Urokodia
| Urokodia Temporal range: | |
|---|---|
 | |
| Fossil specimen with preserved appendages | |
 | |
| Life restoration after study published in 2026 | |
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Clade: | Deuteropoda |
| Genus: | †Urokodia Hou et al, 1989 |
| Type species | |
| Urokodia aequalis Hou et al, 1989 | |
Urokodia is an extinct genus of marine arthropod from the early Cambrian. The only known species is Urokodia aequalis from the Maotianshan Shales of China based on some 15 specimens.[1]
Description
[edit]
Urokodia is an elongated arthropod measuring up to 3.5 cm. The dorsal exoskeleton (tergite) is composed of a cephalon (head) and a pygidium (tail shield) of subequal size. Both were initially thought to be nearly identical, possessing 4 pairs of large lateral spikes. However, subsequent studies revealed that the pygidium had a distinct triangular shape and a parallel row of tiny posterior spikes. Between the cephalon and pygidium was an elongated thorax of 14 or 15 segments.[2] It possesses a pair of stalked eyes.[3][4]
The interpretation of ventral structures other than the eyes differ between redescriptions from two different research teams. Liu et al. 2024 interpreted the first appendages as a pair of stubby antennae. The remaining ventral region possesses subequal appendages with lamellar exopods (lobe-like outer branches), 3 pairs under both cephalon and pygidium and 1 pair under each thoracic segment. All but the pygidial appendages are biramous (two-branched), bearing annulated basipods (limb bases) and segmented, stenopodous endopods (leg-like inner branches).[3]
In contrast, Liu et al. 2026 interpreted the first appendages as a pair of large but relatively short pincer-like appendages. Each have 2 segments developed into the prominent fix and movable finger, with another 2 reduced segments in between their mobile joint. For the remaining appendages, they suggest the head have 5 instead of 3 pairs, and the last 4 thoraxic segments are also lack of endopods. In the case of biramous appendages, they also revealed endites (inner spike) underneath the endopods and 5 layers of additional gill-like exites lying behind each exopod. They also discover a small anterior sclerite underneath the front and a hypostome (ventral mouth plate) fringed by setae.[4]
Taxonomy
[edit]The ventral appendages of Urokodia had been unknown for a long time, and its dorsal features resemble Mollisoniidae (e.g. Mollisonia, Thelxiope, and Corcorania) at first glance. This led to the previous classification of Urokodia as a member of the order Mollisoniida with other mollisoniids. Based on the discovery of Mollisonia appendages in 2019, the group is suggested to be stem-chelicerates.[5]
However, Liu et al. 2024 discovered the appendages of Urokodia differed markedly from those of mollisoniids or of any other chelicerates. Phylogenetic analysis based on the new discovery instead places Urokodia as the most basal member of Artiopoda, an extinct group which includes trilobites and their relatives. The basal position suggest the long, slender body shape of Urokodia represent an ancestral trait before the evolution of compact, widen tergites as seen in most artiopods.[3]
However, Liu et al. 2026 again interpreted Urokodia as a stem-chelicerate, thought not a member of mollisoniids, with Urokodia basal than mollisoniids and represent a transitional form between megacheirans and true chelicerates. For example, the pincer-like appendages of Urokodia with some reduced segments represent an intermediate form between the multi-segmented great appendages of megacheirans and the 2-segmented pincer as seen in the chelicerae of true chelicerates. The exites are numerous than megacheirans which brings it closer to chelicerate book gills. The post-oral appendages are not differentiated between the boundary of head and trunk tagma, just like megacheirans, but the head itself has the amount of fused body segments align with true chelicerates (ocular segment+6) instead of megacheirans (ocular segment+5 or less).[4]
References
[edit]
- ↑ Hou, Xian-guang; Siveter, David J.; Siveter, Derek J.; Aldridge, Richard J.; Cong, Pei-yun; Gabbott, Sarah E.; Ma, Xiao-ya; Purnell, Mark A.; Williams, Mark (12 April 2017). The Cambrian Fossils of Chengjiang, China: The Flowering of Early Animal Life (1 ed.). Chichester, West Sussex, UK: Wiley. Bibcode:2017cfcc.book.....H. doi:10.1002/9781118896372. ISBN 978-1-118-89638-9.
- ↑ Zhang, Xingliang; Han, Jian; Shu, Degan (2002). "New occurrence of the Burgess Shale arthropod Sidneyia in the Early Cambrian Chengjiang Lagerstätte (South China), and revision of the arthropod Urokodia". Alcheringa: An Australasian Journal of Palaeontology. 26 (1): 1–8. Bibcode:2002Alch...26....1Z. doi:10.1080/03115510208619239. ISSN 0311-5518.
- 1 2 3 Liu, Cong; Fu, Dongjing; Wu, Yu; Zhang, Xingliang (July 2024). "Cambrian euarthropod Urokodia aequalis sheds light on the origin of Artiopoda body plan". iScience. 27 (8) 110443. Bibcode:2024iSci...27k0443L. doi:10.1016/j.isci.2024.110443. PMC 11325232. PMID 39148713.
- 1 2 3 Liu, Yu; Lustri, Lorenzo; Mai, Huijuan; Williams, Mark; Liu, Liming; Tang, Yani; Harvey, Thomas H. P.; Hou, Xianguang (2026-07-01). "Urokodia sheds light on the origin of chelicerae and book gills of Chelicerata". Nature. doi:10.1038/s41586-026-10713-2. ISSN 0028-0836.
- ↑ Lerosey-Aubril, Rudy; Skabelund, Jacob; Ortega-Hernández, Javier (2020-04-09). "Revision of the mollisoniid chelicerate(?) Thelxiope, with a new species from the middle Cambrian Wheeler Formation of Utah". PeerJ. 8 e8879. doi:10.7717/peerj.8879. ISSN 2167-8359. PMC 7151752. PMID 32296605.
- Urokodia aequalis Arthropod from Maotianshan Shales - URL retrieved September 11, 2010
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