Parallelisms in the Evolution of Sea Cucumbers (Echinodermata: Holothuroidea).

Abstract

The importance of taking into account parallelisms in the evolution of morphological characters is analyzed for the taxonomy of the class Holothuroidea. The establishment of the order Dactylochirotida and classification of the order Elasipodida serve as examples to illustrate insufficient appreciation of parallelisms in Holothuroidea. The following characters, evolving independently in different groups of sea cucumbers, are considered: a stout skeleton, reduction of the calcareous ring and the body wall sclerites; similarity of body shape; similarity in the shape of tentacles; reduction in the number of tentacles from 12 to 10 in different families and subfamilies of the order Synaptida. Based on the analysis of morphological and molecular data, the family Deimatidae is transferred from the order Elasipodida to the order Aspidochirotida. It is hypothesized that the concave cup_shaped sclerites with three to five rays occurring in the family Laetmogonidae (order Elasipodida) are of paedomorphic origin and correspond to the early growth stages of the laetmogonid wheels; the concave cross_shaped sclerites of the families Elpidiidae and Psychropotidae may have originated from laetmogonid concave cup-shaped sclerites. Emended diagnosis of the order Elasipodida is proposed. The family Vaneyellidae previously synonymized by the author with the family Cucumariidae is reestablished, and its emended diagnosis is also proposed.

Full text

ISSN 00310301, Paleontological Journal, 2016, Vol. 50, No. 14, pp. 1610–1625. © Pleiades Publishing, Ltd., 2016.
1610
INTRODUCTION
Holothurioidea is a class of echinoderms of a pae
domorphic origin (David and Mooi 1996, 1998; Mooi
and David, 1997; Smirnov, 2014, 2015). Due to pae
domorphosis, holothurians managed to escape the
burden of former organization, freeing themselves of
the limitations imposed by a massive skeleton embrac
ing the entire body. In holothurians, the skeleton is
represented only by a calcareous ring and microscopic
sclerites in body wall, tentacles, and sometimes in the
walls of other internal organs. Holothurian sclerites,
certainly evolved through paedomorphosis (Cuénot,
1948; Smirnov, 2015). Apart from the loss of the skel
eton, changes due to paedomorphosis led to the sec
ondary appearance of gradual metamorphosis and to
the arrangement of radial canals of the ambulacral sys
tem. The latter, in turn, caused changes in the mode of
development of epineural neural cords and epineural
canals (Smirnov, 2015). The appearance of the gradual
metamorphosis resulted in an increase in hetero
chrony of the development of characters of different
coordination chains, and in a greater lability in the
evolution of the morphoanatomical structures. Alto
gether, this allowed holothurians to colonize a large
number of biotopes. At present, holothurians live at all
depths of the world’s oceans, from the littoral to the
hadal. In many deep water biocoenoses they are dom
inant forms. Holothurians include epifaunal as well as
infaunal forms. Many holothurians became benthope
lagic and even pelagic (
Pelagothuria natatrix
) (e.g.,
Rogacheva et al., 2012). The colonization of similar
biotopes by different groups of holothurians at differ
ent times and adaptation to similar environments as
well as acquiring a similar lifestyle should have resulted
(and this did happen) in the parallel appearance of
similar morphologies. This in the first hand involved
external morphological characters, such as the body
shape, morphology of tentacles, the development of a
strong skeleton, or on the contrary, reduction of skel
etal elements. The changes also involved the internal
organization of holothurians leading to the parallel
appearance of similar anatomical characters. Not only
origin of holothurians connected with paedomorpho
sis, but it was also very important in their evolution and
among extant holothurians there are some taxa, which
have a paedomorphic origin (Smirnov, 2015). There
fore, parallel characters could also have appeared by
paedomorphosis.
Despite the reduction of skeletons in holothurians,
their skeletal elements are conservative in morphology,
and this allows their use as “prevalent” taxonomic
characters. The structure of the calcareous ring is one
Parallelisms in the Evolution of Sea Cucumbers
(Echinodermata: Holothuroidea)
A. V. Smirnov
Zoological Institute, Russian Academy of Sciences, Universitetskaya nab. 1, St. Petersburg, 199034 Russia
email: sav_11@inbox.ru
Received November 16, 2015; in final form, November 20, 2015
Abstract—
The importance of taking into account parallelisms in the evolution of morphological characters
is analyzed for the taxonomy of the class Holothuroidea. The establishment of the order Dactylochirotida and
classification of the order Elasipodida serve as examples to illustrate insufficient appreciation of parallelisms
in Holothuroidea. The following characters, evolving independently in different groups of sea cucumbers, are
considered: a stout skeleton, reduction of the calcareous ring and the body wall sclerites; similarity of body
shape; similarity in the shape of tentacles; reduction in the number of tentacles from 12 to 10 in different fam
ilies and subfamilies of the order Synaptida. Based on the analysis of morphological and molecular data, the
family Deimatidae is transferred from the order Elasipodida to the order Aspidochirotida. It is hypothesized
that the concave cupshaped sclerites with three to five rays occurring in the family Laetmogonidae (order
Elasipodida) are of paedomorphic origin and correspond to the early growth stages of the laetmogonid
wheels; the concave crossshaped sclerites of the families Elpidiidae and Psychropotidae may have originated
from laetmogonid concave cupshaped sclerites. Emended diagnosis of the order Elasipodida is proposed.
The family Vaneyellidae previously synonymized by the author with the family Cucumariidae is reestablished,
and its emended diagnosis is also proposed.
Keywords
: parallelism, paedomorphosis, calcareous ring, sclerites, holothurians, Elasipodida, Aspidochi
rotida, Dendrochirotida, Dactylochirotida, Deimatidae, Laetmogonidae, Vaneyellidae
DOI: 10.1134/S0031030116140082

PALEONTOLOGICAL JOURNAL Vol. 50 No. 14 2016
PARALLELISMS IN THE EVOLUTION OF SEA CUCUMBERS 1611
of the most important taxonomic characters signifi
cant extant and extinct holothurians taxonomical
studying (Reich, 2015). Unfortunately, little is pres
ently known about its 3_D morphology (Reich and
O’Loughlin, 2011).
Modern molecular genetic methods have revealed
that the complex morphoanatomical structures in
various taxa of holothurians could have originated par
allel. So some taxa, previously considered as mono
phyletic, are really para or polyphyletic. Conse
quently, the taxonomy and phylogeny of the class and
its constituent taxa must be reinterpreted. It has
become apparent that some taxonomists frequently,
and undeservedly, disregarded data on the morpho
anatomical characters of some groups described by
their predecessors. The recognition of simultaneously
appearing parallel characters has always been an
important task for taxonomists because it helped to
diminish mistakes in reconstructing phylogeny of ani
mals. The wide use of cladistics in the taxonomic prac
tice in no way negates this fundamental principle. The
present author did not intend to assemble all examples
of parallel development in holothurians and only
mentioned several instances of the parallel appearance
of morphoanatomical characters affecting the sys
tematics of the class Holothuroidea.
One of the examples of underestimation of the pos
sibility of the parallel appearance of morphological
characters is the establishment of the order Dacty
lochirota (Dactylochirotida) for three families previ
ously placed within the order Dendrochirota (Den
drochirotida) (Pawson and Fell, 1965; Pawson,
1982). The diagnostic characters of the order Dacty
lochirota included: (1) simple unbranched tentacles;
(2) Ushaped body; (3) test comprising imbricating
calcareous plates; and (4) calcareous ring simple,
lacking complex posterior processes.
1. The name of the order Dactylochirotida clearly
suggests one of the major diagnostic characters of the
order. However, unbranched tentacles in modern
holothurians (Figs. 1a, 1b) most certainly derived from
branched ones (Fig. 1d) by reducing their branches,
which is indicated by the presence in some species of
branch traces (Fig. 1c). Ohshima indicated that the
tentacles of
Pseudocucumis dactilicus
(currently
Vaniella dactylica
), assigned by Pawson and Fell to the
family Vaneyellidae in the order Dactylochirota (Paw
son and Fell, 1965), are “seldom with a few knoblike
rudiments of branches” (Ohshima, 1915, p. 272).
Thandar (2001, p. 241) noted that: “Although the ten
tacles of some rhopalodinids may be simple and fin
gerlike with short lateral branches, those of
Rhopalod
inopsis
at least appear to be finely dendritic.” (Fig. 1c).
Simple tentacles, which certainly evolved from den
dritiform tentacles are observed in the species
Psolus
digitatus
Ludwig, 1894 (family Psolidae, order Den
drochirotida). Clearly secondarily simplified simple
tentacles are also observed in other orders of holothu
rians. The paedomorphic genus
Eupyrgus
(family
Eupyrgidae, order Molpadiida) could be a good exam
ple (the similarity in the morphology of the simple
tentacles in this genus and the simple tentacles of
Pso
lus digitatus
was noted by Ludwig (1894, p. 139)).
There are clearly secondary simple tentacles in the
paedomorphic holothurians of the family Synaptidae
(order Synaptida):
Leptosynapta minuta
,
L. brasilien
sis
,
Rhabdomiolgus ruber
, “
Myriotrochus
”
geminiradi
atus
, etc. (Smirnov, 2015). Thus simple tentacles
repeatedly evolved convergently during the evolution
of different orders of holothurians, therefore the pres
ence of simple, unbranched tentacles cannot be con
sidered as a character of the order Dactylochirotida,
distinguishing it from the order Dendrochirotida
sensu
Pawson et Fell, 1965.
2. The Ushaped body is characteristic not only for
Dactylochirotida (Figs. 2c, 2f), but is also found in some
taxa of Dendrochirotida
sensu
Pawson et Fell, 1965, for
instance for representatives of the families Psolidae,
(а) (b) (c) (d)
Fig. 1.
Dendrochirotid holothurian tentacles: (a)
Vaneyella dactylica
(Vaneyellidae, “Dactylochirotida”); (b)
Rhopalodinaria
minuta
(Rhopalodinidae, “Dactylochirotida”); (c)
Rhopalodinopsis capensis
(Rhopalodinidae, “Dactylochirotida”); (d) typical
dendrochirotid tentacle; (a) from Ohshima (1918); (b) and (c) from Thandar (2001); (d) from Pawson (1966).

1612
PALEONTOLOGICAL JOURNAL Vol. 50 No. 14 2016
SMIRNOV
Phyllophoridae, Sclerodactylidae, Heterothyonidae,
Thyonidae, and some others (Figs. 2a, 2b, 2d, 2e, 2g).
Dendrochirotida
s. lato
are suspension feeders, and
most likely this body shape evolved independently in
different families as an adaptation to a burrowing life
style. In the family Rhopalodinidae, the U shaped
body led to a fusion of the descending and ascending
halves, and development of freeliving bottleshaped
forms with 10 ambulacra (Fig. 2h).
3. Reinforcement of the body wall is found in many
holothurians, and is not limited to families included in
the order Dactylochirotida (Figs. 2f, 3d, 3g, 3h), but
(а) (b) (c)
(d) (e) (f)
(g) (h)
Fig. 2.
Ushaped dendrochirotid holothurians: (a)
Stolus rapax
(Thyonidae, Dendrochirotida); (b)
Thorsonia investigatoris
(Thy
onidae, Dendrochirotida); (c)
Echinocucumis hispida
(Cucumariidae, Dendrochirotida (family Ypsilothuriidae, “Dactylochi
rotida”)); (d)
Psolus solidus
(Psolidae, Dendrochirotida); (e)
Psolus phantapus
(Psolidae, Dendrochirotida); (f)
Ypsilothuria
bitentaculata
(Ypsilothuriidae, “Dactylochirotida”); (g)
Heterothyone alba
(Heterothyonidae, Dendrochirotida); (h)
Rhopalodi
naria minuta
(Rhopalodinidae, “Dactylochirotida”); (a) and (b) from Koehler and Vaney (1908); (c) from Sars (1861); (d) from
Massin (1987); (e) from Madsen and Hansen (1984) after Strussenfelt; (f) from Cuénot (1948) after Ludwig (1894); (g) from Paw
son (1963); (h) from Thandar (2001).

PALEONTOLOGICAL JOURNAL Vol. 50 No. 14 2016
PARALLELISMS IN THE EVOLUTION OF SEA CUCUMBERS 1613
also occurs among members of the families remaining
in the order Dendrochirotida
sensu
Pawson et Fell,
1965. In holothurians the reinforcement of the body
wall can occur through the development of dense con
nective tissue, e.g., in the genus
Stichopus
(Stchopo
didae, Aspidochirotida), and due to the presence of
many small body wall sclerites, as observed in many
Dendrochirotida
sensu
Pawson et Fell, 1965. Finally,
the test can be formed by large dense scales, resulting
from extensive growth sclerites. Such a test is charac
teristic of the Ypsilothuriidae (Dactylochirotida)
(Figs. 2f, 3d) and for Psolidae (Dendrochirotida
sensu
Pawson et Fell, 1965) (Figs. 2d, 3c). (In Psolidae the
test of imbricated dense scales envelopes only the dor
sal and lateral sides of the body, whereas the ventral
side in Psolidae is transformed into the attachment
(а)
(b)
(c)
(d)
(e)
(f)
(g) (h) (i) (j)
Fig. 3.
Dendrochirotid holothurians the bodies of which are enclosed in a test formed by overlapping plates (scales): (a)
Pseudoc
nus alcocki
(Cucumariidae, Dendrochirotida); (b)
Heterothyone
alba
(Heterothyonidae, Dendrochirotida), anterior part of the
body; (c)
Psolus
diomedeae
(Psolidae, Dendrochirotida); (d)
Ypsilothuria
bitentaculata
(Ypsilothuriidae, “Dactylochirotida”);
(e)
Psolus
fabricii
(Psolidae, Dendrochirotida), test fragment; (f)
Psolus
phantapus
(Psolidae, Dendrochirotida), test fragment;
(g)
Ypsilothuria
bitentaculata
(Ypsilothuriidae, “Dactylochirotida”), test fragment; (h)
Vaneyella
dactylica
(Vaneyellidae, “Dac
tylochirotida”, test fragment; (i)
Stereoderma
imbricata
(Cucumariidae, Dendrochirotida), test fragment; (j)
Psolus
agulhasicus
(Psolidae, Dendrochirotida), test fragment. (a) from Koehler and Vaney (1908); (b) from Pawson (1963); (c) from Caso (1976);
(d) from Cuénot (1948) after Ludwig (1894); (e) and (f) from Bell (1882); (g) and (j) from Ludwig and Heding, (1935); (h) and
(i) from Ohshima (1918).

1614
PALEONTOLOGICAL JOURNAL Vol. 50 No. 14 2016
SMIRNOV
sole). The development of the test also certainly hap
pened independently in different groups. Among Den
drochirotida
sensu
Pawson et Fell, 1 965
the test is present
in the families Paracucumidae and Heterothyonidae,
and in some members of the family Cucumariidae and in
some other families (Figs. 2a–2b, 3a, 3b, 3i). Note that
the formation of the test and the size of its scales is easily
changed depending on the habitat, which can be
observed in the genus
Psolus
. Most species of the genus
attached by the sole to the hard substrate (rocks or
shells).
P. phantapus
burrows into the substrate and only
its oral and anal cones rise above the seafloor. In this
species, the scales are considerably smaller and far less
dense (Figs. 2e, 3f), than in other species of the genus
(Figs. 2d, 3c, 3e, 3j). Thus, the test appears in parallel in
several groups of dendrochirotid holothurians, and this
character cannot be considered as a feature distin
guishing Dactylochirotida from Dendrochirotida
sensu
Pawson et Fell, 1965.
4. Morphology of the calcareous ring. The families
of Dactylochirotida, Vaneyellidae (Figs. 4a, 4b) and
Ypsylothuriidae (Fig. 4c) have a simple calcareous ring
with no posterior processes on the radial segments
subdivided into pieces, which is similar to Cucumari
idae (Figs. 4e, 4f), Psolidae, Paracucumidae (Fig. 4g)
and some others families of the order Dendrochirotida
sensu
Pawson et Fell, 1965. At the same time, radial
segments of the calcareous ring in the family Rhopal
odinidae of the order Dactylochirotida have posterior
processes, subdivided into pieces (Fig. 4d), which is
similar to Phyllophoridae (Fig. 4h) from the order
Dendrochirotida
sensu
Pawson et Fell, 1965. There
fore, the morphology of the calcareous ring, lacking
subdivided posterior processes of the radial segments,
cannot be considered as a character distinguishing the
order Dactylochirotida from the order Dendrochi
rotida
sensu
Pawson et Fell, 1965. The similarity in the
morphology of the calcareous ring within the order
does not emerge in parallel as with the previous three
characters, but, conversely, indicates the similarity of
the taxa. Among both Dactylochirotida, and Dendro
chirotida
sensu
Pawson et Fell, 1965 there are families
with a calcareous ring lacking posterior processes on
the radial segments and families with a calcareous ring
with complex posterior processes on radial segments.
Thus, none of the above characters supposedly dis
tinguishing the order Dactylochirotida from the order
Dendrochirotida
sensu
Pawson et Fell, 1965 in fact
discriminate between them. It could be suggested that
Dactylochirotida is distinct in having an assemblage of
specific characters rather than isolated characters, but
the first three of the four characters, considered as spe
cific to Dactylochirotida, commonly appear in com
pletely different groups and are apparently connected
with the lifestyle and adaptation to similar environmen
tal constraints, whereas the “prevalent” conservative
characters of the skeleton thought to be more important
taxonomically clearly suggest the polyphyletic charac
ter of the order Dactylochirotida and the connection of
its families with other families of Dendrochirotida
sensu
Pawson et Fell, 1965 (Smirnov, 2012).
The analysis of “prevalent characters”, primarily
skeletal elements clearly showed that the families
Vaneyellidae (Figs. 4a, 4b) and Ypsilothuriidae
(Fig. 4c) are similar to the families Cucumariidae
(Figs. 4e, 4f), Psolidae, Paracucumidae (Fig. 4g) and
several other families in the morphology of the calcar
eous ring lacking long posterior processes on the radial
segments. Hence we proposed to establish for these
taxa the suborder Cucumariina (Smirnov, 2012). In
the morphology of the sclerites the family Vaneyellidae
(Figs. 4i, 4j) is very similar to the family Cucumariidae
(Figs. 4m, 4n). Sclerites of Ypsilothuriidae possessing
an vertical dense reticular process (not homologous to
the spire of the table) (Fig. 4k) are very similar to scler
ites of Paracucumidae (Fig. 4o) and it is possible that
the family Paracucumidae Pawson et Fell, 1965 should
be synonymized with the family Ypsilothuriidae Hed
ing, 1942 (Smirnov, 2012, p. 823). Previously it was
proposed (Smirnov, 2012) that the family Vaneyellidae
Pawson et Fell, 1965 should be synonymized under
Cucumariidae Ludwig, 1894, but presently author
have changed his view. The history of study of the fam
ily Vaneyellidae and its diagnosis are given in the
appendix.
The morphology of the calcareous ring and scler
ites shows the similarity of the families Rhopalod
inidae (Figs. 4d, 4l) and Phyllophoridae
sensu
Smirnov, 2012
.
(Figs. 4h, 4p), which was previously
noted by the authors studying Rhopalodinidae (Hed
ing, 1937; Thandar, 2001). In the system of the class
Holothuroidea proposed by me (Smirnov, 2012) I
placed these two families near to each other in a group
of families characterized by a calcareous ring with pos
terior processes on the radial segments.
The application of cladistic analysis does not
exclude the possibility of mistakes in building a taxo
nomic system, and does not always produce a perfect
result. Kerr and Kim (2001) used a cladistic analysis to
study the taxonomy of the class Holothuroidea, for
which they selected 47 characters and analyzed their
distribution in 25 families of holothurians. Smirnov
(2012) gave a brief analysis of that paper. In the context
of the topic of parallelisms in the class Holothuroidea
described in this paper it is necessary to note that while
most anatomical characters selected by the authors for
the cladistic analysis are selected correctly, eight char
acters are rigidly connected to one another morpho
logically and functionally and represent a single com
plex character. This duplication increases the number
of characters used for the analysis and may skew the
final result. Some of the selected characters clearly
developed in parallel to one another and their use can
lead to wrong results. There are primarily 11 charac
ters related to the morphology and the structure of the
calcareous ring, although parallelisms are also found
among the anatomical characters. In my view, the fol
lowing character states appeared independently in dif

PALEONTOLOGICAL JOURNAL Vol. 50 No. 14 2016
PARALLELISMS IN THE EVOLUTION OF SEA CUCUMBERS 1615
ferent orders and families of holothurians: subdivision
of the longitudinal radial muscle into two longitudinal
bands; presence of ampullae freely hanging in the
body cavity; presence of a prominent sole; elongated
and tapering posterior body end; position of the
mouth (terminalsubterminal, clearly dorsal, or dis
tinctly ventral); position of the anus (terminalsubter
minal, clearly dorsal, or distinctly ventral); the length
of the dorsal interradius (considerably shorter or
longer that the other interradia); imbricated body wall
plates; presence of the increased dorsal papillae; pres
ence of an opening on the radial segments of the cal
careous ring; presence of long posterior processes on
the radial segments of the calcareous ring; proportions
of the radial segments of the calcareous ring (length
and width of the segments are approximately the same
(а) (b) (c) (d)
(e) (f) (g) (h)
(i) (j) (k) (l)
(m) (n) (o) (p)
Fig. 4.
Calcareous ring (a)–(h) and body wall sclerites (i)–(p) of the dendrochirotid holothurians: (a)
Vaneyella digitata
(Vaneyellidae, “Dactylochirotida”); (b)
Mitsukuriella squamulosa
(Vaneyellidae, “Dactylochirotida”); (c)
Ypsilothuria talismani
elegans
(Ypsilothuriidae, “Dactylochirotida”); (d)
Rhopalodinopsis capensis
(Rhopalodinidae, “Dactylochirotida”); (e)
Tra
chythyone bouvetensis
(Cucumariidae, Dendrochirotida); (f)
Heterocucumis steineni
(Cucumariidae, Dendrochirotida);
(g)
Paracucumis turricata
(Paracucumidae, Dendrochirotida); (h)
Phyllophorella robusta
(Phyllophoridae, Dendrochirotida);
(i)
Vaneyella digitata
, (Vaneyellidae, “Dactylochirotida”); (j)
Mitsukuriella squamulosa
(Vaneyellidae, “Dactylochirotida”);
(k)
Ypsilothuria bitentaculata virginiensis
(Ypsilothuriidae, “Dactylochirotida”); (l)
Rhopalodina gracilis
(Rhopalodinidae, “Dac
tylochirotida”); (m)
Cucumaria frondosa
(Cucumariidae, Dendrochirotida); (n)
Cladodactyla senegalensis
(Cucumariidae, Den
drochirotida); (o)
Paracucumis turricata
(Paracucumidae, Dendrochirotida); (p)
Phyllophorus discrepans
(Phyllophoridae, Den
drochirotida). (a), (b), (g)–(j), (l), (o), (p) from Heding, Panning (1954); (c) and (k) from Heding (1942); (d) from Thandar
(2001); (e) and (f) from Ludwig, Heding, (1935); (m) from Panning (1955); (n) from Panning (1957).

1616
PALEONTOLOGICAL JOURNAL Vol. 50 No. 14 2016
SMIRNOV
or the length of a segment is slightly greater than its
width); presence a depression on the anterior margin
of radial plates of the calcareous ring to accommodate
tentacular ampullae; the ratio of the articular surface
of the segments of the ring and the height of the inter
radial segments; presence of platelike sclerites with a
solid vertical spire; presence of dendrochirotid but
tons. The above flaws resulted in Kerr and Kim (2001)
supporting the recognition of the order Dactylochi
rotida. The above supports the evident conclusion that
a cladistic analysis, the usefulness of which I in no way
contest, requires a careful analysis of the selected
characters to avoid their duplication, and also to con
sider their parallel appearance, as the output depends
on what is input.
A far more complex situation is observed in the
existing system of the order Elasipodida. The only
shared character in all Elasipopida is an unusual
arrangement of mesenteries suspending the intestine.
In Elasipodida all three mesenteries suspending the
intestine in the anterior and middle part of the body are
attached to the body wall dorsally (Figs. 5a, 5d) and
only in the posterior part of the body, the mesentery
holding the second descending part of the intestine
enters the right ventral interradius and is attached to
the body wall near the mediadorsal muscular band
(Fig. 5e) (Gebruk, 1990b). In all other holothurians,
the mesentery supporting the second descending part
of the intestine is attached along its entire length in the
right ventral interradius (Figs. 5b, 5c). The second
characters common for Elasipodida is the absence of
the water lungs, also found in the order Synaptida.
Other characters, usually cited for this order diagnosis
are restricted by one or another families and are not
characters typical for the entire order.
The analysis of the morphology of the skeletal ele
ments, their arrangement and some other morpholog
ical characters suggests that the order Elasipodida is
polyphyletic, and the family Deimatidae traditionally
assigned to it should be taken out of the order Elasipo
dida and placed in the order Aspidochirotida. Externally,
Deimatidae (Fig. 6e) are similar to some Laetmogonidae
(а) (b) (c)
(d) (e)
rdr ldr
rvr
mvr lvr
1
2
311
1
1
22
2
2
33
3
3
rdr rdr
rdr
rdr
ldr ldr
ldr
ldr
rvr
rvr
rvr
rvr
mvr mvr
mvr
mvr
lvr
lvr
lvr
lvr
Fig. 5.
Scheme of attachments of mesenteries suspending the intestine to the body wall in different orders of holothurians (a)–(c) and
in different body parts of the holothurians of the order Elasipodida (d) and (e) (oral view): (a) Elasipodida; (b) Aspidochirotida;
(c) Dendrochirotida, (d) Elasipodida, anterior portion of the body; (e) Elasipodida, posterior portion of the body. Designations:
(
ldr
) left dorsal radius; (
lvr
) left ventral; (
mvr
) midventral radius; (
rdr
) right dorsal radius; (
rvr
) right ventral radius; (1) first
descending part of the intestine; (2) ascending part of the intestine; (3) second descending part of the intestine. (a)–(c) from
Ekman (1926); (d) and (e) from Gebruk (1990b), modified.
Fig. 6.
Holothurians of the orders Elasipodida and Aspidochirotida: (a)
Laetmogone maculata
(Laetmogonidae, Elasipodida);
(b)
Amperima naresi
(Elpidiidae, Elasipodida); (c)
Psychropotes mirabilis
(Psychropotidae, Elasipodida); (d)
Pelagothuria natatrix
(Pelagothuriidae, Elasipodida); (e)
Oneirophanta mutabilis
(Deimatidae, Aspidochirotida); (f)
Paroriza grevei
(Synallactidae,
Aspidochirotida); (g)
Mesothuria squamosa
(Mesothuriidae, Aspidochirotida); (h)
Benthothuria distorta
(Synallactidae, Aspi
dochirotida); (i)
Bathyplotes pellucidus
(Synallactidae, Aspidochirotida); (j)
Dendrothuria similis
(Synallactidae, Aspidochi
rotida); (k)
Apostichopus japonicus
(Stichopodidae, Aspidochirotida); (l)
Holothuria tubulosa
(Holothuriidae, Aspidochirotida).
(a) from McBride (1906) after Théel (1882); (b) and (f) from Hansen (1956); (c) and (e) from Hansen (1975); (d) from Kaestner
(1963) after Chun (1903); (g)–(j) from Koehler, Vaney (1908); (k) from Ivanov, Strelkov (1949); (l) from Ludwig (188992).

PALEONTOLOGICAL JOURNAL Vol. 50 No. 14 2016
PARALLELISMS IN THE EVOLUTION OF SEA CUCUMBERS 1617
(а)
(b)
(c)
(d)
(e) (f)
(g) (h)
(i) (j)
(k) (l)

1618
PALEONTOLOGICAL JOURNAL Vol. 50 No. 14 2016
SMIRNOV
(Elasipodida) (Fig. 6a), but also resemble representatives
of the order Aspidochirotida (Figs. 6i–6l). Ludwig
(1894) was the first, who noted that the family Deima
tidae Théel, 1882 can be subdivided into two distinct
groups based on the presence or absence of lateral
papillae. Ekman (1926) established new family Laet
mogonidae to accommodate some members of
Deimatidae, which in contrast to Deimatidae
s. str.
does not have lateral papillae, and has completely dif
ferent sclerites. He also indicated that in the morphol
ogy of their sclerites, Laetmogonidae are nearer to
Elpidiidae than to Deimatidae, to which they were
previously assigned. The family Deimatidae
sensu
Ekman, 1926 is strikingly different from other families
of the order Elasipodida, to which it was previously
assigned, not only in the presence of lateral papillae,
but also orientation of sclerites in the lateral and dorsal
papillae. The longitudinal axis of sclerites in papillae
of Deimatidae is parallel to the longitudinal axis of the
papilla, and in the families of the order Elasipodida
sensu str.
is transversely to it. Perforated plate and spat
ulate sclerites are not found in the families of Elasipo
dida
s. str
. and are more similar to the plates of lung
possessing holothurians. Becher (1909, p. 445, fig. 8)
commented on the resemblance of the plates of
Oneirophanta alternata
(=
O. mutabilis mutabilis
)
(Fig. 7e) (Deimatidae) to those of
Synallactes wood
masoni
(=
Amphigymnas woodmasoni
) (Fig. 7a) (Synal
lactidae). Sclerites of Deimatidae, in the shape of
plates, transformed into cruciform plates and spatu
late rods (Figs. 7d, 7e) are not found in the families of
Elasipodida
s. str
. These plates are very similar to, and
in my opinion, homologous to the tables of Synallac
tidae (Figs. 7a–7c), and are the tables with reduced
spire, in which only the disk has remained.
Unfortunately, little is known of the calcareous ring
of Elasipodida. The families Psychropotidae and Laet
mogonidae (Fig. 8f) have a ring similar to those in
other orders of holothurians, differing in a very low
degree of calcification (Hansen, 1975, p. 186). In the
family Elpidiidae (Fig. 6b) of the order Elasipodida,
the calcareous rings are reduced and composed only of
radial segments. Each segment represented by a star
shaped structure formed by two radiating bunches of
long narrow rodlike processes (Fig. 8g). The calcareous
ring of Deimatidae (Figs. 8d, 8e) is similar to that of
Laetmogonidae (Fig. 8f), but it is also similar to the
calcareous ring of Synallactidae (Figs. 8a–8c). The
current state of knowledge of the calcareous ring in the
order Elasipodida does not allow a positive conclusion
about which ring (Elasipodida
s. str
. or Aspidochirotida)
the calcareous ring of Deimatidae most resembles. The
comparison of the morphology of the ring of Deima
tidae and Synallactidae at least does not contradict the
hypothesis of the close affinity of the families Deima
tidae and Synallactidae, and the transferring of the fam
ily Deimatidae to the order Aspidochirotida.
The first moleculargenetic data received from the
study of the species
Deima validum
showed that this
species is similar to the species
Benthothuria funebris
and
Paroriza prouhoi
(SolísMarín, 2003), which are
currently assigned to the family Synallactidae and
have developed water lungs. It is not important in this
context that the family Synallactidae is polyphyletic in
moleculargenetic data; at least three groups can be
recognized within it (SolísMarín, 2003). It is signifi
cant though, that
Benthothuria funebris
and
Paroriza
prouhoi
both similar to
Deima validum
but have devel
oped water lungs, and that suggests a connection
between the Deimatidae and lung possessing holothu
rians of the order Aspidochirotida.
It is possible, that in Deimatidae, the water lungs
reduced due to their life in a deepwater environment,
since in many deepwater echinoderms, the respira
tory organs have become reduced (A.N. Mironov, pers.
comm.). As for the position of the mesentery supporting
the second descending part of the intestine in the ante
rior and middle parts of the body (Figs. 5a, 5d), it could
happen in parallel in Deimatidae and Elasipodida
s. str
.
The transfer of the family Deimatidae from the
order Elasipodida to the order Aspidochirotida allows
Elasipodida
sensu
Smirnov, 2012 to be considered as a
monophyletic taxon. Supposedly, the cuplike con
cave sclerites occurring in Laetmogonidae with three–
five rays (Figs. 7h, 7o) are of paedomorphic origin and
represent the extensively growing initial stages of a
wheel of the letmogonid type (Fig. 7n). The concave
cuplike sclerites with processes found in Laetmog
onidae could give rise to the curved cruciform sclerites
of Elpidiidae (Fig. 7i) and Psychropotidae (Fig. 7j). As
mentioned previously, families in the order Elasipo
dida (Figs. 6a–6d) are connected to one another by a
series of intermediate characters. Laetmogonid wheel
sclerites characteristic for the family Laetmogonidae
Fig. 7.
Holothurian sclerites of the orders Elasipodida and Aspidochirotida: (a)
Amphigymnas woodmasoni
(Synallactidae, Aspi
dochirotida); (b)
Bathyplotes punctatus
(Synallactidae, Aspidochirotida; (c)
Synallactes heteroculus
(Synallactidae, Aspidochi
rotida); (d)
Deima validum validum
(Deimatidae, Aspidochirotidae); (e)
Oneirophanta mutabilis mutabilis
(Deimatidae, Aspi
dochirotidae); (f) wheel
Laetmogone violacea
(Laetmogonidae, Elasipodida), upper side; (g) wheel
Amperima rosea
(Elpidiidae,
Elasipodida), basal side; (h) cuplike, curved sclerites with 3–5 rays
Laetmogone violacea
(Laetmogonidae, Elasipodida), upper
side; (i) curved cruciform sclerites
Peniagone azorica
(Elpidiidae, Elasipodida); (j) curved cruciform sclerites
Psychropotes longi
cauda
(Psychropotidae, Elasipodida); (k) wheel
Laetmogone violacea
(Laetmogonidae, Elasipodida), basal side; (l) wheel
Laet
mogone violacea
(Laetmogonidae, Elasipodida), lateral view; (m) wheel
Benthogone rosea
(Laetmogonidae, Elasipodida), upper
side; (n) development of wheels
Benthogone rosea
(Laetmogonidae, Elasipodida), basal view; (o) cuplike, curved sclerites with
four rays
Laetmogone violacea
(Laetmogonidae, Elasipodida), lateral view; (a) from Koehler, Vaney (1905); (b) and (c) from Hed
ing (1940); (d)–(j) from Hansen (1975); (k)–(m) from Ekman (1926); (n), (o) from Perrier (1902)

PALEONTOLOGICAL JOURNAL Vol. 50 No. 14 2016
PARALLELISMS IN THE EVOLUTION OF SEA CUCUMBERS 1619
(а)
(b) (c)
(d) (e)
(f) (g) (h)
(i) (j)
(k)
(l)
(m)
(n)
(o)

1620
PALEONTOLOGICAL JOURNAL Vol. 50 No. 14 2016
SMIRNOV
(Figs. 7f, 7k–7m) are also sporadically present in some
species of the family Elpidiidae (Fig. 7g), which may
indicate the similarity of these two families (Ekman,
1926). Concave sclerites of Laetmogonidae (Fig. 7h)
are probably homologous to concave cruciform scler
ites of Elpidiidae (Fig. 7i) and Psychropotidae
(Fig. 7j). The family Psychropotidae (Fig. 6c) is also
connected to the planktonic family Pelagothuriidae
(Fig. 6d) (Gebruk, 1989; 1990a).
The above information allows the diagnosis of the
order Elasipodida to be emended.
Order Elasipodida Théel, 1882
(=Elasmopoda Théel, 1879)
D i a g n o s i s. Holothuroidea with distinct bilat
eral symmetry and welldeveloped ventral sole usually
bound by marginal ventrolateral feet (excluding the
planktonic family Pelagothuriidae). Tentacles peltate.
Radial canals developed. Tubefeet and papillae
(modified tubefeet) present. Papilla usually dorsal. In
some taxa dorsal papillae fused to form specialized
locomotory organs: anterior brim and a caudal swim
ming lobe in the posterior part. Ventral tubefeet often
enlarged and modified into ambulatory feet. In Psy
chropotidae, ventrolateral feet can be fused to form
lateral folds. The modified posterior feet can be fused
to form in some Elpidiidae a posterior swimming lobe.
Ambulacral appendages of some elasipodids of the
families Elpidiidae and Laetmogonidae have strongly
expanded ampullae (typical of Elpidiidae and Laet
mogonidae). Canals of tentacles extend from radial
canals. Radial hemal canals present. Stone canal with
madreporite usually attached to body wall and can
open externally. Ring muscles interrupted by radial
muscles. Radial muscle bands undivided. Mesentery,
(а) (b) (c)
(d)
(e)
(f) (g)
Fig. 8.
Calcareous ring of holothurians of the orders Elasipodida and Aspidochirotida: (a)
Bathyplotes punctatus
(Synallactidae,
Aspidochirotida); (b)
Pseudostichopus mollis
(Synallactidae, Aspidochirotida); (c)
Zygothuria marginata
(Mesothuriidae, Aspi
dochirotida); (d)
Oneirophanta setigera
(Deimatidae, Aspidochirotidae); (e)
Oneirophanta mutabilis
(Laetmogonidae, Elasipo
dida); (f)
Laetmogone maculata
(Laetmogonidae, Elasipodida); (g)
Elpidia glacialis
(Elpidiidae, Elasipodida). (a)–(c), (f) from
Heding (1940), (d), (e), (g) from Hansen (1975).

PALEONTOLOGICAL JOURNAL Vol. 50 No. 14 2016
PARALLELISMS IN THE EVOLUTION OF SEA CUCUMBERS 1621
suspending second descending part of intestine,
attached to body wall anteriorly and medially in right
dorsal interradius. Family Elpidiidae has two pairs of
statocysts arranged near the base of the lateroventral
radial nerves. Respiratory trees absent. Calcareous
ring poorly calcified, sometimes consisting of strong
connective tissue; in some taxa decreasing with age or
absent. Ossicles: laetmogonid wheels (Laetmogonidae
and some Elpidiidae), three–five rays cuplike con
cave sclerites (Laetmogonidae), cruciform psychro
potid crosses (Elpidiidae and Psychropotidae) and rods
and ossicles derived from them. In some forms skeletal
elements are absent. Laetmogonid wheels concave
convex, with smooth rim (in the genus Pannychia, rim
with large inwardfacing denticles), hub usually with
four perforations; on the side opposite to rim branches
extend from edge of hub merging above center of wheel
to form flat or more or less convex cup. Curved cruci
form psychropotid sclerites with one (in the center of
the cross), four (on each ray), or five (in the center of
the cross and rays) orthogonal processes.
The order includes four families: Laetmogonidae
Ekman, 1926, Elpidiidae Théel, 1882, Psychropotidae
Théel, 1882, and Pelagothuriidae Ludwig, 1893.
As in many animal groups, the parallel evolution of
many characters, and first of all characters of external
morphology is related to adaptation to similar habi
tats. Parallel characters can also appear due to paedo
morphosis. In the small paedomorphic species
Tro
choderma elegans
(Myriotrochidae, Synaptida)
(Figs. 9c; 10a, 10c) and
Eupyrgus scaber
(Eupyrgidae,
Molpadiida) (Figs. 9d; 10b, 10d) the body is covered
by a dense layer of tightly packed and even overlapping
sclerites forming a rigid test, while in most species of
these orders have a thin body wall with a few sclerites. It
is possible that in these smallsized taxa of 10–25 mm,
the test appeared by paedomorphosis. This is indirectly
supported by the presence of the test in juvenile
holothurians of other orders: Aspidochirotida (Fig. 9a)
and Dendrochirotida (Fig. 9b), which disappear in
adults (Smirnov, 2015).
Along with the process of strengthening of the body
wall and development of increasingly dense and strong
calcareous skeleton in all orders of holothurians there
is also an opposite trend toward the complete disap
pearance of body wall sclerites and even the calcareous
ring. In deepwater and swimming taxa, the body wall
sclerites are scarce and can by completely absent. For
instance, sclerites are absent in the genera
Benthoturia
,
Paroriza
, species
Paelopatides appendiculata
,
P. atlan
tica
,
P. dissidens
,
P. mammilatus
,
P. mollis
,
P. rotifer
,
Pseudostichopus peripatus
,
P. mollis
, and
Molpadiode
mas depressus
(Synallactidae, Aspidochirotida). Scler
ites can be absent in some specimens of the species
Benthodytes typica
and
B. sanguinolenta
(Psychro
potidae, Elasipodida), and are completely absent in
(а) (b)
(c) (d)
Fig. 9.
Holothurians with a test of sclerites: (a) and (b) juvenile and (c) and (d) mature paedomorphic holothurians: (a)
Aposti
chopus japonicus
juv. (Stichopodidae, Aspidochirotida); (b)
Heterocucumis steineni
juv. (Cucumariidae, Dendrochirotida);
(c)
Trochoderma elegans
(Myriotrochidae, Synaptida), body wall fragment; (d)
Eupyrgus scaber
(Eupyrgidae, Molpadiida);
(a) from Malakhov and Cherkasova (1992); (b) from Ekman (1927); (c) from Théel (1877); (d) from Madsen and Hansen (1994).

1622
PALEONTOLOGICAL JOURNAL Vol. 50 No. 14 2016
SMIRNOV
Achlyonica tui
(Elpidiidae, Elasipodida), and also in
the genus
Gephyrothuria
(Gephyrothuriidae, Gephy
rothuriida).
However, not only body wall sclerites, but also the
massive, large structure of the calcareous ring can
become reduced. The reduction of the calcareous ring
is characteristic of many holothurians of the family
Synallactidae (order Aspidochirotida). A reduced cal
careous ring is present in the benthopelagic species
Bathyplotes natans
, whereas in large specimens of this
species, it can be completely reduced. In the genera
Benthothuria
and
Paelopatides
no calcareous ring is
present. The reduction of the calcareous ring is present
in the order Elasipodida. In the family Laetmogonidae
the calcareous ring is poorly developed, and in the
family Psychropotidae the ring is not calcified but is
composed of connective tissue. As noted above, in the
family Elpidiidae the calcareous ring includes only
radial segments, and each segment represented only
by a starshaped structure formed by two radiating
bunches of long narrow rodlike processes. Naturally,
in the pelagic
Pelagothuria natatrix
(Pelagothuriidae,
Elasipodida) neither the calcareous ring nor the body
wall sclerites are present.
The reduction of skeletal elements in swimming
and deepwater taxa (most swimming holothurians are
deepwater dwellers) is understandable and explained
by adaptation to their lifestyle. Apparently, a similar
lifestyle, i.e., living within the substrate, explains the
thin body wall and the presence of only a few sclerites
in burrowing taxa, which include many members of
the orders Synaptida and Molpadiida. In almost all
representatives of the order Synaptida, sclerites in
their thin body wall are weakly developed, and in some
species are represented only by small rods. In some,
usually monotypic, genera, the body wall sclerites are
completely absent, e.g.
Kolostoneura
(Taeniogyrinae,
Chiridotidae),
Paradota
(Chiridotinae, Chiridotida),
Dactylapta
(Rynkatorpinae, Synaptidae),
Anapta
(Lep
tosynaptinae, Synaptidae),
Rhabdomolgus
(Rhabdo
molginae, Synaptidae), and
Achiridota
(Myriotro
chidae). In some species of holothurians of the genus
Molpadia
(Molpadiidae, Molpadiida), the sclerites
disappear with age. It is interesting that the disappear
ance of sclerites with age is observed in some holothu
rians of the order Dendrochirotida, which have a thick
and dense body wall. For instance, in the species
Cucumaria frondosa
and
Staurocucumis turqueti
(Cucumariidae)
sclerites in the most of the body wall
disappear and are only retained near the anus.
Considering the parallel appearance of morpho
logical characters, it is necessary to take into account
(а) (c)
(b) (d)
Fig. 10.
Paedomorphic holothurians with a test composed of sclerites: (a, b)
Trochoderma elegans
(Myriotrochidae, Synaptida),
(a) general view; (b) body wall fragment; (c, d)
Eupyrgus scaber
(Eupyrgidae, Molpadiida), (c) general view; (d) body wall frag
ment. Photograph by O. Zimina.

PALEONTOLOGICAL JOURNAL Vol. 50 No. 14 2016
PARALLELISMS IN THE EVOLUTION OF SEA CUCUMBERS 1623
that many species of holothurians have a paedomor
phic origin and therefore in some groups it is possible
to encounter “simplified” structures, which appeared
due to paedomorphosis. One of the examples is the
parallel appearance of the following paedomorphic for
order Molpadiida characters in the species
Molpadia
blakei
(family Molpadiidae) and
Eupyrgus
scaber
(family Eupyrgidae) such as the absence free hanging
tentacles ampullae and undivided longitudinal mus
cular bands (Heding, 1935).
The decrease in the number of tentacles in different
families and subfamilies of the same order can also
occur in parallel. The order Synaptida originally had
12 tentacles. However, many taxa in this order are
small, which could be products of paedomorphism. In
different families the reduction of the number of tenta
cles from 12 to 10 was also parallel. Ten tentacles are
present in the genera
Sigmodota
,
Rowedota
,
Scoliorha
pis
,
Kolostoneura
,
Psammothuria
(even eight!) (Taenio
gyrinae, Chiridotidae); a number of species
Leptosyn
apta
(Synaptidae);
Prototrochus
,
Siniotrochus phoxus
,
Parvotrochus
,
Trochoderma
(Myriotrochidae). It is
possible that this process could occur in parallel in
related groups, therefore genera established based
solely on the number of tentacles can be polyphyletic.
In different families of Synaptida, the intestinal loop
disappears in parallel. In
Scoliorhapis
spp.,
Taeniogyrus
havelockensis
,
Kolostoneura novaezealandiae
,
Psam
mothuria ganapatii
(Taeniogyrinae, Chiridotidae),
Lep
tosynapta minuta
,
Rhabdomolgus ruber
(Synaptidae)
the intestine is suspended only by mediodorsal mesen
tery along almost its entire length (Smirnov, 2014;
2015).
The above discussion of Dendrochirotida includes
consideration of the parallel development of simple
tentacles in different families. The structure of the ten
tacles is certainly determined by feeding strategy. The
aspidochirotid holothurians, the majority of which are
deposit feeders, have peltate tentacles used for gather
ing food from the substrate (Figs. 6i, 6k, 6l). The gen
era
Pseudothuria
,
Dendrothuria
and
Scothothuria
(family Synallactidae) have tentacles (Fig. 6g) slightly
resembling tentacles of holothurians of the order Den
drochirotida, which are suspension feeders and use
their dendritic branched tentacles gather plankton and
suspended particles. The manifestation of the conver
gent characters in the above genera is clearly con
nected with the transition to suspension feeding and
possibly, as suggested by Hansen (1978), with an adap
tation to swimming.
A careful study of the development of parallel
structures not only allows the refinement of the taxon
omy and phylogeny of the group, but also shows that
that using modern taxonomic techniques should be
associated with a careful analysis of existing morpho
anatomical data. A careful study of the development of
parallel structures not only allows the refinement of
the taxonomy and phylogeny of the group, but also
shows that using modern taxonomic techniques
should be associated with a careful analysis of existing
morphoanatomical data. This allows focus the study
not only on “uncertain” taxa, but also on groups
which have traditionally been accepted without hesi
tation. The latter is well illustrated by the example of
the family Deimatidae, the morphological characters
of which (primarily its skeletal elements) have been
studied and summarized by Ekman as long ago as
1926. The analysis of parallelisms in evolution certainly
facilitates a deeper understanding of the external and
internal causes and factors of the evolution of organ
isms, and helps to finding interesting model objects to
study patterns of evolutionary biology.
APPENDIX
Family Vaneyellidae Pawson et Fell, 1965
The genera
Vaneyella
and
Mitsukuriella
were
described by Heding and Panning (1954) and together
with the genus
Paracucumis
Mortensen, 1925 assigned
to “group B” of the multitentacle subfamily Thyoni
diinae they recognized in the family Phyllophoridae,
because they had 12–20 tentacles. Pawson and Fell
(1965) proposed the family Vaneyellidae to accommo
date the genera
Vaneyella
and
Mitsukuriella
which
have 15–20 tentacles, and assigned the genus
Paracu
cumis
to their new family Paracucumidae. Later, the
8tentacle genus
Psolidothuria
Thandar, 1998 with test
composed by simple imbricated plates was also
assigned to the family Vaneyellidae. Smirnov (2012)
proposed to consider the family Vaneyellidae Pawson
et Fell, 1965 a synonym of the family Cucumariidae
Ludwig, 1894, based on the similarity of the morphol
ogy of the calcareous ring and body wall sclerites. Cur
rently, I consider that proposal to be somewhat prema
ture. The family Vaneyellidae should be retained for
the multitentacle genera
Vaneyella
and
Mitsukuriella
,
while the question of the separation of the family
Vaneyellidae should await further revision of the sub
order Cucumariina. The 8tentacled genus
Psolidot
huria
Thandar, 1998 with the species
P. octodactyla
Thandar, 1998, and
P. yasmeena
Thandar, 2006
assigned to Vaneyellidae are clearly of paedomorphic
origin. These species are smallsized—28 and 8 mm,
respectively. It is possible that their poorly developed
test was inherited from a juvenile stage because in
many dendrochirotids, juveniles have a test composed
of ossicles (Smirnov, 2015) (Fig. 9b). This hypothesis
is in my opinion also supported by the morphology of
sclerites in
Psolidothuria
which represent simple per
forated plates, the development of which was arrested
at an early stage of development. This hypothesis is
also supported by the morphology of the underdevel
oped simple tentacles of
Psolidothuria octodactyla
,
which are not very simple, and have short lateral
branches (Thandar, 1998, p. 80), whereas in
P. yas
meena
, the tentacles are fingershaped but finely
branched (Thandar, 2006, p. 44, fig. 15H). Some
holothurians of the family Cucumariidae, for example

1624
PALEONTOLOGICAL JOURNAL Vol. 50 No. 14 2016
SMIRNOV
C. frondosa
and
C. vegae
have two underdeveloped
ventral tentacles and considering the trend to reduce
the ventral tentacles in some Cucumariidae it is possi
ble to suggest that in the genus
Psolidothuria
thus
reduction was complete. Considering the above, I pro
pose to assign the genus
Psolidothuria
to the family Cucu
mariidae, and to retain in the family Vaneyellidae the two
multitentacle genera
Vaneyella
Heding et Panning, 1954
and
Mitsukuriella
Heding et Panning, 1954, which were
placed in this family when it was established (Pawson
and Fell, 1965).
D i a g no s i s: Cucumariina with 15–20 tentacles.
Body Ushaped, covered by test of plates. Tube feet
arranged along radii. Segments of calcareous ring with
a high central part and low lateral parts. The lower
edge of segments with small depression in center and
lacking processes. Ring segments not subdivided into
pieces. Segments connected by lateral regions corre
sponding to lower lateral regions of ring of other
holothurians. Ring sinusoidal. Sclerites simple mono
layered, plated with numerous relatively large perfora
tions.
ACKNOWLEDGMENTS
The author is deeply grateful to Olga Zimina for
providing previously unpublished photographs, to
Antonina Kremenetskaya (Rogacheva) (Institute of
Oceanology, Russian Academy of Sciences, Moscow)
for the information on the holothurian orders Elasipo
dida and Aspidochirotida with reduced skeletal ele
ments, and to S.V. Rozhnov (Paleontological Insti
tute, Russian Academy of Sciences, Moscow) for his
constant interest and support.
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