''Hallucigenia'' is a genus of

Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...

animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage ...

resembling worms, known from articulated fossils in Burgess Shale-type deposits in Canada and China, and from isolated spines around the world. The generic name reflects the type species' unusual appearance and eccentric history of study; when it was erected as a genus, ''H. sparsa'' was reconstructed as an enigmatic animal upside down and back to front. ''Hallucigenia'' was later recognized as a

lobopodian The lobopodians, members of the informal group Lobopodia (from the Greek, meaning "blunt feet"), or the formally erected phylum Lobopoda Cavalier-Smith (1998), are panarthropods with stubby legs called lobopods, a term which may also be used a ...

, a grade of

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ...

panarthropods from which the

velvet worm Onychophora (from grc, ονυχής, , "claws"; and , , "to carry"), commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, '' Peripatus ...

water bear Tardigrades (), known colloquially as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them Kleiner Wasserbä ...

s, and

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...

s arose.

Description

20210000 Hallucigenia diagrammatic reconstruction

''Hallucigenia'' is a long tubular animal with up to ten pairs of slender legs (

lobopods The lobopodians, members of the informal group Lobopodia (from the Greek, meaning "blunt feet"), or the formally erected phylum Lobopoda Cavalier-Smith (1998), are panarthropods with stubby legs called lobopods, a term which may also be used as ...

). The first 2 or 3 leg pairs are slender and featureless, while the remaining 7 or 8 pairs each terminate with 1 or 2 claws. Above the trunk region are 7 pairs of rigid conical sclerites (spines) corresponding to the 3rd–9th leg pairs. The trunk is either featureless (''H. sparsa'') or divided by heteronomous annulations (''H. fortis'' and ''H. hongmeia''). The "head" and "tail" end of the animal are difficult to identify; one end extends some distance beyond the legs and often droops down as if to reach the substrate. Some specimens display traces of a simple gut. Research in the mid-2010s clarified that the longer end is a head with anteroventral mouth and at least a pair of simple eyes. The shape of head differs between species – elongated in ''H. sparsa'', rounded in ''H. fortis'', while those of ''H. hongmeia'' remain unknown. At least in ''H. sparsa'', the head possesses radial teeth and

pharyngeal teeth Pharyngeal teeth are teeth in the pharyngeal arch of the throat of cyprinids, suckers, and a number of other fish species otherwise lacking teeth. ''Hallucigenia'' spines are made up of one to four nested elements. The spine surface of ''H. sparsa'' is covered in an ornament of minute triangular 'scales', while the spine surface of ''Hallucigenia hongmeia'' is a net-like texture of microscopic circular openings, which can be interpreted as the remains of Papillae.

History of study

20210917 Hallucigenia sparsa interpretations

''Hallucigenia'' was originally described by Charles Walcott as a species of the polychaete worm '' Canadia''. In his 1977 redescription of the organism,

Simon Conway Morris Simon Conway Morris (born 1951) is an English palaeontologist, evolutionary biologist, and astrobiologist known for his study of the fossils of the Burgess Shale and the Cambrian explosion. The results of these discoveries were celebrated in ...

recognized the animal as something quite distinct, establishing the new genus. No specimen was available that showed both rows of legs, and as such Conway Morris reconstructed the animal walking on its spines, with its single row of legs interpreted as tentacles on the animal's back. A dark stain at one end of the animal was interpreted as a featureless head. Only the forward tentacles could easily reach to the 'head', meaning that a mouth on the head would have to be fed by passing food along the line of tentacles. Conway Morris suggested that a hollow tube within each of the tentacles might be a ''mouth''. This raised questions, such as how it would walk on the stiff legs, but it was accepted (with reservations) as the best available interpretation. An alternative interpretation considered ''Hallucigenia'' to be an appendage of a larger, unknown animal. There had been precedent for this, as ''

Anomalocaris ''Anomalocaris'' ("unlike other shrimp", or "abnormal shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group arthropods. The first fossils of ''Anomalocaris'' were discovered in the ''Ogygopsis'' Shale of the Stephen F ...

'' had been originally identified as three separate creatures before being identified as a single huge (for its time) creature. In 1991, Lars Ramskold and Hou Xianguang, working with additional specimens of a "hallucigenid", '' Microdictyon'', from the lower

Maotianshan shales The Maotianshan Shales are a series of Early Cambrian deposits in the Chiungchussu Formation, famous for their '' Konservat Lagerstätten'', deposits known for the exceptional preservation of fossilized organisms or traces. The Maotianshan Shales ...

of China, reinterpreted ''Hallucigenia'' as a

, a legged worm-like taxon which were still thought to be exclusively related to

onychophoran Onychophora (from grc, ονυχής, , "claws"; and , , "to carry"), commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, '' Peripatus ...

(velvet worm) at that time. They inverted it, interpreting the tentacles, which they believe to be paired, as walking structures and the spines as protective. Further preparation of fossil specimens showed that the 'second legs' were buried at an angle to the plane along which the rock had split, and could be revealed by removing the overlying sediment. Ramskold and Hou also believe that the blob-like 'head' is actually a stain that appears in many specimens, not a preserved portion of the anatomy. This stain may be an artifact of decomposition.

Affinity

Since the revisions around 1990s, ''Hallucigenia'' is unquestionably a lobopodian panarthropod, although the relationship with other panarthropods remain controversial. ''Hallucigenia'' has long been interpreted as a stem-group onychophoran (velvet worms) – a position that has found support from multiple

phylogenetic analysis In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

. A key character demonstrating this affinity is the cone-in-cone construction of ''Hallucigenia'' claws, a feature shared only with modern onychophorans. On the other hand, some analysis rather support the position of ''Hallucigenia'' as a basal panarthropod outside of onychophoran stem-group. Under this scenario, the cone-in-cone structure shared between ''Hallucigenia'' and onychophorans represent panarthropod

plesiomorphy In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades. Plesiomorphy, symplesiomorphy, apomorphy, and ...

. ''Hallucigenia'' also exhibits certain characters inherited from the ancestral ecdysozoan, but lost in the modern onychophorans – in particular its distinctive foregut armature. Below is a cladogram for ''Hallucigenia'' according to Yang ''et al.'', 2015:

Diversity

In 2002,

Desmond Collins Desmond H. Collins is a Canadian paleontologist, associate professor of zoology at the University of Toronto and retired curator of invertebrate paleontology at the Royal Ontario Museum The Royal Ontario Museum (ROM) is a museum of art, world c ...

informally suggested that new ''Hallucigenia'' fossils from the Burgess Shale showed male and female forms, one with "a rigid trunk, robust neck and a globular head" and the other thinner, and with a small head. Three species of ''Hallucigenia'' have been described. The first specimen, ''Hallucigenia'' ''sparsa'', was discovered in Canada. Two other species, ''H. fortis'' and ''H. hongmeia'', are represented by the

Maotianshan Shales The Maotianshan Shales are a series of Early Cambrian deposits in the Chiungchussu Formation, famous for their '' Konservat Lagerstätten'', deposits known for the exceptional preservation of fossilized organisms or traces. The Maotianshan Shales ...

' fossils of Chengjiang.

Distribution

''Hallucigenia'' was first described from the Burgess Shale in southeastern

British Columbia British Columbia (commonly abbreviated as BC) is the westernmost province of Canada, situated between the Pacific Ocean and the Rocky Mountains. It has a diverse geography, with rugged landscapes that include rocky coastlines, sandy beaches, for ...

, Canada. 109 specimens of ''Hallucigenia'' are known from the Greater

Phyllopod bed The Phyllopod bed, designated by USNM locality number 35k, is the most famous fossil-bearing member of the Burgess Shale fossil ''Lagerstätte''. It was quarried by Charles Walcott from 1911–1917 (and later named Walcott Quarry), and was t ...

, where they comprise 0.3% of the community. ''Hallucigenia'' also forms a minor component of Chinese

lagerstätte A Lagerstätte (, from '' Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that exhibits extraordinary fossils with exceptional preservation—sometimes including preserved soft tissues. These f ...

n. Isolated hallucigeniid spines, however, are widely distributed in a range of Cambrian deposits, preserved both as carbonaceous and mineralized fossils.

References

External links

*
''Hallucigenia sparsa'' at Smithsonian Museum of Natural History
– (

Internet Archive The Internet Archive is an American digital library with the stated mission of "universal access to all knowledge". It provides free public access to collections of digitized materials, including websites, software applications/games, music, ...

)
Fossils found in Burgess Shale
(in Spanish)

{{Taxonbar, from=Q132406 Burgess Shale fossils Maotianshan shales fossils Xenusia Taxa named by Simon Conway Morris Cambrian genus extinctions Wheeler Shale