A new wing skeleton of the Jehol tapejarid Sinopterus and its implications for ontogeny and paleoecology of the Tapejaridae

Abstract

The tapejarid pterosaurs flourished in the Jehol Biota with an abundance of immature individuals and a rarity of individuals at skeletal maturity. Most of these individuals plot well on an ontogenetic series based on the proportions of limb elements, but this has lacked histological evidence until now. Here, a new wing skeleton of Sinopterus was thin-sectioned to provide the first histological data about the ontogeny of the Jehol tapejarids. Histologically, the new specimen is an immature individual at a late juvenile stage prior to sexual maturity. It is grouped with medium-sized and medium-crested individuals, which are distinct from the small-sized and crestless individuals as well as the rare large-sized and large-crested individuals at skeletal maturity, supporting the presence of the premaxillary crest as an ontogenetic feature in the Jehol tapejarids. Furthermore, this histology indicates that the largest skeletally immature individuals might have reached the sexual maturity. Enigmatically, there is a size gap between sexual and skeletal maturity, which is at about 79% of the large size, implying a ontogenetic strategy comparable with Pteranodon and possibly with the Brazilian tapejarid Caiuajara. This size gap is consistent with lack of the larger sexually mature individuals in the Jehol Biota, which is hypothesized to be a migratory habitat for the Jehol tapejarids.

A taxonomic revision of the Sinopterus complex (Pterosauria, Tapejaridae) from the Early Cretaceous Jehol Biota, with the new genus Huaxiadraco

“…Monographical descriptions are beyond the scope of the present paper and will be provided elsewhere. Specimens PMOL-AP00030 (Liu et al, 2015), SDUST-V1012 ( Zhou, Niu & Yu, 2022 ) and SDUST-V1014 ( Zhou et al, 2022 ) are not included in the present reassessment due to their rather incomplete nature. The holotype of Nemicolopterus crypticus , which may be a hatchling tapejarid ( Witton, 2013 ; Naish, Witton & Martin-Silverstone, 2021 ), is also not included due to its very immature nature and disputed identification, and is thus discussed separately further below in the Discussion section.…”

“…This specimen has been referred to S. dongi by Shen et al (2021) , as accepted by Zhou et al (2022) and not commented on the literature any further so far. Shen et al (2021) noticed that D3072 shares with the holotype of S. dongi similar limb proportions as well as a reduced metatarsal I (shorter than metatarsals II and III), which has been considered a diagnostic apomorphy for S. dongi within the expansive taxonomic scheme of the Sinopterus complex ( Zhang et al, 2019 ).…”

“…Furthermore, it resembles tapejarids due to a combination of several features, most importantly edentulousness, a downturned rostrum, a slender and subvertical lacrimal process of the jugal, a jaw joint ventral to the anterior half of the orbit, and relatively elongate hindlimbs ( Naish, Witton & Martin-Silverstone, 2021 ). We further note that one of the proposed diagnostic features of Nemicolopterus crypticus , a penultimate phalanx of pedal digit 4 longer than the first ( Wang et al, 2008 ), is a feature it shares with Jiufotang tapejarids ( e.g ., Shen et al, 2021 ; Zhou et al, 2022 ).…”

“…We have coded in our dataset 14 relatively complete Jiufotang tapejarid specimens: the holotypes of Sinopterus dongi (IVPP V 13363, Wang & Zhou, 2003 ), Huaxiapterus jii (GMN-03-11-001, Lü et al, 2005), Huaxiapterus corollatus (ZMNH M813, Lü et al, 2006a ), Huaxiapterus benxiensis (BXGM V0011, Lü et al, 2007 ), Sinopterus lingyuanensis (JPM-2014-005, Lü et al, 2016 ) and Huaxiapterus atavismus (XHPM 1009, Lü et al, 2016 ), as well as specimens D2525 ( Lü et al, 2006b ), IVPP V 23388 ( Zhang et al, 2019 ), D3702 ( Shen et al, 2021 ), and five new specimens which are presented here for the first time (D4019, BPMC 103, BPMC 104, BPMC 105, and BPMC 107). The holotype of Sinopterus gui (see Li, Lü & Zhang, 2003 ), specimens PMOL-AP00030 ( Liu et al 2014 ), SDUST-V1012 ( Zhou, Niu & Yu, 2022 ), and SDUST-V1014 ( Zhou et al, 2022 ), as well the new specimen BPMC 106, were not included in the morphometric dataset due to their high level of incompleteness. The holotype of Nemicolopterus crypticus is also not included due its extremely young age (most likely a near-hatchling; Naish, Witton & Martin-Silverstone, 2021 ) and the incompleteness of its wings ( Wang et al, 2008 ).…”

“…(late Barremianearly Aptian), a great abundance of tapejarids is found in the Jiufotang Formation (Aptian). In total, 15 specimens of Jehol tapejarids have been formally described in the literature ( Wang & Zhou, 2003 ; Li, Lü & Zhang, 2003 ; Lü & Zhang, 2005 ; Lü & Yuan, 2005 ; Lü et al, 2006a , 2006b , 2006c , 2007 , 2016 ; Liu et al, 2014 ; Zhang et al, 2019 ; Shen et al, 2021 ; Zhou, Niu & Yu, 2022 ; Zhou et al, 2022 ). Under the accounts of Shen et al (2021) , the total number of recovered specimens, scattered around Chinese institutions, must be close to a hundred.…”