Haifanggou Formation

The age of the Daohugou bed has been debated, and a number of studies, using different methodologies, have reached conflicting conclusions. Various papers have placed the fossils here as being anywhere from the Middle Jurassic period (169 million years ago) to the Early Cretaceous period (122 ma).^[4] One of the first studies on the age of the Daohugou beds, published in 2004 by He et al., found them to be Early Cretaceous, only a few million years older than the overlying Jehol beds of the Yixian Formation.^[5] The 2004 study primarily used Argon–argon dating of a tuff within the Daohugou Beds to determine its age.

However, subsequent studies cast doubt on this relatively recent age. In a 2006 study, Gao & Ren criticized He et al. for not including enough specifics and detail in their paper, and also took issue with their radiometric dating of the Daohugou tuff. The tuff, Gao and Ren argued, contains crystals with a variety of diverse radiometric ages, some up to a billion years old, so using dates from only a few of these crystals could not determine the overall age of the deposits. Gao and Ren went on to defend a Middle Jurassic age for the beds based on biostratigraphy (the use of index fossils), and the bed's relationship to a layer that is known to mark the Middle Jurassic-Late Jurassic boundary.^[6]

Another study, published in 2006 by Wang et al., argued that the 159-164 million years old Tiaojishan Formation underlies, rather than overlies, the Daohugou Beds. Unlike the earlier study by Gao and Ren, Wang et al. found an overall similarity between the fossil animals found in the Daohugou Beds and those from the Yixian Formation. The authors stated that

"vertebrate fossils such as Liaoxitriton, Jeholopterus and feathered maniraptorans show much resemblance to those of the Yixian Formation. In other words, despite the absence of Lycoptera, a typical fish of the Jehol Biota, the Daohugou vertebrate assemblage is closer to that of the Early Cretaceous Jehol Biota than to any other biota."

Wang et al. concluded that the Daohugou probably represents the earliest evolutionary stages of the Jehol Biota, and that it "belongs to the same cycle of volcanism and sedimentation as the Yixian Formation of the Jehol Group."^[7] However, a later study by Ji et al. argued that the key indicator of the Jehol biota are the index fossils Peipiaosteus and Lycoptera. Under this definition, the earliest evolutionary stage of the Jehol Biota is represented by the Huajiying Formation, and the Daohugou Formation is excluded due to the absence of Lycoptera fossils.^[8] Later in 2006, Liu et al. published their own study of the age of the Daohugou beds, this time using Zircon Uranium-lead dating on the volcanic rocks overlying and underlying salamander-bearing layers (salamanders are often used as index fossils). Liu et al. found that the beds formed between 164 and 158 million years ago, in the Middle to Late Jurassic.^[1][9] A 2012 study by Gao and Shubin agreed with this assessment, and reported an Argon–argon date of 164 plus or minus 4 million years ago for the Daohugou horizon.^[10]

At least 760 insect species have been described from the Haifanggou Formation and associated deposits, making it one of the most diverse insect-bearing strata from the Jurassic period.^[11]

Insects
Genus	Species	Year	Abundance	Notes	Images
Ahirmoneura	A. neimengguensis[12]	2008		A tangle-veined fly
Aneuroderma	A. oiodes[13]	2021	2 specimens	An earwig
Applanatiforceps	A. angustus[14]	2023	3 specimens	An earwig
Archaboilus	A. musicus[15]	2012	1 specimen	A stem-katydid
Ekpagloderma	E. gracilentum[14]	2023	1 specimen	An earwig
Formosibittacus	F. macularis[16]	2008	1 specimen	A hangingfly
Fortiholcorpa	F. paradoxa[17]	2013	1 specimen	A Mecopteran
Juracimbrophlebia	J. ginkgofolia[18]	2012	1 specimen	A hangingfly
Jurahylobittacus	J. astictus[16]	2008	1 specimen	A hangingfly
Mesobunus	M. dunlopi[19]	2012	1 specimen	A harvestman
Miriholcorpa	M. forcipata[17]	2013	1 specimen	A Mecopteran
Mongolarachne	M. jurassica[20]	2011		A spider
Mongolbittacus	M. daohugoensis[21]	2007	1 specimen	A hangingfly
Pseudomantina	P. baijuyii[22]	2023		A cockroach
Pycnopolystoechotes	P. striatus[23]	2023		A lacewing
Sinojuraphis	S. ningchengensis[24]	2008		An aphid
Sinopalaeodermata	S. concavum[13]	2021	1 specimen	An earwig
Stictopolystoechotes	S. sparsulus[23]	2023		A lacewing

Based largely on.^[25]

Genus	Species	Year	Notes	Images
Chunerpeton[26]	C. tianyiensis	2003	A salamander
Jeholotriton	J. paradoxus		A salamander
Neimengtriton	N. daohugouensis		A salamander
Pangerpeton	P. sinensis		A salamander
Pedopenna	daohugouensis		A maniraptoran dinosaur
Epidexipteryx	hui		A scansoriopterygid dinosaur
Epidendrosaurus	ningchengensis
Volaticotherium	V. antiquum		A gliding eutriconodont mammal
Castorocauda[27]	C. lutrasimilis	2006	An aquatic docodont mammaliaform
Microdocodon	M. gracilis		A docodont mammaliaform
Agilodocodon	A, scansorius		A docodont mammaliaform
Megaconus	M. mammaliaformis		A possible allotherian mammal
Mirusodens[28]	M. caii		an arboreal/climbing euharamiyidan mammal/mammaliaform
Pseudotribos	P. robustus		A shuotheriid mammaliaform
Liaosteus[29]	L. hongi	1995	A peipiaosteid fish
Yanliaomyzon[30]	Y. ingensdentes	2023	A lamprey

• Geology of Inner Mongolia
• Jurassic Asia and Jurassic System of Asia — period's Asian prehistoric life, geological formations, fossil record
• Yixian Formation
• Callovian formations
  • Tiourarén Formation, fossiliferous formation of Niger
  • Qiketai, Toutunhe, Shishugou and Qigu Formations, fossiliferous formations of the Junggar Basin, Xinjiang
• Oxfordian formations
  • Oxford Clay, England
  • Tendaguru Formation, fossiliferous formation of Tanzania
  • Cañadón Calcáreo Formation, fossiliferous formation of the Cañadón Asfalto Basin, Argentina

• Liaoning Stratigraphic Group. 1978. The Stratigraphic Tables of Liaoning. Geology Press 1-296