| Palaeos | |
Panarthropoda |
| Ecdysozoa | Lobopodia (2) |
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Abbreviated Dendrogram
ECDYSOZOA |==SCALIDOPHORA `--+--NEMATOIDA | `--PANARTHROPODA |--Aysheaia `--+--TARDIGRADA `--+--Xenusiidae |--ONYCHOPHORA `--+--Paucipodia |--Microdictyon |--Archonychophora | |--Cardiodictyon | `--+--Hallucigenia | `--+--Onychodictyon | `--Luolishaniidae |--Diania `--+==Siberiidae `--+==DINOCARIDIDA `--ARTHROPODA |
Contents
Overview |
Paucipodia inermis Chen et al., 1995
Horizon: Yuanshan Member, Qiongzhusi Formation, Maotianshan Shales, Chengjiang Lagerstätte, Yunnan, South China, (Atdabanian age - Early Cambrian)
Phylogeny: Panarthropoda : Mureropodia + (Aysheaia + (Tardigrada + (Xenusiidae + Orstenotubulus + Onychophora + ((paraphyletic Siberiidae + (Kerygmachela + (Dinocaridida + (Schinderhannes + Arthropoda)))) + Microdictyon + Archonychophora + Diania + *)))
Comments Appears to be a very simple lobopodian, unarmoured, with only six, or alternatively nine, pairs of appendages (hence the name), and a proboscis-like head area (a primitive feature). Most cladograms place it in a basal position (although more derived than the apparently more complicated Aysheaia). But regularily spaced paired pigment patches along the presumed nerve cord could be ganglia, if they are not taphonomic artifacts. Their regular spacing in relation to the limbs is closer to the segmental ganglia of arthropods than to the unganglionated nerve cord of onychophorans. This would mean that Paucipodia is more derived than extant Onychophora, despite its otherwise primitive features (Edgecombe 2009 pp.185-6), an example of mosaic evolution. MAK120427
Microdictyon Bengtson et al. 1981 emend. Chen, Hou & Lu 1989 : M. effusum, M. rhomboidale, M. robisoni, M. sinicum, M. sphaeroides, M. tenuiporatum, etc
Range: Early Cambrian (Atdabanian to Toyonian), Cosmopolitan
Phylogeny: Panarthropoda : Mureropodia + (Aysheaia + (Tardigrada + (Xenusiidae + Orstenotubulus + Onychophora + ((paraphyletic Siberiidae + (Kerygmachela + (Dinocaridida + (Schinderhannes + Arthropoda)))) + Paucipodia + Archonychophora + Diania + *)))
Comments: an armoured lobopodian, very similar to, and perhaps closely related to Hallucigenia (Ramsköld & Chen (1998)) but without the spines. Isolated sclerites have long been known for example from Siberia as components of the small shelly fauna, and as with many such structures, their nature was puzzling. The discovery of Microdictyon sinicum (Chen, Hou and Lu, 1989) in the Chengjiang Lagerstätte (Early Cambrian Maotianshan Shale) of Yunnan China, solved the problem, as it was realised that the sclerites were attached to a lobopodian-like animal. Microdictyon sinicum is a typical lobopodian, with ten pairs of prominent net- or grate-like structure of flat trunk sclerites on the sides, matched to a pair of long slender limbs. The head and posterior are tubular and featureless (Wikipedia). Unlike other armoured lobopodians, in which there are generally an additional one or two limb pairs behind the head, in Microdictyon every limb pair is associated with a pair of sclerites. Ramsköld & Chen (1998), pointing out the similaritiy to Hallucigenia, note that if the first two sclerite pairs are removed, Microdictyon would have the same configoration of sclerites and limbs as Hallucigenia. So either these two taxa are closely related, as the authors argue, or else rthis is a shared primitive feature (p[lesiomorphy) of armoured lobopodians.
The suggestion that the sclerites may be eyes or eye-like structures, or a supportive structure for lateral paired eyes (Dzik 2003, Gehring 2012), is unlikely to be the case, as true small single eyes have been figured from Miraluolishania, Hallucigenia and Cardiodictyon (Steiner et al 2012 p.121, although this does not deny the possibility that arthropod compound eyes (as opposed to simple lobopodian eyes) may have developed from proto-arthropod sclerite-like structures (a bit like mammalian ear bones developing from therapsid jaw bones).
the following species are listed (Wikipedia), many from isolated sclerites, although no attempt here is made to work out their phylogeny
Placed in the monotypal Family Eoconchariidae Hou & Shu 1987 (= Microdictyonidae Chen, Hou & Lu 1989) ref), in the monotypal Order Scleronychophora by Hou & Bergström (1995). Such taxonomic speculation does little to elucidate evolutionary relationships. The wide distribution of these tiny structures shows that lobopodians were an important element of early Cambrian faunas. Of course, not all need necessarily belong to the genus Microdictyon itself.
Microdictyon sinicum and the related Paucipodia inermis seem to have lived in close association with other animals like sponges or the enigmatic jelyfish ecomorph Eldonia. It is not clear whether they were parasitic on live Eldonia, feeding on carcasses of eldoniids on the sea bottom (Chen et al 2007), or had some other relation, perhaps mutualist or commensualist. MAK120427
Archonychophora Hou & Bergstrom, 1995 sensu Ma et al 2010
Horizon: Early to Middle Cambrian
Phylogeny: Panarthropoda : Mureropodia + (Aysheaia + (Tardigrada + (Xenusiidae + Orstenotubulus + Onychophora + ((paraphyletic Siberiidae + (Kerygmachela + (Dinocaridida + (Schinderhannes + Arthropoda)))) + Paucipodia + Microdictyon + Diania + * : Cardiodictyon + (Hallucigenia + (Onychodictyon + Luolishaniidae)))))
Characters: Differentiation of head from body, a set of anterior appendages ventrally at the base of the head (Liu et al 2011); trunk bears spine-shaped sclerites, which are often differentiated with regard to trunk tagmosis.
Comments Ma et al 2009 and 2010 distinguish two Cambrian lobopodian clades. Microdictyon, Paucipodia, Jianshanopodia, Hadranax and Xenusion (which would be family Xenusiidae), Megadictyon, and finally as the most derived forms extant onychophora, are placed in the first clade. The characteristics of this clade are more likely to be convergences or plesiomorphic. Large body size and tapering body shape are not features found in Microdictyon, Paucipodia, or extant onychophores. Limbs shortening towards one end of the body is found in both clades, The absence of spine-shaped sclerites in the Xenusiidae may be actual or simply be due to poor poreservation, while Paucipodia could be better understood as secondarily unarmoured. Lack of a distinct head and body tagmosis is a plesiomorphic feature, and in any case does not apply to Jianshanopodia or Megadictyon, both of which are transitional to protoarthropods. Their second clade, for which they used Hou & Bergstrom's taxon Archonychophora (but in a different definition) seems to be better defined. It includes Cardiodictyon, Hallucigenia, Onychodictyon, Luolishania, and the "Collins' Monster". and features many arthropodian and protoarthropodian characteristics. In contrast, Liu et al 2011 recognised only a single lobopodian clade, but consider Jianshanopodia and Megadictyon to be stem arthropods rather than stem euonychophora. For now we are tentatively positing a monophyletic Archonychophora intermediate between basal Xenusian klobopodians and Siberiid protodinocaridid lobopodians, but this may also be a paraphyletic grade, with some forms such as Luolishania and the "Collins' Monster" (= family Luolishaniidae) being mnore derived and arthropod-like than standard armoured lobopodians like Cardiodictyon, Hallucigenia, and Onychodictyon. The heavily armoured Diania may represent a third lineage, intermediate between these two. MAK120430
Cardiodictyon catenulum Hou et al., 1991
Horizon: Yuanshan Member, Qiongzhusi Formation, Maotianshan Shales, Chengjiang Lagerstätte, Yunnan, South China, (Atdabanian age - Early Cambrian)
Phylogeny: Archonychophora : (Hallucigenia + (Onychodictyon + Luolishaniidae)) + *
Comments This most elongate of all the Cambrian lobopodians, with 23 to 25 pairs of appendages on the trunk (more than twice the usual number), along with pairs of unique triangular sclerites that change in ontogeny from V-shaped in small adyults to shield shaped in large adults. The legs are finely and densely annulated, each with two curved, pointed terminal claws. There are at least one pair of set of anterior appendages at the base of the head (Ramsköld & Chen (1998), Liu et al 2011). The existence of head sclerites was reported by Hou & Bergström (1995), but Ramsköld & Chen (1998) found no evidence of any in the specimens they examined. MAK120501
Onychodictyon ferox Hou, Ramsköld, and Bergström, 1991
Horizon: Yuanshan Member, Qiongzhusi Formation, Maotianshan Shales, Chengjiang Lagerstätte, Yunnan, South China, (Atdabanian age - Early Cambrian)
Phylogeny: Archonychophora : Cardiodictyon + (Hallucigenia + (Luolishaniidae + *))
Comments robust, short-limbed lobopodian, consisting of a head with large papillae and an elongate trunk. The trunk with 10 segments, each with a pair of dorsal round plates and lobe-like appendages on the ventro-lateral surface. Terminal claws prominent, indicative of an adapation to specific lifestyle in the epibenthos, perhaps climbing on algae or sponges. In addition to the standard trunk sclerites, head sclerites have been postulated by Ramsköld & Chen (1998). There are two species, the larger Onychodictyon ferox Hou, Ramsköld, and Bergström, 1991, 5 to 7 cm long, and the smaller Onychodictyon gracilis Liu et al 2008 upto 5 cm long, although it is possible these sexual dimorphs. (Liu et al 2008b). Although Onychodictyon ferox doesn't show any sign of anterior appendages, Onychodictyon gracilis shows one pair (Liu et al 2011) , so it is likely that the absense in O. ferox is due to poor preservation. As with Halluciginia, elongated spines protruding from the plates, (Liu et al 2011). Hou & Bergström 1995 consider Onychodictyon to be the nearest to extant onychophorans of any of the other Cambrian lobopodians, and Onychodictyon is often reconstructed to look like a squat, spiny Peripetus. Subsequent cladograms and phylogenies place it near Cardiodictyon, Hallucigenia, and Luolishania ( Ma et al 2009, Liu et al 2011, strict consensus of the latter's data by Mounce & Wills 2011 and Legg 2011 result in a (Cardiodictyon + (Onychodictyon + Hallucigenia)) clade with Luolishania resolving separately), despite the others all being long-limbed forms. It may in addition to (Dzik 2011) or apart from, those taxa also be close to Diania. MAK120427
Diania cactiformis Liu et al 2011
Horizon: Yuanshan Member, Qiongzhusi Formation, Maotianshan Shales, Chengjiang Lagerstätte, Yunnan, South China, (Atdabanian age - Early Cambrian)
Phylogeny: Panarthropoda : Mureropodia + (Aysheaia + (Tardigrada + (Xenusiidae + Orstenotubulus + Onychophora + ((paraphyletic Siberiidae + (Kerygmachela + (Dinocaridida + (Schinderhannes + Arthropoda)))) + Paucipodia + Microdictyon + Archonychophora + *))))
Comments nicknamed the "walking cactus" because of its appearance. The generic name is from Dian, a linguistic abbreviation in Chinese of the Yunnan province, where the Chengjiang fossil Lagerstatte is located. The species name speaks for itself.
Although similar to other Cambrian lobopodians in most respects, Diania is remarkable in being densely covered on most of the trunk and espcially the limbs by numerous spinose sclerites. Sclerotization is usually not seen in the limbs of lobopodians except in the paired trunk sclerites and sclerotized claws. The microstructure of sclerites in Diania also differs from the net-like microstructure of trunk sclerites in other xenusian lobopodians
As well as a protective function, massive sclerotization of the outer cuticle enhanced body rigidity, and required the development of jointed legs. is the most basal panarthropod to possess articulated limbs (all other lobopodians had soft cuticle and hence naturally flexible limbs). Compared to puny limbs of Hallucigenia or Hallucigenia, or the stumpy ones of Aysheaia or recent onychophores the limbs are both exceptionally robust (in the illustration they appear to be as thick as or thicker than the trunk) and long, and it is easy to envisage this animal crowling along the sea floor.
In terms of limb morphology, Diania is closer to the arthropod condition than any other lobopodian known to date. Phylogenetic analysis by Liu et al (2011) empahsised the exceptional sclerotization and arthropod-like limbs of this taxon, and recovered it as the sister taxon to the Arthropoda proper, above ebven the dinocaridida (protoarthropods). The authors suggest that arthropodization (sclerotization of the limbs) preceded arthrodization (sclerotization of the body)., and that taxa such as Kerygmachela, Jianshanopodia, and Megadictyon1 constitute a paraphyletic series with different taxa expressing different grades of arthropodization. In their scenario the heavier sclerotization and partial arthropodization found in Diania led to the sclerotized exoskeleton of euarthropods.
Perhaps because of the obviously primitive grade of organisation of Diania in every way except arthropodisation, Liu et al (2011) were strongly criticised (by Mounce & Wills 2011 and Legg 2011, to which they responded Liu et al. 2011b. Other workers such as Ma et al 2009 and Dzik 2011 have provided alternative phylogenies in which different taxa (Archonychophora and Siberiidae respectively) represent uintermediate forms, although ironically each considers the others candidates much more basal for Ma et al 2009, siberiid lobopodians (although they did nort use that term as it had not yet been coined) were stem Euonychophora, whereas for Dzik 2011 archonychophora were derived Cambrian lobopodians,that either went nowhere or became pentostomids. We feel a more reasonable hypothesis than thes ethree is that there was a tremendous amount of parallelism among panarthropods, witha number of different lines evolving arthropodlike features while retaining various primitive traits. A similar situation can be seen with Permo-Triassic therapsids as they evolve from a reptilian to a mammalian condition. This makes determining phylogeny difficult. We have for now placed Diania as a standard lobopodian that independently aquired arthropodisation. It may or may not belong in the Archonychophora, a group taht culminated in the proto-arthropod-like Luolishaniid filter feeders. The Siberiids are another group that were independently evolving towards the dinocaridid and ultimately arthropod condition. For this reason we have placed Diania among the lobopodians rather than the protoarthropods. MAK120430
Reference and illustration: Liu et al (2011)
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