Tlayúa Formation

Stratigraphy

The Tlayúa Formation consists of two stratigraphic units (Tlayúa and Barranca Abuelo)^[4] and three members. The Lower Member is lower Albian in age. It consists of bioturbated limestones, and is characterised by the presence of the non-rudist bivalve Chondrodonta and the rudist bivalve Toucasia polygyra. The Middle Member, corresponding to the Tlayúa Quarry, is the most fossiliferous.^[3] Its age has been difficult to ascertain, as the foraminiferan Spiroloculina cretacea is known exclusively from lower Cenomanian strata,^[5] though it has since been determined to be Upper Albian.^[3] The Middle Member consists of fine-grained lithographic limestones, interspersed with hematitic layers that preserve its vertebrate fauna. The Upper Member is Cenomanian in age, and consists of a sequence of dolomites. It is characterised by the presence of the miliolid foraminiferan Dicyclina schlumbergi.^[3]

Depositional environments

The depositional environment of the Tlayúa Formation was likely a shallow, coastal lagoon,^[6] with some freshwater influence, as indicated by the presence of crocodilian and turtle fossils.^[6][7] The Lower Member of the Tlayúa Formation represents a carbonate environment which apparently had stressed biodiversity.^[4] The depositional environment of the Middle Member was likely arid for much of the year, with the exception of seasonal rains and storms.^[7] The Upper Member likely represents a tidal flat.^[4] The Tlayúa Formation may have been part of an island, though a connection to the North American mainland cannot be discounted.^[7]

Archelosaurs

The remains of several indeterminate archelosaur taxa have been recovered from the Tlayúa Formation. Indeterminate crocodilians and turtles have been identified.^[6][7] Partial, articulated wings of an unidentified pterosaur have been discovered from the formation. Though tentatively assigned to Pteranodon sp. and Nyctosaurus sp. in 2008,^[12] they likely represent either an early azhdarchoid or an indeterminate ornithocheiromorph.^[13]

Lepidosaurs

Fish

Mainly after González-Rodríguez (2016)^[18] and Applegate et al. (2006)^[3]

More information Fish, Genus ...

Fish
Genus	Species	Higher taxon	Notes	Images
Amblysemius?	cf. A. sp	Caturidae	Only non-amiid amiiform from Tlayúa[18]
Araripichthys	A. weberi	Teleostei	Possibly related to Late Cretaceous genera from North America?[19]
Armigatus	A. felixi, A. carrhenoae	Ellimmichthyiformes	First records of Armigatus from America, alongside the oldest species in the genus,[20] the two species from Tlayúa also seem to clade closest to each other[21]
Aspidorhynchidae spp.	Inapplicable	Teleostei	Undescribed species of Vinctifer and Belonostomus have been recorded[18]
Axelrodichthys	A. cf. A. araripensis	Mawsoniidae	Only known specimen was lost before description[18]
Bananogmius?	cf. B. sp	Tselfatiiformes	One of the few Mexican members of the clade other than Tselfatia[18]
Cipactlichthys	C. scutatus	Halecomorphi	Bears large plate-like scutes along its midline[22]
Ellimmichthys	E. sp[23]	Ellimmichthyiformes	Type genus of its order
Elopomorpha spp.	Unapplicable	Teleostei	Possible species of Brannerion, Paraelops and Megalops are known[18]
Epaelops	E. martinezi	Elopiformes	Related to the Jurassic Anaethalion[24]
Gonorynchiformes indet.	Unapplicable	Ostariophysi	One specimen reported in the 1990s, but has not been studied since[23]
Lycoptera?	cf. L. sp	Lycopteridae	Only osteoglossiform fossil from Cretaceous Mexico[18]
Macrosemiocotzus	M. americanus	Macrosemiidae	Similar to Macrosemius, but differs in a divided dorsal fin[25]
Michin	M. csernai	Pachyrhizodontidae	Likely the most basal pachyrhizodontid[26]
Neoproscinetes	N. sp[18]	Pycnodontidae	Related to Proscinetes
Notagogus	N. novomundi	Macrosemiidae	Closest to Notagogus helenae[27]
Nusaviichthys	N. nerivelai	Pachyrhizodontoidei	Formerly misidentified as Notelops fossils[28]
Pachyamia	P. mexicana	Vidalamiinae	Also known from the Middle East[29]
Paraclupea	P. seilacheri	Ellimmichthyiformes	First fossil of the genus outside of China[30]
Ranulfoichthys	R. dorsonudus	Clupeomorpha	Somewhat resembles Ellimmichthyiformes, but lacks their dorsal scutes, therefore it is likely a basal clupeomorph[31]
Quetzalichthys	Q. perilliatae	Ionoscopiformes (Halecomorphi)	Likely closest to Oshunia[32]
Tahnaichthys	T. magnuserrata	Pycnodontidae	Bears two humps in front of its dorsal fin, a novel trait in pycnodonts[33]
Teoichthys	T. kallistos, T. brevipina[34]	Ophiopsidae (Halecomorphi)	Name of the type species (Teoichthys kallistos) translates to "God's most beautiful fish", in reference to the exquisite preservation[35]
Tepexichthys	T. aranguthyorum	Pycnodontidae	Likely fed on coral, as shown by gut contents and the large amount of wear on its teeth[36]
Tlalocbatos	T. applegatei	Rhinopristiformes	Only chondrichthyan known from Tlayúa[37]
Tlayuamichin	T. iztli	Semionotiformes	Specific name translates to "obsidian", after the colour of the scales[38]
Unamichthys	U. espinosai	Ichthyodectiformes	Discovered alongside two other currently undescribed ichthyodectiforms[39]
Yabrudichthys?	cf. Y. sp[18]	Aulopiformes	Likely an enchodontid, mainly known from the Ein Yabrud site (hence its generic name)

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Arthropods

More information Arthropods, Genus ...

Arthropods
Genus	Species	Higher taxon	Notes	Images
Archaeoniscus	A. aranguthyorum	Sphaeromatidea?	Bears an unusual axial pouch unlike all extant isopods, may have been a fish ectoparasite or scavenger like modern cymothoids[40]
Atocatle	A. ranulfoi	Mesothelae?	Indeterminate family due to poor preservation, likely within Mesothelae due to its distinct abdominal segments,[40] but more recently its affinity as a spider is questioned[41]
Ixtahua	I. benjamini	Anisoptera?	Only known from a nymph, matches dragonfly nymphs in shape[40]
Protaegla	P. miniscula	Aeglidae	Earliest aeglid fossil known[40]
Tepexicarcinus	T. tlayuaensis	Tepexicarcinidae (basal Eubrachyura)	Formerly assigned questionably to Homolidae due to an absence of clear carapace regions,[40] but then separated into its own family as a basal eubrachyuran[42]
Tipulidae indet.	Unapplicable	Tipuloidea	Only known from a wing pair[40]

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Echinoderms

More information Arthropods, Genus ...

Arthropods
Genus	Species	Higher taxon	Notes	Images
Astropecten	A. sp	Astropectinidae	Known from juvenile fossils preserving both dorsal and ventral sides[43]
Echinaster	E. sp	Echinasteridae	Only known from a fossil in dorsal view[43]
Ophiactis	O. applegatei	Ophiactidae	One of the few articulated ophiuroids from the Early Cretaceous[44]
Ophidiaster	O. sp	Ophidiasteridae	One of the few starfish from this formation which is not known from juvenile specimens[43]
Paleopentacta	P. alencasterae	Cucumariidae	Likely an infaunal suspsension feeder based on extant relatives[45]
Parapsolus	P. tlayuensis	Psolidae	Likely a suspension-feeder attached to hard substrates based on extant psolids[45]
Plutonaster	P. sp	Astropectinidae	Known from a juvenile specimen[43]
Tamaria	T. sp	Ophidiasteridae	Known from a juvenile[43]

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