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Tricarina gadvanensis is an isopod: reinterpretation of an alleged lobster from the Lower Cretaceous of Iran

Published online by Cambridge University Press:  28 August 2019

Matúš Hyžný,
Denis AudoOpen the ORCID record for Denis Audo[Opens in a new window],
Rodney M. Feldmann and
Guenter Schweigert
Matúš Hyžný
Affiliation:
Comenius University, Faculty of Natural Sciences, Department of Geology and Palaeontology, Mlynská dolina, Ilkovičova 6, 842 15Bratislava, Slovakia Naturhistorisches Museum Wien, Burgring 7, 1010Vienna, Austria
Denis Audo*
Affiliation:
Yunnan Key Laboratory for Palaeobiology, Yunnan University, China MEC International Joint Laboratory for Palaeobiology and Palaeoenvironment, Yunnan University, China
Rodney M. Feldmann
Affiliation:
Department of Geology, Kent State University, KentOH44242, USA
Guenter Schweigert
Affiliation:
Staatliches Museum für Naturkunde, Rosenstein 1, 70191 Stuttgart, Germany
*
*Corresponding author
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Abstract

An incomplete arthropod fossil, Tricarina gadvanensis Feldmann et al., 2007, from the Lower Cretaceous (Barremian–Aptian) of Iran, originally interpreted as a lobster (Malacostraca, Eucarida, Decapoda), is reinterpreted herein as a representative of a seroloid isopod crustacean (Malacostraca, Peracarida, Isopoda). The body orientation of the fossil is reversed, and the alleged cephalothoracic shield is reinterpreted as being the pleotelson. Tricarina gadvanensis, with its unique configuration of pleonite fusion (pleonites 1–4 are free, whereas pleonite 5, with still visible epimera, is fused with the pleotelson), is suggested to represent a close relative of Serolidae. The preservation of T. gadvanensis in deepwater shales speaks for conservativeness in environmental preferences of seroloid isopods for at least 125 million years.

Information

Type
Articles
Information
Journal of Paleontology , Volume 94 , Issue 2 , March 2020, pp. 304 - 310
Copyright
Copyright © 2019, The Paleontological Society

Introduction

Body orientation of extinct animals is sometimes difficult to interpret. Tricarina gadvanensis Feldmann et al., Reference Feldmann, Kolahdouz, Biranvand and Schweigert2007 (Fig. 1) is an incomplete arthropod fossil recovered from a borehole in Iran and dated as Lower Cretaceous (Barremian–Aptian). The fossil was originally interpreted as the cephalothoracic shield (carapace) of a decapod crustacean and was attributed to a new genus, species, and family of achelatan lobsters (Eucarida, Decapoda, Achelata; Fig. 1.1). Later it was transferred to polychelidans (Eucarida, Decapoda, Polychelida; Fig. 1.3) by Karasawa et al. (Reference Karasawa, Schweitzer and Feldmann2013). Drawing on detailed comparison with isopod crustaceans, we propose herein yet another interpretation: the specimen in question actually represents a posterior exoskeletal portion of a seroloid isopod (Peracarida, Isopoda, Seroloidea; Fig. 1.4). In addition, the configuration of preserved somites in T. gadvanensis allows some comparisons with sphaeromatidean isopods. The variations we observe may be useful for a phylogenetic analysis, which is, however, beyond the scope of this paper.

Figure 1.

The discussed affinities of Tricarina gadvanensis. The assignment is largely based on the antero-posterior orientation of the fossil (upper part / lower part of the figure). A large, flattened structure is interpreted to be a (1, 3) carapace, (2) telson or (4) pleotelson. Feldmann et al. (Reference Feldmann, Kolahdouz, Biranvand and Schweigert2007) assigned the species to (1) Achelata and compared it to (2) Stomatopoda. Karasawa et al. (Reference Karasawa, Schweitzer and Feldmann2013) reassigned the species to (3) Polychelida. The assignment to (4) Isopoda is presented herein. Abbreviations: a = basal segment of antenna; als = anterolateral spine; c = carapace; p7 = pereiopod 7; pls = posterolateral spine; plt = pleotelson; ps4–ps5 = pleonal somites 4–5; ta = thoracic appendage; u = uropod.

From lobster to isopod

Background of Tricarina gadvanensis

More than a decade ago, Feldmann et al. (Reference Feldmann, Kolahdouz, Biranvand and Schweigert2007) erected a new family, genus, and species of an achelatan lobster based on an incomplete arthropod fossil. The single specimen of Tricarina gadvanensis was extracted from a well, and the possibility of finding additional material is remote. The fragmentary fossil consists of a large flattened structure adorned with three carinae positioned longitudinally in respect to the supposed anterior-posterior body axis. To this structure, a short succession of segments is attached. A pair of fragmentary appendages seems to be articulated to the large structure, opposite to the segments. An additional appendage fragment lies parallel to the lateral margin of the larger structure. The fossil apparently is lacking both ends, as they were sawn off in the process of drilling.

In the original description, the specimen was interpreted as coming from a peculiar decapod lobster lacking orbits and having anterolateral and posterolateral spines separated by a straight, smooth lateral margin. The authors admitted that a pleon lacking an axial keel and having very long, curving pleura “is unlike that of other decapod taxa” (Feldmann et al., Reference Feldmann, Kolahdouz, Biranvand and Schweigert2007, p. 406). What appear to be strange characters for a decapod pleon are a very common characteristic of the pleon in isopod crustaceans.

Feldmann et al. (Reference Feldmann, Kolahdouz, Biranvand and Schweigert2007, p. 407) compared the enigmatic fossil to a stomatopod (Eucarida, Hoplocarida, Stomatopoda; Fig. 1.2) and stated that “when rotated 180°, the specimen bears a superficial resemblance to the abdomen and telson of a stomatopod.” After argumentation against the “stomatopod hypothesis,” the authors stated that the “systematic position of the specimen as a decapod crustacean is assured.” We argue that more options are at hand and that deciphering the nature of T. gadvanensis does not depend on maneuvering between two possibilities.

Placement of Tricarina gadvanensis

The monotypic family Tricarinidae Feldmann et al., Reference Feldmann, Kolahdouz, Biranvand and Schweigert2007 was initially assigned to Palinuroidea Latreille, 1802 (largely due to the apparent absence of a claw in the preserved appendage) although in the original description this placement was not considered fully convincing (Feldmann et al., Reference Feldmann, Kolahdouz, Biranvand and Schweigert2007, p. 407). The alternative ‘stomatopod hypothesis’ was rejected because of the shape of the segment proximal to that of the large flattened structure, which they interpreted as a carapace. Indeed, in stomatopods, the well-developed sixth pleonal segment bears the articulation for the uropod (Manning, Reference Manning, Carpenter and Niem1998), as correctly pointed out by Feldmann et al. (Reference Feldmann, Kolahdouz, Biranvand and Schweigert2007), who also noted that this segment in T. gadvanensis has a reduced length and shorter tergopleura. Following this view, these characteristics are similar to the first pleonite of ‘macruran’ decapods (cf. Glaessner, Reference Glaessner and Moore1969; McLaughlin, Reference McLaughlin1980). Feldmann et al. (Reference Feldmann, Kolahdouz, Biranvand and Schweigert2007) interpreted the two appendages opposite to the pleonites as parts of antennae. Finally, they interpreted the fragment of appendage lateral to the large structure as an achelate appendage.

Due to the development of the alleged cephalothoracic carinae, T. gadvanensis and its family were ascribed to Eryonoidea (= Polychelida) by Karasawa et al. (Reference Karasawa, Schweitzer and Feldmann2013), and as such it was included in the Revised Treatise Online section dedicated to polychelidans (Feldmann et al., Reference Feldmann, Schweitzer, Karasawa, Schweigert and Garassino2013). Polychelidans were revised by Audo (Reference Audo2014, p. 255) in his doctoral thesis, in which the ‘polychelidan hypothesis’ and the interpretation of the orientation were rejected because of the overlapping pleonites, the orientation of tergopleura, the rather straight (instead of curved) margin between the large element and pleonites, and the supposed antennae that do not look like antennae in known polychelidans. Audo (Reference Audo2014) returned to the ‘stomatopod hypothesis’ and considered the large structure to represent the telson of a mantis shrimp (Fig. 1.2). The overlap of segments and the orientation of their tergopleura were taken as crucial characters. In this respect, Audo (Reference Audo2014) also recalled a case of Eryon yehoachi Remy and Avnimelech, Reference Remy and Avnimelech1956, a stomatopod telson being mistaken for a decapod crustacean cephalothorax (Hof, Reference Hof1998).

The isopod hypothesis

Several aspects of the original interpretation of T. gadvanensis as the anterior part of a decapod crustacean (Feldmann et al., Reference Feldmann, Kolahdouz, Biranvand and Schweigert2007) are problematic. First, the segments are visibly inverted, with the tergopleura pointing in the wrong direction. In most malacostracans, tergopleura are pointing toward the posterior portion of the body or are oriented downward (ventrally). A notable exception to this aspect is known in some species of slipper lobsters (Holthuis, Reference Holthuis1991; Lavalli and Spanier, Reference Lavalli, Spanier, Lavalli and Spanier2007). In the original interpretation of T. gadvanensis, the pleura point forward, which is, nevertheless, highly unusual.

The tergite of the alleged first segment is slightly overlain by that of the ‘second,’ which is overlain by that of the ‘third.’ However, in arthropods, the anterior margin of each pleonite is overlain by the posterior margin of the preceding segment.

Feldmann et al. (Reference Feldmann, Kolahdouz, Biranvand and Schweigert2007) interpreted the structures attached to the alleged cephalothorax as the basal elements of the antennae. These structures, however, are positioned rather distant from one another for being antennae. In general, the pair of antennae is carried by an antennal somite belonging to the cephalic region (McLaughlin, Reference McLaughlin1980), which implies that the bases of antennae are expected to be positioned quite close to each other and near the front of the carapace.

Finally, the spine that in the original interpretation is positioned on the posterolateral margin of the carapace and points forward is quite unusual for a decapod crustacean. All these characters were used as a basis for description of not only a new genus and species of a lobster, but also for a family on its own. Whereas we do not doubt the overall uniqueness of the fossil, we think that a far more elegant and logical interpretation of this strange fossil is possible.

Clearly, the superposition of segments and the orientation of tergopleura suggest that the correct orientation of the specimen in question is in fact opposite to that proposed by Feldmann et al. (Reference Feldmann, Kolahdouz, Biranvand and Schweigert2007); thus, the large structure is located at the posterior end of the animal, not the anterior end. Instead of being an incomplete cephalothorax of a decapod with a missing anteriormost portion, the large structure is interpreted herein to represent a near-complete pleotelson of an isopod (Fig. 2). Longitudinal carinae often are present in a number of isopod taxa. A well-developed median carina is present in some Cirolanidae (Bruce, Reference Bruce1986; Lowry and Dempsey, Reference Lowry and Dempsey2006; Hyžný et al., Reference Hyžný, Pasini and Garassino2019) and Sphaeromatidae (Woodward, Reference Woodward1898; Bachmayer, Reference Bachmayer1955), whereas a combination of three (or even more) carinae is not uncommon in some serolids (Brandt, Reference Brandt1988; Poore and Storey, Reference Poore and Storey2009; Fig. 3.3, 3.4).

Figure 2.

Tricarina gadvanensis Feldmann et al., Reference Feldmann, Kolahdouz, Biranvand and Schweigert2007 interpreted as a seroloid isopod and its comparison with selected serolid taxa. (1, 2) T. gadvanensis, part and counterpart of the holotype CM 54197. (3) Acutiserolis spinosa (Kussakin, Reference Kussakin1967), NIWA 24311. (4) Brucerolis nowra Poore and Storey, Reference Poore and Storey2009, paratype NMV J55674. Scale bars = 10 mm.

Figure 3.

Tricarina gadvanensis Feldmann et al., Reference Feldmann, Kolahdouz, Biranvand and Schweigert2007 interpreted as (1) a seroloid isopod, and its comparison with posterior body portions of (2) a mantis shrimp Squilla mantis (Linnaeus, Reference Linnaeus1758) and serolid isopods (3) Acutiserolis spinosa (Kussakin, Reference Kussakin1967), NIWA 23526, and (4) Brucerolis nowra Poore and Storey, Reference Poore and Storey2009, holotype NMV J58261. mc = median carina; lc = lateral carina; u = uropod.

A careful comparison of T. gadvanensis with representatives of extant serolid genera Acutiserolis Brandt, Reference Brandt1988 and Brucerolis Poore and Storey, Reference Poore and Storey2009 (Figs. 2, 3) demonstrates that the enigmatic ‘lobster’ from the Lower Cretaceous of Iran fits very well with the usual morphology of seroloid isopods. In this interpretation, the tergopleura represent pleonites. The alleged posterolateral spine represents a tergopleuron of the fifth pleonite segment that is fused with the pleotelson; in sphaeromatoid and seroloid isopods the fusion of various numbers of pleonites with the pleotelson is a common characteristic (Brandt and Poore, Reference Brandt and Poore2003). Note that the pleotelson itself is a structure originating from the fusion of the telson and the last pleonite; therefore, the uropods are attached to the pleotelson (Kensley and Schotte, Reference Kensley and Schotte1989). This allows reinterpreting the ‘antennae’ of T. gadvanensis as being the basal segments of uropods. Finally, the ‘achelatan’ appendage is simply the seventh thoracopod (pereiopod), which in flabelliferan isopods is commonly long enough to protrude from behind the pleon and/or pleotelson (Kensley and Schotte, Reference Kensley and Schotte1989).

Materials and methods

The present study is based on reexamination of the part and counterpart of the holotype and the only known specimen of Tricarina gadvanensis from the Lower Cretaceous Gadvan Formation (James and Wynd, Reference James and Wynd1965; Alavi, Reference Alavi2004) of Iran. The specimen was documented both in natural light and coated with ammonium chloride before photography. For comparison, high-resolution photographs of selected extant serolid isopods were studied.

Repositories and institutional abbreviations

CM: Carnegie Museum of Natural History, Pittsburgh, Pennsylvania; NMV: Museum Victoria, Melbourne, Australia; NIWA: Museum of Tropical Queensland, Queensland Museum, formerly National Institute of Water and Atmosphere, Wellington, New Zealand.

Systematic paleontology

Class Malacostraca Latreille, Reference Latreille1802
Superorder Peracarida Calman, Reference Calman1904
Order Isopoda Latreille, Reference Latreille and Cuvier1817
Suborder Sphaeromatidea Wägele, Reference Wägele1989
Superfamily Seroloidea Dana, Reference Dana1852
Family Tricarinidae Feldmann et al., Reference Feldmann, Kolahdouz, Biranvand and Schweigert2007

Emended diagnosis

As for the single known species, Tricarina gadvanensis.

Remarks

Tricarinidae differs significantly from all other seroloid families, with pleonite 5 epimera still protruding laterally. From modern taxa, the uropodal insertion on the posterolateral margin of the pleotelson is present only in Thysanoserolis Brandt, Reference Brandt1991; all other serolid genera have the uropodal insertion on the anterolateral margin of the pleotelson or on its midlength.

Type species

Tricarina gadvanensis by monotypy.

Emended diagnosis

As for the single known species, Tricarina gadvanensis.

Tricarina gadvanensis Feldmann et al., Reference Feldmann, Kolahdouz, Biranvand and Schweigert2007
Figures 1, 2.1, 2.2, 3.1, 4.2
Reference Feldmann, Kolahdouz, Biranvand and Schweigert2007

Tricarina gadvanensis Feldmann et al., p. 406, fig. 2.

Reference Karasawa, Schweitzer and Feldmann2013

Tricarina gadvanensis; Karasawa, Schweitzer, and Feldmann, p. 98, fig. 7C.

Reference Feldmann, Schweitzer, Karasawa, Schweigert and Garassino2013

Tricarina gadvanensis; Feldmann et al., p. 8, fig. 6.

Reference Audo2014

Tricarina gadvanensis; Audo, p. 255, fig. 94.

Figure 4.

Variation in fusion of pleonites (labeled with numerals) with the pleotelson in seroloid isopod crustaceans. (1) Bathynataliidae (Bathynatalia gilchristi Kensley, Reference Kensley1978) with all five free pleonites. (2) Tricarinidae (Tricarina gadvanensis Feldmann et al., Reference Feldmann, Kolahdouz, Biranvand and Schweigert2007) with four free pleonites. (3) Serolidae (Serolis reptans Brandt, Reference Brandt1988) with three free pleonites.

Holotype

CM 54197, by monotypy, from the Barremian-Aptian Gadvan formation of Iran, deposited in the Carnegie Museum of Natural History, Pittsburgh, Pennsylvania.

Emended diagnosis

Pleonites 1–4 free and nonoverlapping, pleonite epimera proportionally short, with acute tip. Pleonite 5 fused with pleotelson, pleonite 5 epimera still present and protruding from the anterolateral margin of pleotelson. Uropodal insertion on posteroventral margin of pleotelson. Pleotelson with strong median longitudinal carina and two less-developed lateral carinae.

Emended description

Pleonites 1–4 free, near-equally long and nonoverlapping. Pleonites 3–5 epimera proportionally short, with acute tip directed slightly posteriorly; pleonite 4 epimera shortest. Pleonite 5 fused with pleotelson; pleonite 5 epimera present, demarcated with furrow, and protruding distinctly from the anterolateral margin of pleotelson. Pleotelson subhexagonal in outline; lateral margins diverging posteriorly; posterior margin not preserved. Pleotelson with strong median longitudinal carina and two less-developed lateral carinae; median carina with suboval bulge anteriorly. Pleotelson surface between carinae moderately concave. Uropodal insertion on posteroventral margin of pleotelson, insertions projecting into small spine. Uropodal basipod short and robust.

Remarks

From extant serolids, T. gadvanensis is rather close to Acutiserolis spinosa (Kussakin, Reference Kussakin1967); both species share the subhexagonal pleotelson with three distinct longitudinal carinae. The uropodal insertion projecting into a small spine in T. gadvanensis is similar to that in Bathynatalia gilchristi Kensley, Reference Kensley1978.

In comparison with extant seroloid isopods, T. gadvanensis has a large pleotelson, more than 3 cm wide and equally long. The size of the whole animal is difficult to estimate. However, in many seroloids the pleotelson length is approximately one-third of the entire body length. If postulating a similar length ratio for T. gadvanensis, the total length of the animal would be approximately 10 cm or more. Such a size can be directly compared to representatives of the cirolanid genus Bathynomus Milne-Edwards, Reference Milne-Edwards1879 (Lowry and Dempsey, Reference Lowry and Dempsey2006) with a known Cenozoic fossil record (Wieder and Feldmann, Reference Wieder and Feldmann1989, Reference Wieder and Feldmann1992; Feldmann, Reference Feldmann1990; Kato et al., Reference Kato, Kurihara and Tokita2016; Hyžný et al., Reference Hyžný, Pasini and Garassino2019). Although seroloid isopods usually do not reach such size, being in general several centimeters long (Kussakin, Reference Kussakin1967; Brandt, Reference Brandt1988; Bruce, Reference Bruce2009), there are giants among serolids living today. The males of deepwater Ceratoserolis Cals, Reference Cals1977 may attain up to nearly 8 cm in length (Wägele, Reference Wägele1986). This genus, together with a cirolanid Bathynomus, are examples of gigantism often reported in deep-sea dwellers. As discussed further in the following, T. gadvanensis was also an inhabitant of such an environment, and its size should not be taken as a surprise.

Discussion

Systematic position

Confusing isopods for decapods as documented herein for Tricarina gadvanensis is not exceptional. For example, Ibacus cottreaui Roger, Reference Roger1946, originally described as an achelatan lobster, was reassigned to cirolanid isopods by Feldmann and Charbonnier (Reference Feldmann and Charbonnier2011), who turned the animal downside-up, so to speak. It seems that in contrast to decapod crustaceans, recognizing a correct body orientation in isopod fossils is less straightforward given much less attention to the latter group from both scholars and amateur fossil hunters. Incomplete fossil isopod specimens require careful evaluation before further taxonomic implications are developed. Because of different tagmatization patterns in decapods and isopods and variable configuration of the fused segments in the latter (Brandt and Poore, Reference Brandt and Poore2003), the correct assignment of fossils to higher malacostracan taxa can be surprisingly tricky. Therefore, we encourage the use of more comparative material of extant taxa in describing fossils. Crustaceans exhibit a great disparity in overall body morphology (McLaughlin, Reference McLaughlin1980) and diverse ontogenies (Martin et al., Reference Martin, Olesen and Høeg2014) across and within the taxa. In many cases, a thorough screening of published literature suffices, although direct first-hand observation of extant animals for comparative purposes is invaluable.

Fusion of pleonites in Sphaeromatidea

In the evolution of Sphaeromatidea, various states of fusion between the pleonites and pleotelson occur (Brandt and Poore, Reference Brandt and Poore2003). In extant representatives of sphaeromatideans, lateral pleonal pleura are not recognizable when already fused with the pleotelson. In this respect, T. gadvanensis is unique because it shows the state when the pleonal pleura are still present after initial fusion with the pleotelson. This state helps to interpret the four free (not fused) pleonal segments of T. gadvanensis as being pleonites 1–4, whereas pleonite 5 is fused with the pleotelson.

In many sphaeromatideans, the anterior pleonites often are partly overlapped or concealed completely by the last pereonite (Jacobs, Reference Jacobs1987; Wall et al., Reference Wall, Bruce and Wetzer2015). In addition, some pleonites may be overlapped by other pleonites (Bruce, Reference Bruce1986; Brusca et al., Reference Brusca, Coelho, Taiti and Carlton2007). In T. gadvanensis, not only are pleonites 3–4 free, but no preceding pleonite overlaps any succeeding one, a character quite commonly found in some isopod groups (Brusca et al., Reference Brusca, Coelho, Taiti and Carlton2007).

In sphaeromatideans, the pleotelson progressively fuses with pleonites (Brandt and Poore, Reference Brandt and Poore2003). In this respect, Tricarinidae with four free, nonoverlapping pleonites (Fig. 4.2) shows a state of the pleon somewhat intermediate between Bathynataliidae Kensley, Reference Kensley1978, with all five free pleonites (Kensley, Reference Kensley1978; Fig. 4.1), and Serolidae Dana, Reference Dana1852, with three free pleonites (Brandt and Poore, Reference Brandt and Poore2003; Fig. 4.3). Thus, T. gadvanensis, with its unique state of fusion of pleonites with pleotelson, fills this gap in the evolution of the tagmatization in sphaeromatideans (see other cases of gradual evolution in Haug and Haug, Reference Haug and Haug2017). Nevertheless, in representatives of Bathynataliidae, some pleonites are overlapped by others, a characteristic not present in T. gadvanensis.

Comparisons

The evolutionary significance of Mesozoic fossils that can be assigned to Sphaeromatidea was highlighted by Brandt et al. (Reference Brandt, Crame, Polz and Thomson1999), who discussed the affinities of the Tithonian isopod Schweglerella strobli Polz, Reference Polz1998. Its peculiar pattern of body segment fusion allowed interpreting the monotypic family Schweglerellidae as a sister group to Bathynataliidae + Serolidae (Brandt et al., Reference Brandt, Crame, Polz and Thomson1999). As discussed, T. gadvanensis is morphologically even closer to seroloid isopods, and as such it is here suggested to be the sister taxon to Serolidae (Fig. 4).

Serolid isopods are characterized by a number of characters, including the presence of uropodal insertion on the anteroventral margin of the pleotelson. In T. gadvanensis, the uropodal insertion is positioned on the posteroventral margin of the pleotelson, which is considered a synapomorphy of Tricarinidae. Otherwise, the posterior portion of the exoskeleton of T. gadvanensis is remarkably similar to representatives of serolid genera Acutiserolis, Brucerolis, and Thysanoserolis. Especially, the longitudinal carinae on the pleotelson of Acutiserolis spinosa are very close to those in T. gadvanensis.

Environment preferences in seroloid isopods

The fissile black shale with the remains of Early Cretaceous T. gadvanensis was interpreted to have been deposited in an offshore, deepwater setting (Alavi, Reference Alavi2004; see also Feldmann et al., Reference Feldmann, Kolahdouz, Biranvand and Schweigert2007). Seroloid isopods commonly inhabit such environments today, often deeper than 500 m (Kensley, Reference Kensley1978; Poore and Storey, Reference Poore and Storey2009). Tricarina gadvanensis apparently was a deepwater inhabitant of muddy-bottom settings, and as such it documents rather conservative environmental preferences of seroloid isopods for at least 125 million years.

Conclusions

Tricarina gadvanensis from the Lower Cretaceous (Barremian–Aptian) of Iran, originally interpreted as a lobster (decapod crustacean), is reinterpreted herein as a representative of isopod crustaceans, with the alleged cephalothoracic shield being the pleotelson. The monotypic family Tricarinidae exhibits a unique configuration of pleonite fusion; pleonites 1–4 are free, whereas pleonite 5, with still visible epimera, is fused with the pleotelson. Tricarinidae is considered a distinct family within Seroloidea. Due to its pleonite 5 being fused, but still distinguishable, it seems to have strong affinities to extant Serolidae. The preservation of T. gadvanensis in deepwater shales speaks for conservativeness in environmental preferences of seroloid isopods for at least 125 million years.

Acknowledgments

G.C. Poore (Museums Victoria, Melbourne, Australia) provided the photographs of extant isopods used for comparative purposes. We are grateful to S. Charbonnier and an anonymous reviewer for their constructive comments on an earlier version of this manuscript. The research of MH has been supported by the Slovak Research and Development Agency under contract no. APVV-17-0555. The research of DA has been supported by the China Postdoctoral Science Foundation, Yunnan Province Postdoctoral Science Foundation, NSFC grant 41861134032, and Yunnan Provincial Research Grants 2018FA025 and 2018IA073.

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Figure 0

Figure 1.The discussed affinities of Tricarina gadvanensis. The assignment is largely based on the antero-posterior orientation of the fossil (upper part / lower part of the figure). A large, flattened structure is interpreted to be a (1, 3) carapace, (2) telson or (4) pleotelson. Feldmann et al. (2007) assigned the species to (1) Achelata and compared it to (2) Stomatopoda. Karasawa et al. (2013) reassigned the species to (3) Polychelida. The assignment to (4) Isopoda is presented herein. Abbreviations: a = basal segment of antenna; als = anterolateral spine; c = carapace; p7 = pereiopod 7; pls = posterolateral spine; plt = pleotelson; ps4–ps5 = pleonal somites 4–5; ta = thoracic appendage; u = uropod.

Figure 1

Figure 2.Tricarina gadvanensis Feldmann et al., 2007 interpreted as a seroloid isopod and its comparison with selected serolid taxa. (1, 2) T. gadvanensis, part and counterpart of the holotype CM 54197. (3) Acutiserolis spinosa (Kussakin, 1967), NIWA 24311. (4) Brucerolis nowra Poore and Storey, 2009, paratype NMV J55674. Scale bars = 10 mm.

Figure 2

Figure 3.Tricarina gadvanensis Feldmann et al., 2007 interpreted as (1) a seroloid isopod, and its comparison with posterior body portions of (2) a mantis shrimp Squilla mantis (Linnaeus, 1758) and serolid isopods (3) Acutiserolis spinosa (Kussakin, 1967), NIWA 23526, and (4) Brucerolis nowra Poore and Storey, 2009, holotype NMV J58261. mc = median carina; lc = lateral carina; u = uropod.

Figure 3

Figure 4.Variation in fusion of pleonites (labeled with numerals) with the pleotelson in seroloid isopod crustaceans. (1) Bathynataliidae (Bathynatalia gilchristi Kensley, 1978) with all five free pleonites. (2) Tricarinidae (Tricarina gadvanensis Feldmann et al., 2007) with four free pleonites. (3) Serolidae (Serolis reptans Brandt, 1988) with three free pleonites.