Platylomaspis

Platylomaspis Temporal range: Telychian, [1] 438.6–432.9  Ma PreꞒ Ꞓ O S D C P T J K Pg N
Restoration of the headshield of Platylomaspis serratus
Life reconstruction of Platylomaspis serratus
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Infraphylum:	Agnatha
Class:	† Galeaspida
Order:	† Polybranchiaspiformes
Family:	† Gumuaspidae
Genus:	† Platylomaspis Gai et al., 2018 [1]
Species:	†P. serratus
Binomial name
†Platylomaspis serratus Gai et al., 2018 [1]

Platylomaspis is an extinct genus of polybranchiaspiform galeaspid known from the Telychian age (Llandovery, Silurian) of the Tataertag Formation (Kalpin County, Xinjiang, northwest China). ^{[1] [2] [3]} As such, it is regarded as the oldest known polybranchiaspiform. ^{[2] [4] [5]} The genus has associated to it the holotype GMC V 2415.1, a nearly complete ventral head shield, and the paratype GMC V 2415.1, an incomplete dorsal head shield. ^[1]

Etymology

The generic name is a composite of the Greek platy (broad), lom (brim) and aspis (shield), referencing the broad ventral rim of the head shield. The specific epithet serratus, Latin for "serrated", highlights the serrated lateral margin of the head shield. ^[1]

Description

Paratype (A) and holotype (B) of Platylomaspis serratus, with a close-up of the latter's subcutaneous vascular plexi (C)

The head shield of Platylomaspis takes a broad pear shape, with a dome-shaped depression at its centre surrounded by a wide flattened marginal band that tapers rostrally into a long rostral process. The shield could reach a maximum width of 60-70 mm at the middle point and a length of 70–80 mm, excluding the rostral process. It is ornamented by coarse-granular tubercles and bears along its lateral marginal 5-6 serrations every 10 mm. ^[1]

The rostral process of both specimens is only proximally preserved, but it is possible to interpret an anterior tapering of the same into a rod-like structure. ^[1]

The flattened ventral region of the body, the ventral rim, is diagnostic in Platylomaspis for its dimension. It accounts for approximately one-fourth of the head-shield length at 20.0 mm. This is exceptionally broad for polybranchiaspiformes, which typically have it account for only one-tenth. ^[1]

Although they are not preserved in the available material, certain features characteristic of galeaspids are expected to have been present in the dome-shaped depression, including orbital openings, where the eyes would be located, a mediodorsal opening, through which the intake of respiratory water would occur when the animal was buried in the sand, and a sensory canal system. ^[1]

The dome encloses the oralo-branchial chamber that opens ventrally into the oralo-branchial fenestra, embraced by the ventral rim. This fenestra is anteriorly composed of a subtriangular form, the anterior oral fenestra and where the mouth would be located, that posteriorly widens into a large suborbicular shape, the posterior branchial fenestra, with 5–7 successive notches on each side that constitute the external branchial openings. In this way, Platylomaspis does not display the polybranchic condition (bearing a considerable number of gills) seen in many extinct jawless fishes. ^{[1] [5]}

Phylogeny

An analysis by Gai and colleagues in 2018 obtained the following phylogenetic tree by strict consensus of 3 maximum parsimony trees: ^[1]

Agnatha

Ateleaspis Galeaspida Changxingaspis Hanyangaspis Dayongaspis Eugaleaspiformes ⊞ ⊟ Shuyu zhejiangaspis 'Sinogaleaspidae' Anjiaspis Meishanaspis 'Shuyu' xikengensis Shuyu shankouensis Eugaleaspidae Yunnanogaleaspis Nochelaspis Eugaleaspis Pterogonaspis Polybranchiaspidida Geraspididae Geraspis Kwangnanaspis Polybranchiaspiformes Gumuaspidae Gumuaspis Platylomaspis Nanningaspis Pentathyraspidae Microhoplonaspis Pentathyraspis Duyunolepidae Duyunolepis Lopadaspis Polybranchiaspidae Bannhuanaspis Dongfangaspis Polybranchiaspis Damaspis Laxaspis Zhaotongaspididae Wenshanaspis Zhaotongaspis Huananaspiformes ⊞ ⊟ Sanchaspidae Antiquisagittaspis Sanchaspis Gantarostrataspidae Gantarostrataspis Wumengshanaspis Sanqiaspidae Sanqiaspis Huananaspidae Asiaspis Nanpanaspis Stephaspis Huananaspis Macrothyraspinae Lungmenshanaspis Qingmenaspis Macrothyraspis Sinoszechuanaspis

References

1. Gai, Zhikun; Lu, Liwu; Zhao, Wenjin; Zhu, Min (2018-09-19). Peng, Zuogang (ed.). "New polybranchiaspiform fishes (Agnatha: Galeaspida) from the Middle Palaeozoic of China and their ecomorphological implications". PLOS ONE. 13 (9) e0202217. Bibcode:2018PLoSO..1302217G. doi: 10.1371/journal.pone.0202217 . ISSN   1932-6203. PMC   6145596 . PMID   30231026.
2. Liu, Y H; Zhu, M; Lin, X H; Lu, L W; Gai, Z K (2019). "A reappraisal of the Silurian galeaspids (stem Gnathostomata) from Tarim Basin, Xinjiang". Vertebrata PalAsiatica. 57 (4): 253–273. doi:10.19615/j.cnki.1000-3118.190524.
3. Shan, X R; Zhao, W J; Lin, X H; Chen, Y; Li, Q; Zhu, M; Gai, Z K (2022). "The correlations of the lower red beds of early Telychian (Llandovery, Silurian) in China from the palaeoichthyological evidence" 中国志留纪兰多维列世特列奇早期下红层对比之古鱼类化石证据. Journal of Stratigraphy (in Chinese). 46 (2): 138–153. doi:10.19839/j.cnki.dcxzz.2022.0011 – via ResearchGate.
4. Gai, Z K; Shan, X R; Sun, Z X; Zhao, W J; Pan, Z H; Zhu, M (2020). "A redescription of the Silurian Sinogaleaspis shankouensis (Galeaspida, stem-Gnathostomata) from Jiangxi, China". Vertebrata PalAsiatica. 58 (2): 85–99. doi:10.19615/j.cnki.1000-3118.191105.
5. Meng, Xin-Yuan (2023-04-03). "First Middle Devonian galeaspid from the Haikou Formation in Yunnan Province". Vertebrata PalAsiatica. 60 (3): 184. doi:10.19615/j.cnki.2096-9899.220613. ISSN   2096-9899.