Tlayúa Formation

The Tlayúa Formation was first described by Jerjes Pantoja-Alor in 1992.^[2] It is located in the Tlayúa ravine, which itself lies in the southern portion of Puebla, Central Mexico, near Tepexi de Rodríguez,^[1] and consists of a series of limestone quarries that has seen commercial use for decades.^[3]

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]

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]

The formation contains a diverse array of vertebrate and invertebrate fossils. For this reason, it is considered a Konservat-Lagerstätte.^[4][8] About 70% of the macrofossils are osteichthyan fish.^[9] Other vertebrates include chelonians, pterosaurs, lepidosaurs, and crocodiles.^[10] Cyanobacteria, foraminifera, algae, gymnosperms, sponges, cnidarians, annelids, gastropods, ammonites, bivalves, arachnids, insects, isopods, anomurans, brachyurans, crinoids, echinoids, holothuroids, stelleroids, and ophiuroids, have also been recovered from the Tlayúa Formation.^[10][11][12]

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,^[13] they likely represent either an early azhdarchoid or an indeterminate ornithocheiromorph.^[14]

Mainly after González-Rodríguez (2016)^[19] and Applegate et al. (2006)^[3]

Fish
Genus	Species	Higher taxon	Notes	Images
Amblysemius?	cf. A. sp	Caturidae	Only non-amiid amiiform from Tlayúa[19]
Araripichthys	A. weberi	Teleostei	Possibly related to Late Cretaceous genera from North America?[20]
Armigatus	A. felixi, A. carrhenoae	Ellimmichthyiformes	First records of Armigatus from America, alongside the oldest species in the genus[21], the two species from Tlayúa also seem to clade closest to each other[22]
Aspidorhynchidae spp.	Inapplicable	Teleostei	Undescribed species of Vinctifer and Belonostomus have been recorded[19]
Axelrodichthys	A. cf. A. araripensis	Mawsoniidae	Only known specimen was lost before description[19]
Bananogmius?	cf. B. sp	Tselfatiiformes	One of the few Mexican members of the clade other than Tselfatia[19]
Cipactlichthys	C. scutatus	Halecomorphi	Bears large plate-like scutes along its midline[23]
Ellimmichthys	E. sp[24]	Ellimmichthyiformes	Type genus of its order
Elopomorpha spp.	Unapplicable	Teleostei	Possible species of Brannerion, Paraelops and Megalops are known[19]
Epaelops	E. martinezi	Elopiformes	Related to the Jurassic Anaethalion[25]
Gonorynchiformes indet.	Unapplicable	Ostariophysi	One specimen reported in the 1990s, but has not been studied since[24]
Lycoptera?	cf. L. sp	Lycopteridae	Only osteoglossiform fossil from Cretaceous Mexico[19]
Macrosemiocotzus	M. americanus	Macrosemiidae	Similar to Macrosemius, but differs in a divided dorsal fin[26]
Michin	M. csernai	Pachyrhizodontidae	Likely the most basal pachyrhizodontid[27]
Neoproscinetes	N. sp[19]	Pycnodontidae	Related to Proscinetes
Notagogus	N. novomundi	Macrosemiidae	Closest to Notagogus helenae[28]
Nusaviichthys	N. nerivelai	Pachyrhizodontoidei	Formerly misidentified as Notelops fossils[29]
Pachyamia	P. mexicana	Vidalamiinae	Also known from the Middle East[30]
Paraclupea	P. seilacheri	Ellimmichthyiformes	First fossil of the genus outside of China[31]
Ranulfoichthys	R. dorsonudus	Clupeomorpha	Somewhat resembles Ellimmichthyiformes, but lacks their dorsal scutes, therefore it is likely a basal clupeomorph[32]
Quetzalichthys	Q. perilliatae	Ionoscopiformes (Halecomorphi)	Likely closest to Oshunia[33]
Tahnaichthys	T. magnuserrata	Pycnodontidae	Bears two humps in front of its dorsal fin, a novel trait in pycnodonts[34]
Teoichthys	T. kallistos, T. brevipina[35]	Ophiopsidae (Halecomorphi)	Name of the type species (Teoichthys kallistos) translates to "God's most beautiful fish", in reference to the exquisite preservation[36]
Tepexichthys	T. aranguthyorum	Pycnodontidae	Likely fed on coral, as shown by gut contents and the large amount of wear on its teeth[37]
Tlalocbatos	T. applegatei	Rhinopristiformes	Only chondrichthyan known from Tlayúa[38]
Tlayuamichin	T. iztli	Semionotiformes	Specific name translates to "obsidian", after the colour of the scales[39]
Unamichthys	U. espinosai	Ichthyodectiformes	Discovered alongside two other currently undescribed ichthyodectiforms[40]
Yabrudichthys?	cf. Y. sp[19]	Aulopiformes	Likely an enchodontid, mainly known from the Ein Yabrud site (hence its generic name)

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[41]
Atocatle	A. ranulfoi	Mesothelae?	Indeterminate family due to poor preservation, likely within Mesothelae due to its distinct abdominal segments[41]
Ixtahua	I. benjamini	Anisoptera?	Only known from a nymph, matches dragonfly nymphs in shape[41]
Protaegla	P. miniscula	Aeglidae	Earliest aeglid fossil known[41]
Tepexicarcinus	T. tlayuaensis	Tepexicarcinidae (basal Eubrachyura)	Formerly assigned questionably to Homolidae due to an absence of clear carapace regions[41], but then separated into its own family as a basal eubrachyuran[42]
Tipulidae indet.	Unapplicable	Tipuloidea	Only known from a wing pair[41]

Arthropods
Genus	Species	Higher taxon	Notes	Images
Astropecten	A. sp	Astropectinidae	Known from juvenile fossils preserving both dorsal and ventral sides[43]
Ophiactis	O. applegatei	Ophiactidae	One of the few articulated ophiuroids from the Early Cretaceous[44]
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]