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| Phrynolambrus sagittalis Ferratges, Luque, Domínguez, Ossó, Aurell & Zamora, 2023 Reconstruction by Hugo Salais (Metazoa Studio). |
Abstract
Eubrachyurans, or ‘higher’ true crabs, are the most speciose group of decapod crustaceans and have a rich fossil record extending into the Early Cretaceous. However, most extant families are first found in the fossil record in the Palaeogene, and particularly in the Eocene. Unfortunately, fossils of many early eubrachyuran groups are often fragmentary, and only a few studies have combined extinct and extant taxa in a phylogenetic context using different optimality criteria. Here, we report the dairoidid crab Phrynolambrus sagittalis sp. nov., an enigmatic eubrachyuran from the upper Eocene of Huesca (northern Spain), whose completeness and exquisite preservation permit examination of its anatomy in a phylogenetic context. Dairoidids have previously been considered among the oldest stone crabs (Eriphioidea) or elbow crabs (Parthenopoidea), two disparate and distantly related groups of true crabs living today. Mechanical preparation and computed tomography of the fossil material revealed several diagnostic features that allow a detailed comparison with families across the crab tree of life, and test hypotheses about its phylogenetic affinities. Phrynolambrus sagittalis is the first record of the genus in the Iberian Peninsula, and represents one of the oldest crown parthenopoidean crabs worldwide, expanding our knowledge of the biogeographical distribution of elbow crabs during the Palaeogene, as well as their early origins, anatomical diversity and systematic affinities. Understanding the disparity of Eocene eubrachyurans is pivotal to disentangling the systematic relationships among crown families, and interpreting the spatio-temporal patterns leading to the evolution of modern faunas.
Keywords: eubrachyuran, fossil crab, phylogeny, Priabonian, Pyrenees, Iberian Peninsula
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| Paratypes of Phrynolambrus sagittalis sp. nov. from the upper Eocene (Pamplona Fm) of Huesca, Spain. A–B, paratype MPZ 2022/763 in dorsal view; B, detail of the cuticle with mushroom-shaped tubercles. C, MPZ 2022/203. D–F, paratype MPZ 2022/204 in: D, ventral; E, frontal; F, dorsal views. G, isolated sternum (MPZ 2022/767) assigned to the new species due to similarities with the modern Dairoides and other parthenopoids. Abbreviations: a.s., anterolateral spine; e.l., epimeral line; ep.s., epibranchial spine (equivalent to lateral angle); h.n., hepatobranchial notch; h-o.n., hepato-orbital notch; h.s., hepatic spine; i.d., interorbital depression; i.s., inter-antennular spine; o, orbit; p.b., postbranchial spine; p.g., pterygostomial groove; p.r., pterygostomial ridge; p.t., protogastric tubercles; s.g., subhepatic groove; se.r., subepibranchial region. All scale bars represent 5 mm, except G (= 2 mm). |
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| 3D artist's reconstruction of Phrynolambrus sagittalis sp. nov. based on the type material and the CT scan in: A, dorsal; B, ventral; C, frontal; D, posterior; E–F, lateral view. The ambulatory legs and colour pattern are based on the modern genus Dairoides. Drawings by Hugo Salais (Metazoa Studio). |
Infraorder BRACHYURA Latreille, 1802
Section EUBRACHYURA de Saint Laurent, 1980
Superfamily PARTHENOPOIDEA MacLeay, 1838
Family PARTHENOPIDAE MacLeay, 1838
Subfamily DAIROIDINAE Števčić, 2005
Type genus: Dairoides Stebbing, 1920.
Included genera: Aragolambrus Ferratges et al., 2019; Dairoides; Phrynolambrus Bittner, 1893.
Genus PHRYNOLAMBRUS Bittner, 1893
Type species: Phrynolambrus corallinus Bittner, 1893, by original designation.
Included species: Phrynolambrus corallinus; P. italicus De Angeli & Beschin, 2008; P. sagittalis.
Emended diagnosis: Carapace rhomboid (triangular?) anteriorly narrowing, rostrum short, projected; hepatic region convex laterally, remainder of anterolateral margin slightly convex, then extending into stout, triangular lateral spine; posterolateral margin convex; strong epibranchial spine, oriented obliquely posteriorly; posterior margin straight; carapace region broadly inflated and ornamented with mushroom-shaped tubercles. (Modified after Schweitzer et al. 2020a)
Phrynolambrus sagittalis sp. nov.
Derivation of name: From the Latin sagitta, in reference to its arrowhead carapace shape.
Diagnosis: Carapace triangular, wider than long; projected frontal margin; orbits inflated; anterolateral margins almost straight, with small conical spine; posterolateral margin converging posteriorly, with strong spine and marked concavity behind epibranchial spine; armed epibranchial spine, oriented obliquely posteriorly; straight posterior margin; depressed postbranchial region; carapace surface densely tuberculated by mushroom-shaped tubercles.
Occurrence: The genus Phrynolambrus is known only from the upper Eocene of Italy, Hungary and Romania (Bittner 1893; Lörenthey & Beurlen 1929; Beschin et al. 2009) and the Oligocene of Italy (De Angeli & Beschin 2008). In this work we expand its distribution to the upper Eocene of the Iberian Peninsula.
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| Reconstruction of Phrynolambrus sagittalis sp. nov. in the lower Priabonian (upper Eocene) benthos of the Pamplona Formation (Huesca province, Spain), on a hypothesized muddy bottom and seagrass meadow. Parts not preserved in the fossil are interpreted by comparison with the closest modern representative. Reconstruction by Hugo Salais (Metazoa Studio). |
CONCLUSION:
Phrynolambrus sagittalis, from the upper Eocene of Spain, is the oldest record of fossil crabs of the enigmatic subfamily Dairoidinae. The completeness and exquisite preservation of the new fossil material enabled us to examine aspects of the anatomy of Phrynolambrus and Dairoides (the type and only genus of Dairoidinae previously known) and to place them in a phylogenetic context. We conclude that Dairoides is indeed a type of elbow crab, or Parthenopoidea, rather than a type of stone crab, superfamily Eriphioidea, to which it is only superficially similar due to convergence.
The recognition of Phrynolambrus as sister group to Dairoides pushes back the origins of the subfamily Dairoidinae more than 48 myr into at least the early Eocene and provides a reliable calibration point of the subfamily for molecular phylogenetic studies. This description of the new species is also the second report of Eocene elbow crabs from the Iberian Peninsula, and it is one of the oldest parthenopid crabs discovered to date.
Our results: (1) expand our understanding of the stratigraphic and palaeobiogeographic distribution of Palaeogene elbow crabs; (2) highlight the Eocene as a time of rapid evolution of higher eubrachyuran groups; and (3) shed light on the early origins, anatomical diversity and systematic affinities of ancient Parthenopoidea, one of the most emblematic and anatomically distinctive groups of true crabs. Understanding the anatomical diversity or disparity of Palaeogene eubrachyurans is pivotal to disentangle the systematic relationships of several of the main crown families, and to investigate spatio-temporal patterns leading to the evolution of modern faunas.
Fernando A. Ferratges, Javier Luque, José Luis Domínguez, Àlex Ossó, Marcos Aurell and Samuel Zamora. 2023. The Oldest dairoidid Crab (Decapoda, Brachyura, Parthenopoidea) from the Eocene of Spain. Papers in Palaeontology. DOI: 10.1002/spp2.1494
