Sea spider

Sea spiders
Temporal range: Late Cambrian–present
Left to right, top to bottom: Palaeoisopus problematicus, Flagellopantopus blocki, Haliestes dasos, Austrodecus bamberi (Austrodecidae), Colossendeis sp. (Colossendeidae), Pycnogonum stearnsi (Pycnogonidae), Ammothea hilgendorfi (Ammotheidae), Endeis flaccida (Endeinae), Nymphon signatum (Nymphonidae)
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata
Latreille, 1810
Class: Pycnogonida
Gerstaecker, 1863
Type genus
Pycnogonum
Brünnich, 1764
Orders and Families

See text.

Synonyms

Arachnopoda Dana, 1853

Sea spiders are marine arthropods of the class Pycnogonida,[1] hence they are also called pycnogonids (/pɪkˈnɒɡənədz/;[2] named after Pycnogonum, the type genus;[3] with the suffix -id). The class include the only now-living order Pantopoda[4] (lit. ‘all feet’[5]), alongside a few fossil species which could trace back to the early or mid Paleozoic.[6] They are cosmopolitan, found in oceans around the world. The over 1,300 known species have leg spans ranging from 1 mm (0.04 in) to over 70 cm (2.3 ft).[7] Most are toward the smaller end of this range in relatively shallow depths; however, they can grow to be quite large in Antarctic and deep waters.

Despite their name and brief resemblance, "sea spiders" are not spiders, nor even arachnids. While some literatures around 2000s suggests they may be a sister group to all other living arthropods,[8][9] their traditional classification as a member of chelicerates alongside horseshoe crabs and arachnids regained wide support in subsequent studies.[10][11][6]

Morphology

Callipallene brevirostris

Many sea spiders are recognised by their enormous walking legs in contrast to a reduced body region, resulting into the so-called "all legs" or "no body" appearance. The body segments (somites) are generally interpreted as 3 main section (tagma): cephalon (head, aka cephalosoma), trunk (aka thorax) and abdomen.[12][13] However, the definition of cephalon and trunk might differ between literatures (see text), and some studies might follow a prosoma (=cephalon+trunk)-opisthosoma (=abdomen) definition, align to the tagmosis of other chelicerates.[14][15] The exoskeleton of the body are tube-like, lacking the dorsoventral division (tergite and sternite) as seen in most other arthropods.[13]

The cephalon was formed by the fusion of ocular somite and 4 anterior segments behind it (somite 1-4). It consists of an anterior proboscis, a dorsal ocular tubercle with eyes, and up to 4 pairs of appendages (chelifores, palps, ovigers and first walking legs). Although some literatures might consider the segment carrying the first walking leg (somite 4) to be part of the trunk,[6] it is completely fused to the remaining head section to form a single cephalic tagma.[14][13] The proboscis has three-fold symmetry, ended with a typically Y-shaped mouth (vertical slit in Austrodecidae[16]). It usually has fairly limited dorsoventral and lateral movement. However In those species that have reduced chelifores and palps, the proboscis is well developed and flexible, often equipped with numerous sensory bristles and strong rasping ridges around the mouth.[17] The proboscis is unique to pycnogonids and its exact homology with other arthropod mouthparts is enigmatic, as well as its relationship with the absence of labrum (preoral upper lip of ocular somite) in pycnogonid itself.[13] The ocular tubercles has up to two pairs of simple eyes (ocelli) on it, though sometimes the eyes can be reduced or missing, especially among species living in the deep oceans. All of the eyes are median eyes in origin, homologous to the median ocelli of other arthropods, while the lateral eyes (e.g. compound eyes) as seen in most other arthropods are completely absent.[18]

  • Pseudopallene pachycheira, showing robust chelifores and the absence of palps.
    Pseudopallene pachycheira, showing robust chelifores and the absence of palps.
  • Pycnogonum litorale, showing the absence of both chelifores and palps. Ovigers are absent in female.
    Pycnogonum litorale, showing the absence of both chelifores and palps. Ovigers are absent in female.
  • Colossendeis sp., showing the absence of chelifores but otherwise elongated proboscis, palps and ovigers.
    Colossendeis sp., showing the absence of chelifores but otherwise elongated proboscis, palps and ovigers.

In adult pycnogonids, the chelifores (aka cheliphore[12]), palps and ovigers (aka ovigerous legs[19]) are variably reduced or absent depend on taxa and sometime sexes. Nymphonidae is the only family where all of the 3 pairs are always functional. The ovigers can be reduced or missing in females, but are present in almost all males.[20] In a functional condition, the chelifore terminated with a pincer (chela) formed by 2 segments (podomeres), just like the chelicerae of most other chelicerates. The scape (peduncle) behind the pincer is usually unsegmented, but could be 2-segmented in some species, resulting into a total of 3 or 4 chelifore segments.[21][6] The palps and ovigers have up to 9 and 10 segments respectively, but can be fewer even in a functional condition.[22][23] The palps are rather featureless and never have claws in adult Pantopoda, while the ovigers may or may not possess a terminal claw and rows of specialised spines on its curved distal segments (strigilis).[23] The chelifores were used for feeding and the palps were used for sensing and manipulating food items,[24] while the ovigers were used for cleaning themselves, with additional function of carrying offsprings in male.[19]

Conditions of chelifores, palps, and ovigers by family[16][25][23][26]
appendages
families
 chelifores palps ovigers
Austrodecidae absent functional functional
(absent in some male of Austrodecus)
Rhynchothoracidae absent functional functional
Pycnogonidae absent absent absent in female
(both sexes in Nulloviger)
Colossendeidae absent
(functional in polymerous genera) 		functional functional
Endeidae absent absent absent in female
Phoxichilidiidae functional absent absent in female
Pallenopsidae functional reduced functional
Ammotheidae reduced functional functional
Ascorhynchidae reduced functional functional
Callipallenidae functional absent
(functional in some male) 		functional
Nymphonidae functional functional functional
Decolopoda australis, showing 10 legs and 4-segmented chelifores (upper left).
Tibia 2 (distal portion), tarsus, propodus and claws of various pantopod pycnogonids.

The leg-bearing somites (somite 4 and all trunk somites, the alternatively defined "trunk/thorax") are either segmented or fused to each other, carring the walking legs via a series of lateral processes (lateral tubular extension of the somites). In most species, the legs are more much larger than the body in both length and volume, only being exceptionally short and slender than the body in Rhynchothoracidae. Each leg typically compose of 8 tubular segments, commonly known as coxa 1, 2 and 3, femur, tibia 1 and 2, tarsus and propodus.[15] These terminology component with 3 coxae, no trochanter and using the term "propodus" is unusual for arthropods. However, based on muscular system and serial homology to the podomeres of other chelicerates, they are most likely coxa (=coxa 1), trochanter (=coxa 2), prefemur/basifemur (=coxa 3), postfemur/telofemur (=femur), patella (=tibia 1), tibia (tibia 2) and 2 tarsomeres (=tarsus and propodus) in origin.[27] The leg segmentation of Paleozoic taxa are a bit different, noticeably they have annulated coxa 1 and further divided into 2 types: one with flatten distal (femur and beyond) segments and first leg pair with one less segment than the other leg pairs (e.g. Palaeoisopus, Haliestes), and another one with immobile joint between the apparently fourth and fifth segment which altogether might represent a divided femur (e.g. Palaeopantopus, Flagellopantopus).[6] Each leg terminated with a main claw (aka pretarsus/apotele, the true terminal segment), which may or may not have a pair of auxiliary claws on its base. Most of the joints move vertically, except the joint between coxa 1-2 (coxa-trochanter joint) which provide lateral mobility (promotor-remotor motion), and the joint between tarsus and propodus did not have muscles, just like the subdivided tarsus of other arthropods.[15][23] There are usually 8 (4 pairs) legs in total, but a few species have exceptionally 5 to 6 pairs. These are called polymerous (i.e., extra-legged) species, which are distributed among 6 genera discovered in the families Pycnogonidae (5 pairs in Pentapycnon), Colossendeidae (5 pairs in Decolopoda and Pentacolossendeis, 6 pairs in Dodecolopoda) and Nymphonidae (5 pairs in Pentanymphon, 6 pairs in Sexanymphon).[28][15]

Several alternatives had been proposed for the position homology of pycnogonid appendages, such as chelifores being protocerebral/homologous to the labrum (see text)[9] or ovigers being duplicated palps.[29] Conclusively, the classic, morphology-based one-by-one alingment to the prosomal appendages of other chelicerates was confirm by both neuroanatomic and genetic evidences.[29][30] Noticeably, the order of pycnogonid leg pairs are mismatched to those of other chelicerates, starting from the ovigers which are homologous to the 1st leg pair of arachnids. While the 4th walking leg pair was considered align to the variably reduced 1st opisthosomal segment (somite 7, also counted as part of the prosoma based on different studies and/or taxa) of euchelicerates, the origin of the additional 5-6th leg pairs in the polymerous species are still enigmatic.[31][13] Together with the cephalic position of 1st walking legs, the anterior and posterior boundary of pycnogonid leg pairs are not align to those of euchelicerate prosoma and opisthosoma, nor the cephalon and trunk of pycnogonid itself.[14]

somites
taxa
 0
(ocular somite) 		1 2 3 4 5 6 7
Euchelicerates labrum chelicerae pedipalps leg 1 leg 2 leg 3 leg 4 chilarium in horseshoe crabs, appendage absent in arachnids
Pycnogonids ? chelifores palps ovigers leg 1 leg 2 leg 3 leg 4

The abdomen (aka trunk end[21]) does not have any appendages. In Pantopoda it is also called anal tubercle,[31][32] which is always unsegmented, highly reduced and almost vestigial, simply terminated by the anus. It is consider to be a remnant of opisthosoma/trunk of other chelicerates, but it is unknown which somite (s) it actually aligned to. So far only Paleozoic species have segmented abdomen (at least up to 4 segments, presumably somite 8-11 which aligned to opisthosomal segment 2-5 of euchelicerates), with some of them even terminated by a long telson (tail).[12][33][6]

Internal anatomy and physiology

Digestive tract (yellow highlight) of a pantopod pycnogonid
Female Ammothella longipes with pedal gonads full of eggs
  • Sagittal section of an ascorhynchid pycnogonid, showing pharynx (F), mid gut (H) and central nervous system (B).
    Sagittal section of an ascorhynchid pycnogonid, showing pharynx (F), mid gut (H) and central nervous system (B).
  • Transverse section of a pycnogonid leg, showing gut diverticulum (C, D) and reproductive gland (E)
    Transverse section of a pycnogonid leg, showing gut diverticulum (C, D) and reproductive gland (E)

A striking feature of pycnogonid anatomy is the distribution of their digestive and reproductive system. The pharynx inside the proboscis was lined by dense setae, which is possibly related to their feeding behaviour.[17] A pair of gonads (ovaries in female, testes in male) located dorsally in relation to the digestive tract, but the majority of these organs are branched diverticula throughout the legs because its body is too small to accommodate all of them alone. The midgut diverticula are very long, usually reach beyond the femur (variably down to tibia 2, tarsus or propodus) of each leg, except in Rhynchothoracidae which only reach coxa 1. Some species have additional branches (in some Pycnogonum) or irregular pouches (in Endeis) on the diverticula. There is also a pair of anterior diverticula which corresponded to the chelifores or inserted to the proboscis in some chelifores-less species. The palps and ovigers never contain diverticula, although some might possess a pair of small diverticula near the bases of these appendages.[34] The gonad diverticula (pedal gonad) reaching each femur and opened via a gonopore located at coxa 2.[35] The structure and number of the gonopores might differ between sexes (e.g. larger in female, variably absent at the anterior legs of some male).[19] In males, the femur or both femur and tibia possess cement glands.[19]

Pycnogonids do not require a traditional respiratory system (e.g. gills). Instead, gasses are absorbed by the legs via the non-calcareous, porous exoskeleton and transferred through the body by diffusion.[36] The morphology of pycnogonid creates an efficient surface-area-to-volume ratio for respiration to occur through direct diffusion. Oxygen is absorbed by the legs and is transported via the hemolymph to the rest of the body with an open circulatory system.[37] The small, long, thin pycnogonid heart beats vigorously at 90 to 180 beats per minute, creating substantial blood pressure. The beating of the heart drives circulation in the trunk and in the part of the legs closest to the trunk, but is not important for the circulation in the rest of the legs.[37][38] Hemolymph circulation in the legs is mostly driven by the peristaltic movement of the gut diverticula that extends into every leg, a process called gut peristalsis.[37][38] In the case of taxa without a heart (e.g. Pycnogonidae), the whole circulatory system was presume to be solely maintained by gut peristalsis.[34]

The central nervous system of pycnogonid largely retain a segmented ladder-like structure. It consisting of a dorsal brain (supraesophageal ganglion) and a pair of ventral nerve cords, intercepted by the esophagus. The former is a fusion of the first and second brain segments (cerebral ganglia): protocererum and deutocerebrum, corresponded to the eyes/ocular somite and chelifores/somite 1 respectively. The whole section was rotated, as the protocerebrum goes upward and the deutocerebrum shifted forward.[39] The third brain segment, tritocerebrum (corresponded to the palps/somite 2), were fused to the oviger/somite 3 ganglia instead, which was followed up by a series of leg ganglia (somite 4 and so on). The leg ganglia might shift anteriorly or even clustered together, but never highly fused into the ring-like synganglion of other chelicerates.[34] The abdominal ganglia are vestigal, absorb by the preceeded leg ganglia during juvenile development.[31]

Distribution and ecology

Nymphon leptocheles grazing on a hydroid

Sea spiders live in many different oceanic regions of the world, from Australia, New Zealand, and the Pacific coast of the United States, to the Mediterranean Sea and the Caribbean Sea, to the north and south poles. They are most common in shallow waters, but can be found as deep as 7,000 metres (23,000 ft), and live in both marine and estuarine habitats. Pycnogonids are well camouflaged beneath the rocks and among the algae that are found along shorelines.

Sea spiders are benthic in general, usually walk along the bottom with their stilt-like legs, but they also capable of swimming by using an umbrella pulsing motion,[40] and some Paleozoic species with flatten legs might even have a nektonic lifestyle.[12][6] Sea spiders are mostly carnivorous predators or scavengers that feed on soft-bodied invertebrates such as cnidarians, sponges, polychaetes, and bryozoans, by inserting their proboscis into targeted prey item. Although they are known to feed on sea anemones, most sea anemones survive this ordeal, making the sea spider a parasite rather than a predator of sea anemones.[24] A few species such as Nymphonella tapetis are specialised endoparasites of bivalve mollusks.[41]

Not much is known about the primary predators of sea spiders, if any. At least some species have obvious defensive methods such as amputating and regenerating body parts,[42][43] or making itself unpleasant meal via high level of ecdysteroids (ecdysis hormone).[44] On the other hand, sea spiders are known to be infected by parasitic gastropod and bivalve mollusks[45][46][47] or hitch‐rided by sessile animals such as goose barnacles, which may negatively affect their locomotion and respiratory efficiency.[48]

Reproduction and development

Tanystylum californicum with eggs, ventral view.

All sea spiders have separate sexes, except the only known hermaphroditic species Ascorhynchus corderoi and some extremely rare gynandromorph cases.[19] Among all extant families, the Colossendeidae and Austrodecidaeare the only two that still lacking any observations on their reproductive behaviour and life cycle.[19][32] Reproduction involves external fertilisation when male and female stack together (usually male on top), exceeding sperm and eggs from the gonopores of their leg coxae.[19] After fertilisation, males glue the egg cluster with cement glands and using their ovigers (the oviger-lacking Nulloviger using only the ventral body wall) to take care of the laid eggs and young.[19]

Protonymphon larva of Achelia spinosa

In most cases, the offsprings hatched as a distinct larval stage known as protonymphon. It has a blind gut and the body consists of a cephalon and its first 3 pairs of cephalic appendages only: the chelifores, palps and ovigers. In this stage, The chelifores usually have attachment glands, while the palps and ovigers are subequal, 3-segmented appendages known as palpal and ovigeral larval limbs. When the larvae moult into the postlarval stage, they undergoing transitional metamorphosis: the leg-bearing segments develop and the 3 pairs of cephalic appendages further develop or reduce. The postlarva eventually metamorph into a juvenile that looks like a miniature adult, which will continue to moult into adult with fix number of walking legs.[32][49] In Pycnogonidae, the ovigers reduced in juveniles but reappeared in oviger-bearing adult males.[14]

These kind of "head-only" larvae and its anamorphic metamorphosis resemble crustacean nauplius larvae and megacheiran larvae, all together might reflects how the larvae of a common ancestor of all arthropods developed: starting its life as a tiny animal with a few head appendages, while new body segments and appendages were gradually added as it was growing.[14][50]

Further details of the postembryonic developments of sea spiders vary, but their categorization might differ between literatures. As of 2010s, there are 5 types being identified as follows:[32]

Type
Characteristics
 1 2 3 4 5
Also known as typical protonymphon attaching larva (partially), lecithotrophic protonymphon atypical protonymphon encysted larva attaching larva (partially)
Hatch as protonymphon protonymphon protonymphon protonymphon postlarva
Palpal and ovigeral larval limbs functional, claw-like functional, claw-like functional, claw-like functional, filament-like variably reduced or absent
Hatching with walking leg buds no no no no at least leg 1-2 present
Walking leg development sequential sequential synchornized for all legs synchornized for leg 1-3 remaining legs sequential
Instar leaving father protonymphon postlarva with at least leg 1-2 protonymphon protonymphon postlarva with at least leg 1-2
Postlarval life cycle parasite of cnidarians and rarely mollusks lecithotrophic on ovigers, thereafter free living ectoparasites of mollusks and polychates endoparasite of hydrozoans lecithotrophic on oviger, thereafter free living
Occured taxa Ammotheidae, Ascorhynchidae, Endeidae, Nymphonidae, Pallenopsidae, Pycnogonidae Ammotheidae, Nymphonidae Ammotheidae Ammotheidae, Phoxichilidiidae Callipallenidae, Nymphonidae, Pallenopsidae

The type 1 (typical protonymphon) is the most common and possibly an ancestral one. When the type 2 and 5 (attaching larva) hatches it immediately attaches itself to the ovigers of the father, where it will stay until it has turned into a small and young juvenile with 2 or 3 pairs of walking legs ready for a free-living existence. The type 3 (atypical protonymphon) have limited observations. The adults are free living, while the larvae and the juveniles are living on or inside temporary hosts such as polychaetes and clams. The type 4 (encysted larva) is a parasite that hatches from the egg and finds a host in the shape of a polyp colony where it burrows into and turns into a cyst, and will not leave the host before it has turned into a young juvenile.[51][19][32]

Taxonomy

Phylogenetic position

Best-supported position of Pycnogonida

Pycnogonida

Cormogonida
Cormogonida hypothesis

Sea spiders had been interpreted as some kind of arachnids or crustaceans in historical studies.[52] However, after the concept of Chelicerata being established in 20th century, sea spiders have long been considered part of the subphylum, alongside euchelicerate taxa such as Xiphosura (horseshoe crabs) and Arachnida (spiders, scorpions, mites, ticks, harvestmen and other lesser-known orders).[53]

A competing hypothesis around 2000s proposes that Pycnogonida belong to their own lineage, sister to the lineage lead to other extant arthropods (i.e. euchelicerates, myriapods, crustaceans and hexapods, collectively known as Cormogonida). This Cormogonida hypothesis was first indicated by early phylogenomic analysis aroud that time,[8] followed by another study suggest that the sea spider's chelifores are not positionally homologous to the chelicerae of euchelicerates (originated from the deutocerebral segment/somite 1), as was previously supposed. Instead, the chelifore nerves were thought to be innervated by the protocerebrum, the first segment of the arthropod brain which corresponded to the ocular somite, bearing the eyes and labrum. This condition of having paired protocerebral appendages is not found anywhere else among arthropods, except in other panarthropods such as onychophoran (primary antennae) and contestably[54] in Cambrian stem-group arthropods like radiodonts (frontal appendages), which was taken as evidence that Pycnogonida may be basal than all other living arthropods, since the protocerebral appendages were thought to be reduced and fused into a labrum in the last common ancestor of crown-group arthropods, and pycnogonids did not have a labrum coexist with the chelifores. If that's true, it would have meant the sea spiders are the last surviving (and highly modified) members of an ancient, basal arthropods that originated in Cambrian oceans.[9] However, the basis of this hypothesis was immediately refuted by subsequent studies using Hox gene expression patterns, demonstrated the developmental homology between chelicerae and chelifores, with chelifore nerves innervated by a deuterocerebrum that has been rotated forwards, which was misinterpreted as protocerebrum by the aforementioned study.[30][55][39]

Alignment of anterior somites and appendages of extant panarthropods, with chelifores (Chf) indicated as deutocerebral (D, yellow) somite 1 appendages. Dark grey indicating head somites.

Since 2010s, the chelicerate affinity of Pycnogonida regain wide support as the sister group of Euchelicerata. Under the basis of phylogenomics, this is one of the only stable topology of chelicerate interrelationships in contrast to the uncertain relationship of many euchelicerate taxa (e.g. poorly resolved position of arachnid orders other than tetrapulmonates and scorpions; non-monophyly of Arachnida in respect to Xiphosura).[56][57][58][59][60] This is consistent with the chelifore-chelicera homology, as well as other morphological similarities and differences between pycnogonids and euchelicerates.[52] However, due to pycnogonid's highly modified anatomy and lack of intermediate fossils, their evolutional origin and relationship with the basal fossil chelicerates (such as habeliids and Mollisonia) are still difficult to compare and interpret.[61]

Interrelationship

?

stem-groups (e.g. Palaeoisopus, Flagellopantopus, Palaeopantopus)

Pantopoda
Stiripasterida

Austrodecidae

Eupantopodida
Internal phylogeny of Pycnogonida.[25][26]

The class Pycnogonida comprises over 1,300 species, which are split into over 80 genera. All extant genera are considered part of the single order Pantopoda, which was subdivided into 11 families. Historically there are only 9 families, with species of nowadays Ascorhynchidae placed under Ammotheidae and Pallenopsidae under Callipallenidae. Both were eventually separated after they are considered distinct from the once-belonged families.[16]

Phylogenomic analysis of extant sea spiders was able to establish a backbone tree for Pantopoda, revealing some consistent relationship such as the basal position of Austrodecidae, monophyly of some major branches (later redefined as superfamilies[26]) and the paraphyly of Callipallenidae in respect to Nymphonidae.[16][62][25] The topology also suggest Pantopoda undergoing multiple times of cephalic appendage reduction/reappearance and polymerous species acquisition, contray to previous hypothesis on pantopod evolution (cephalic appendages were thought to be progressively reduced along the branches, and polymerus condition were though to be ancestral).[26] On the other hand, the position of Ascorhynchidae and Nymphonella are less certain across multiple results.[25][26]

The position of Paleozoic pycnogonids are poorly examined, but most, if not, all of them most likely represent members of stem-group basal than Pantopoda (crown-group Pycnogonida), especially those with segmented abdomen, a feature that was most likely ancestral and reduce in the Pantopoda lineage.[63][33][64][26][6] While some phylogenetic analysis placing them within Pantopoda, this result is questionable as they have low support values and based on outdated interpretation of the fossil taxa.[31][65][66]

According to the World Register of Marine Species, the Class Pycnogonida is subdivided as follows[67] (with subsequent updates on fossil taxa after Sabroux et al. (2023[23], 2024[6])):

Fossil record

Reconstruction of the larva Cambropycnogon klausmuelleri
Reconstruction of Palaeoisopus problematicus
Fossil of Colossopantopodus boissinensis

The fossil record of pycnogonids is scant, represented only by a handful of fossil sites with exceptional preservation (Lagerstätte). While most of them are discovered from Paleozoic era, unambiguous evidence of crown-group (Pantopoda) only restricted to Mesozoic era.[6]

The earliest fossils are Cambropycnogon discovered from the Cambrian 'Orsten' of Sweden (ca. 500 Ma). So far only its protonymphon larvae had been described, featuring some traits unknown from other pycnogonids such as paired anterior projections, gnathobasic larval limbs and annulated terminal appendages.[68] Due to its distinct morphology, some studies have argued that this genus is not a pycnogonid at all.[25]

Ordovician pycnogonids are only known by Palaeomarachne (ca. 450 Ma), a genus found in William Lake Provincial Park, Manitoba and described in 2013. It only preserve possible moults of the fragmental body segments, with one showing an apparently segmented head region.[73] However, just like Cambropycnogon, its pycnogonid affinity was questioned by some studies as well.[26]

The Silurian Coalbrookdale Formation of England (Haliestes, ca. 425 Ma) and the Devonian Hunsrück Slate of Germany (Flagellopantopus, Palaeopantopus, Palaeoisopus, Palaeothea and Pentapantopus, ca. 400 Ma) include unambigious fossil pycnogonids with exceptional preservation. The latter is by far the most diverse community of fossil pycnogonids in terms of both species number and morphology. Some of them are significant in that they possess something never seen in pantopods: annulated coxae, flatten swimming legs, segmented abdomen and elongated telson. These provide some clues on the evolution of sea spider bodyplan before the arose and diversification of Pantopoda.[12][65][6]

Fossil of Mesozoic pycnogonids are even rare, and so far all of them are Jurassic pantopods. Historically there are two genus (Pentapalaeopycnon and Pycnogonites) from the Solnhofen Limestone (ca. 150 Ma) of Germany being described as such, which are in fact misidentified phyllosoma larvae of decapod crustaceans.[68] The actual first report of Mesozoic pycnogonids was described by researchers from the University of Lyon in 2007, discovering 3 new genus (Palaeopycnogonides, Colossopantopodus and Palaeoendeis) from La Voulte-sur-Rhône of Jurassic La Voulte Lagerstätte (ca. 160 Ma), south-east France. The discovery fill in an enormous fossil gap in the record between Devonian and extant sea spiders.[74][75] In 2019, a new species of Colossopantopodus and a specimen possibly belong to the extant genus Eurycyde were discovered from the aforementioned Solnhofen limestone.[76]

References

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