a longer than wide odontophore.

Downey (1972) stated that the “calcareous

ring” is composed of the massive, tumid, fused

first ambulacrals and the oral plates, that are

smaller than the first adambulacral plates; in

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fact, the first ambulacrals are not fused, and the

oral plates are bigger than first ambulacrals and

adambulacrals. Their apparent “fusion” might

be due to the presence of thick, hard attachment

muscles both between first ambulacral and oral

plates and within them.

We also remark that the outer wall of the

disk is not made up of the second ambulacral

and second adambulacral plates together with

the first marginal plates, as Downey (1972),

Downey (1973), Downey (1986) mentioned;

instead, it is composed of the first adambulacral

plates. However, we do confirm that the second

ambulacral plates are highly compressed, as

Downey (1972) noted, having almost unno-

ticeable wings. With respect to the first mar-

ginal plates, in this study we noticed that they

are joined to the odontophore, and they do

not completely separate the first adambulacral

plates as Downey described.

Concerning pedicellariae distribution,

Downey (1972), Downey (1973), Downey

(1986) and Clark and Downey (1992) affirmed

that there are no pedicellariae on the abactinal

surface of the disk. However, we found large

crossed pedicellariae with C-shaped valves

scattered on the abactinal membrane of the

disk, which were previously only noticed in

the ambulacral furrow. Clark and Downey

(1992) provided illustrations of complete large

and tiny pedicellariae, and even though the

basal plate was not presented, the general

morphology of both pedicellariae was shown.

They illustrated C-shaped valves pedicellariae

with 17 teeth on the medial projection and an

angular base; however, SEM photos showed

that the number and development of these teeth

might be variable, having up to 25 teeth and

some marked spaces between them, and the

base is slightly rounded. Likewise, Clark and

Downey (1992) described tiny pedicellariae

with a robust appearance, and valves with an

extensive medial projection and canine area.

On the contrary, our SEM images of these pedi-

cellariae revealed valves of a slender shape and

shorter denticulation areas.

Observations on brisingids morphol-

ogy: Comprehensive descriptions about bris-

ingids ossicle, spination and pedicellariae have

been given for only a few species: Brisinga

costata Verrill, 1884; Freyella elegans (Verrill,

1884); Brisingaster robillardi de Loriol, 1883;

Novidinia helenae Rowe, 1989; Novodinia

antillensis (A. H. Clark, 1934) and Odinella

nutrix Fisher, 1940 (Ludwig, 1897; Emson &

Young, 1994; Mah, 1999; Gale, 2011; Fau &

Villier, 2018; Fau & Villier, 2019), with only B.

coronata being fully described by Sars (1875).

Due to the low availability of specimens of M.

xandaros and descriptions of full skeletons of

brisingids, a morphological comparison within

Brisingida was not carried out. Therefore, the

following comments are restricted to the gen-

eral morphology of a brisingid, adding new

observations which will be useful for future

comparative studies.

The oral frame had been considered as an

internal, immobile ring whose ossicles are a

partially or completely fused ring (Spencer &

Wright, 1966; Blake, 1987; Gale, 1987; Mah,

1999; Gale, 2011). In addition to this, Fau and

Villier (2019) stated that oral, first ambulacral

plates and odontophore are partially fused,

remarking that the odontophore is not fused

with the other two plates, and explaining that

the understanding of a fused oral frame might

be due to the difficulty of dissociating their

plates. However, later in their discussion they

mentioned that these ossicles are imbricated

rather than fused. We also observed an odon-

tophore separated from the first ambulacral

and oral plates, being the first ossicle to be

isolated, but also well-defined, separated first

ambulacrals from the oral plates for M. xanda-

ros. Therefore, we agree with them that the oral

frame ossicles are not fused.

According to Fau and Villier (2018, 2019),

the iioa articulation reinforces the rigidity of

the oral frame due to its rugose surface which

is above to aciim on the orals. The oral plate

of M. xandaros in interradial view also shows

both iioa and abiim with extensive surface, and

an actinal, narrow aciim. Likewise, the odonto-

phore has the general brisingid shape “longer

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than wide”, being attached to oral plates by

only poda mentioned by them.

Sars (1875) illustrated a fragment of the

oral frame of B. coronata, showing an attach-

ment between the “dorsal marginal plates” and

the “wedge plate” which represents the odon-

tophore. This attachment was also observed in

the oral frame of M. xandaros; therefore, we

consider odomim as the articulation joining

the odontophore with the first inferomarginal

plates. Gale (2011) stated that the axillary ossi-

cle is homologous to the odontophore, which

articulates with both oral and marginal ossicles

through ima (inferomarginal articulation on

axillary/odontophore) in the extinct Calliaster-

ella mira (Trautschold, 1879) but absent in

neoasteroid odontophores. We suggest further

study on the articulation between odontophores

and inferomarginal plates to evaluate their cor-

respondence with ima.

Fau and Villier (2019) mentioned that

Brisingida does not have an adoral carina even

the first adambulacral plates may be abutted,

here we also noticed that the first and second

adambulacral plates are abutted interradially

through the inferomarginal plates. In addition

to this, they also noted that superambulacral

and actinal plates are absent, and the crest on

the adambulacral plates separates the almost

equal expanded padam and dadam as we

observed in M. xandaros.

Articulations between abactinal plates on

the arm of asteriids and zoroasterids have

been recognized. Blake (1987) stated that one

or more articular flanges are present both

in irregular or cruciform ossicles of asteri-

ids. Likewise, node and connecting ossicles

were defined for the abactinal mesh of Aste-

rias rubens Linnaeus, 1758 which are joined

through processes (Schwertmann et al., 2019).

Fau and Villier (2019) considered these two

types of ossicles as primary and secondary

plates, respectively, remarking that primary

plates hold spines and overlap other abactinal

plates through four or more articular facets,

also called lobes, in Forcipulatacea Blake,

1987. For example, the abactinal plates of the

arms of Zoroaster fulgens Thomson, 1873 have

two lateral articulation facets which overlap to

carinal and marginal plates (Fau and Villier,

2018). Following this assumption, they named

primary abactinals to the plates of the costae

rows in brisingids. Since brisingids do not have

carinal series (Fau and Villier, 2019), here the

primary abactinals are called costae plates to

remark their correspondence in costae rows.

Therefore, each costae plate has two lateral

costcost articulations which allow the connec-

tion with the adjacent costae plate, forming the

transverse costae rows.

Regarding the robustness of adambulacral,

ambulacral and inferomarginal plates, it was

observed that the second ambulacral plate is the

most compressed one, and the first ambulacral

plates are the highest, most angular. Beyond

the gonadal region, the ambulacral plates have

more conspicuous wings and shaft, but their

apex is less developed; meanwhile, the adam-

bulacral plates are longer than wide, quite nar-

row transversally with an inconspicuous crest,

and inferomarginal plates are almost tubercles.

Spencer and Wright (1966), Gale (1987), Blake

(1987) and Fau and Villier (2019) mentioned

that brisingids have a weak skeleton, such as

in the abactinal plates of the disk, and that the

furrow of ambulacral plates is irregular. Par-

ticularly, M. xandaros ossicles tend to decrease

in size and height, shortening or elongating in

shape in the case of adambulacral plates, fur-

ther from the base of the arm; additionally, the

distal ambulacral plates have an almost incon-

spicuous furrow.

As Downey (1972), Downey (1986), Clark

and Downey (1992) and Fau and Villier (2019)

remarked, there is only one row of marginal

plates called inferomarginals; this study fol-

lows the same name for brisingids marginal

plates. Likewise, Fau and Villier (2019) men-

tioned that these plates do not overlap with

each other, which might be due to separate

or incomplete costae rows; nevertheless, both

proximal and gonadal inferomarginal plates

of M. xandaros abut by imim articulation and

lied on ambulacral plates through amim. This

means that inferomarginals are not necessar-

ily joined to each costae plate because of the

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presence of other small inferomarginal plates.

Distal to the gonadal region, the inferomarginal

plates alternate with ambulacral plates.

Lastly, straight pedicellariae were not

observed, only recognizing two types of

crossed pedicellariae: large, C-shaped, and

tiny, spatulate-shaped. Apart from the lack of

straight pedicellariae, the presence of a medial

projection with teeth and less than three rows of

distal teeth has been noted as unique characters

of brisingids (Gale, 1987; Fau & Villier, 2019).

It has been demonstrated that ophiuroids

ossicle morphology provides useful taxonom-

ic information from order to specific level

(Martynov, 2010; Pineda-Enríquez, Bribiesca-

Contreras, Solís-Marín, Laguarda-Figueras, &

O’Hara, 2017; O’Hara, Stöhr, Hugall, Thuy, &

Martynov, 2018). In the class Asteroidea, Vil-

lier, Blake, Jagt and Kutscher (2004) remarked

on the taxonomic importance of ossicle mor-

phology; however, most of the studies about

asteroids ossicles have provided insights at an

order level, and in the particular case of brisin-

gids, there is still a significant gap in informa-

tion about their skeleton and spination.

Fau and Villier (2019) stressed that more

studies on oral frame structure are needed, but

we also recommend undertaking comparative

descriptions focused on marginal, adambula-

cral and ambulacral plates, since the position

of articulation and muscles such as dada,

dadam and padam, and crest, sturdiness and

sharpness of these plates seems to be variable

along the arm. As the costae rows are absent

in Freyellidae, we suggest carrying out a com-

plete description of a species of this family in

order to get a holistic understanding of the arm

of Brisingida.

In addition to the description of these

ossicles, presented for the first time is the mor-

phology of abactinal, first adambulacral, first

and subsequent inferomarginal, and the distal

arm plates, as well as abactinal spines and

C-shaped crossed pedicellariae for a brisingid

species. Therefore, the present study expands

on the morphological description of M. xanda-

ros and contributes to the knowledge of brisin-

gids morphology, providing the most extensive

morphological description for a brisingid spe-

cies using SEM to date which gives insights for

future taxonomic studies of this order.

Ethical statement: authors declare that

they all agree with this publication and made

significant contributions; that there is no con-

flict of interest of any kind; and that we fol-

lowed all pertinent ethical and legal procedures

and requirements. All financial sources are

fully and clearly stated in the acknowledge-

ments section. A signed document has been

filed in the journal archives.

ACKNOWLEDGMENTS

We would like to thank to Ma. Esther

Diupotex Chong and Alicia Durán González

for their technical support at the CNE, ICML,

UNAM. To Ma. Berenit Mendoza Garfias

(IB, UNAM) and Laura E. Gómez Lizárraga

(ICML, UNAM) for the SEM photographs,

to Gloria I. Rosales-Contreras for her help

to take some additional photos. To Susana

Guzmán Gómez (IB, UNAM) for her technical

support with the multifocal photography. Mag-

dalena De los Palos-Peña would like to thank

to CONACYT for the scholarship (number

929010) and to Dave Pawson, William Moser

and the Invertebrate Zoology’s, Smithsonian

staff for their support. Finally, the authors

would like to thank Matthew G. Lovegrove,

Andrea A. Caballero-Ochoa and Duncan A.

Purdie for their valuable comments on the

manuscript’s English.

RESUMEN

Morfología del endoesqueleto y ornamentación

de la estrella de mar Midgardia xandaros

(Brisingida: Brisingidae) en el Golfo de México

Introducción: Las estrellas de mar de profundidad

del orden Brisingida tienen una distribución casi global,

sin embargo, han sido poco estudiadas debido a su profun-

da distribución batimétrica y esqueleto frágil. Objetivo:

Describir la morfología externa e interna de Midgardia

xandaros incluyendo el arreglo de las placas del esqueleto

mediante técnicas de microscopía multifocal y electrónica

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de barrido (MEB). Métodos: Se examinó un total de 21

ejemplares, incluyendo 27 fragmentos de brazos, prove-

nientes del Golfo de México y Honduras. Dos de estos

ejemplares fueron disectados. Resultados: Se presenta

la descripción de pedicelarios; placas abactinales, inter-

costales, inferomarginales, adambulacrales, ambulacrales,

orales y odontóforo, y sus espinas, enfatizando los sitios

de articulaciones e inserción de músculos. Se reconocieron

pedicelarios con valvas con forma de C, y pequeños pedi-

celarios cuyas valvas poseen áreas de denticulación cortas.

Conclusiones: La descripción morfológica de M. xandaros

es ampliada, presentado por primera vez la morfología de

las placas abactinales, primera adambulacrales, primera

y subsecuentes inferomarginales, espinas abactinales y

pedicelarios con valvas con forma de “C”, así como las

placas distales de los brazos para una especie del orden

Brisingida.

Palabras clave: osículos; morfología interna; pedicelarios;

espinas; mar profundo.

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APPENDIX 1

Terminology and abbreviations used in this study. *Definitions modified.

Term Definition Author/Present study

Abactinal view View of the upper surface opposite to mouth (above marginals)

of the ossicle

Clark and Downey (1992)

abd

Abductor muscle on crossed forcipulate pedicellariae Emson and Young (1994)

abiim

Interoral abactinal muscle Fau and Villier (2019)

abtam

Transverse abactinal interambulacral muscle Fau and Villier (2019)

aciim

Interoral actinal muscle Fau and Villier (2019)

actam

Transverse actinal interambulacral muscle Fau and Villier (2019)

Actinal view View of the lower surface opposite to anus (below marginals)

of the ossicle

Clark and Downey (1992)

add

Adductor muscle on crossed forcipulate pedicellariae Emson and Young (1994)

Adradial extension Adambulacral carinae on the plate margin Fau and Villier (2018)

amim

Ambulacral-inferomarginal articulation Present study

Articular ridge Projection of the basal plate where crossed forcipulate pedicellariae

valves rest

Emson and Young (1994)

Basal plate Ossicle joining two valves and attaching abductor and adductor

muscles located in the basal part of the crossed forcipulate

pedicellariae

Emson and Young (1994)

Base (of the spines/

spinelets)

Coarse, proximal end of the spine that articulates to the boss

of the ossicle

Present study

Base (of the

ambulacrals)

Inferior part that articulates with two adjacent adambulacral ossicles

through a transverse articular ridge on its actinal surface and proximal

and distal wing-like processes on which proximal (padam) and distal

(dadam) adambulacral-ambulacral muscles insert

Gale (2011); Fau and Villier

(2019)

Body Actinal part of the ossicle bearing the spines, the odom and aciim

muscles on the interradial side, and the iioa articulation

Fau and Villier (2019)

Boss A mound-like projection for spine attachment. Turner and Dearborn (1972)

Spine pit A central depression in some bosses. A notch completely or partially

surrounded by an articulation area in a boss.

Turner and Dearborn (1972).

Fau and Villier (2019) (as a

pustule).

Canine Canine-like tooth on upper part of crossed forcipulate

pedicellariae valves

Chia & Amerongen (1975)

costcost

Costae-costae ossicle articulation Present study

Crest Ridge separating the muscle insertions, padam and dadam on the

adambulacrals (brisingids and zoroasterids). Ridge or elevation

*Fau and Villier (2019)

dada

Ambulacral/adambulacral articulation (distal on the ambulacral,

proximal on the adambulacrals)

Fau and Villier (2019)

dadam

Distal ambulacral/adambulacral muscle, on the ambulacrals Fau and Villier (2019)

Diastema Gap between the distal teeth and median teeth on crossed forcipulate

pedicellariae valves

Fau and Villier (2019)

Distal Direction toward the tip of the arm Fau and Villier (2018)

dicoa

Trace on the oral plates of the articulation between the oral and the

first ambulacral ossicle

Fau and Villier (2019)

dicoam

Oral/first ambulacral distal muscle insertion, on the oral Fau and Villier (2019)

doda

Distal oral/odontophore articulation Fau and Villier (2019)

Furrow Furrow on the distal process of the first ambulacrals and on the shaft

of ambulacrals

Fau and Villier (2019)

Head (of the

ambulacrals)

Superior part that articulates with the corresponding ambulacral plate

of the opposing column across the mid-radial line by means of an

interdigitating dentition

Gale (2011)

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Term Definition Author/Present study

iioa

Interoral articulation Fau and Villier (2019)

imabt

Inferomarginal-abactinal articulation *Fau and Villier (2018)

imim

Inferomarginal-inferomarginal articulation Present study

interadam

Interadambulacral muscle Fau and Villier (2019)

Interradial view View of the surface of the ossicle that faces away from the radial

plane (with respect to the midradial line along the arms)

*Fau and Villier (2018)

lia

Longitudinal interambulacral articulation Fau and Villier (2019)

lim

Longitudinal interambulacral muscle Fau and Villier (2019)

Medial projection Mid process with teeth on crossed forcipulate pedicellariae valves Chia & Amerongen (1975)

Median teeth Transversal tooth on upper part of crossed forcipulate pedicellariae

valves

Chia & Amerongen (1975)

odom

Odontophore-oral muscle Fau and Villier (2019)

odomim

Odontophore-inferomarginal articulation Present study

pada

Ambulacral-adambulacral articulation, (proximal on the ambulacral,

distal on the adambulacral)

Fau and Villier (2019)

padam

Proximal ambulacral-adambulacral muscle, on the ambulacral Fau and Villier (2019)

Plateau Flat area at the end of the abactinal ramus edge of the oral ossicle,

generally distinct from the latter by a change of slope, bearing the

doda articulation on the interradial side, and the complex dicoa/

dicoam on the radial side

Fau and Villier (2019)

poda

Proximal odontophore-oral articulation Fau and Villier (2019)

procoa

Proximal oral-first ambulacral articulation, on the orals Fau and Villier (2019)

procoam

Proximal oral-first ambulacral muscle on the oral Fau and Villier (2019)

Proximal Direction toward the center of the disk Fau and Villier (2018)

R Radio mayor (del centro del disco a la punta del brazo) Clark and Downey (1992)

r Radio menor (del centro del disco a un borde interradial) Clark and Downey (1992)

Radial view View of the surface that faces towards the radial plane (with respect to

the midradial line along the arms)

*Fau and Villier (2018)

Ramus Abactinal extension of the oral ossicle, bearing the abiim muscle and

the poda articulation on the interradial side, and the complex procoa/

procoam on the radial side of the ossicle

Fau and Villier (2019)

riom

Interoral muscle Fau and Villier (2019)

rng

Passageway of the nervous oral ring Fau and Villier (2019)

rvg

Groove along the oral ossicles in which lies the ring canal of the

ambulacral system

Fau and Villier (2019)

Shaft (of the

ambulacrals)

Middle part of the ambulacrals Gale (2011); Fau and Villier

(2019).

Shaft (of the spines) Long section of the spines Gale (2011)

Teeth (of the ossicles) Imbricating teeth and socket structures, on the ambulacral head.

Similar structure can appear on the interradial side of the orals and of

adambulacrals of the adoral carina.

Fau and Villier (2019)

Teeth (of the

pedicellariae)

Sharply pointed spines on the valves of a pedicellariae Emson and Young (1994)

Terminal peg Extension of the basal plate to interlock closely with the crossed

forcipulate pedicellariae valves

Emson and Young (1994)

Trabeculae Parallel columns along the spine/spinelet Gale (2011)

Wing Process at the base of ambulacrals on which proximal (padam) and

distal (dadam) muscles insert

Gale (2011); Fau and Villier

(2019)