Lower Cretaceous Dinosaur Tracks from Puebla , Mexico

Dinosaur tracks have been identified near San Martı́n Atexcal, southern Puebla, Mexico, within the sedimentary sequence of the San Juan Raya Formation of Lower Cretaceous (Albian) age. The tracksite, located in the bed of the Magdalena River, reveals six different ichnofossiliferous levels identified within a 9 m thick sedimentary sequence. The inferred environment is that of a tidal (marginal marine) mudflat (Level I). Level I preserves three theropods trackways (?Allosauroidea), additionally, isolated tracks belonging to iguanodontids (Ornithopoda). Level II preserves faint iguanodontid tracks. Levels III to V preserve sauropod tracks. Younger level VI preserves, although morphologically different, a track belonging to Ornithopoda. The dinosaur tracks from San Martı́n Atexcal support the existence of continental facies within the San Juan Raya Formation; they represent the second record of dinosaur tracks from the Lower Cretaceous of Mexico and are part of an important but little documented record of Lower Cretaceous dinosaurs in Mexico.


Introduction
Since the first report on dinosaur tracks from Mexico, the number of reported localities has increased considerably [1][2][3].To date, this record includes three reports of Jurassic dinosaur tracks, plus several dinosaur tracks and trackways of Late Cretaceous dinosaurs; most of these localities are distributed from the central through northern parts of the country [3].
However, the evidence of Mexican dinosaurs of Lower Cretaceous age is now restricted to the site reported here (see [3,4]).This track site is located near the small town of San Martin Atexcal in southern Puebla, central Mexico (Figure 1), and it represents the first known dinosaur track site of this age from Mexico.Two other small outcrops, recently discovered, reveal more dinosaur tracks within the San Juan Raya Formation; however, these remain unstudied.
The site herein reported reveals dinosaur tracks in at least six different stratigraphic levels in the upper part of the San Juan Raya Formation.The objective of this paper is to document this dinosaur track site, to comment on the observed ichnofaunal succession, and place the information regarding the tracks in the broader context of the Lower Cretaceous dinosaur paleoichnology.

Study Area and Geological Context
This Lower Cretaceous dinosaur tracksite is located in southern Puebla, 1 km west of the town of San Martín Atexcal and within the stream channel of the La Magdalena River (Figure 1(b)).
The tracks were originally registered on grayish shale; however, these are mostly preserved as convex hyporeliefs in brownish, fine to medium-grained sandstone layers.These rocks belong to the upper portion of the stratigraphic sequence of the San Juan Raya Formation of Lower Cretaceous age.
The inferred paleoenvironment for the San Juan Raya Formation is that of a shallow lagoonal, tidal-flat setting, between reefs and the coast line; this interpretation is confirmed Puebla México U .S .A .by the presence of alternating layers of shale, sandstone, and calcareous shale [5,6].The known paleofauna is composed mostly of invertebrates such as corals, gastropods, cephalopods, pelecypods, echinoderms, and crustaceans; a Lower Cretaceous age, Aptian, is supported by some elements of this invertebrate fauna [5,6].The San Juan Raya Formation is overlayed by the Cipiapa Formation of Albian-Cenomanian age and is underlain by the late Barremian Zapotitlán Formation [5,6].

Material and Method
A map of the distribution of the tracks over the main surface (or Level I, see below) was made on a plastic sheet, drawing the track outlines with a water-proof ink marker.Track measurements were taken with a standard metric tape, and the angles between digits as well as pace angulations were measured with a standard 360 • protractor.The trackways were identified using the nomenclature proposed by recognized vertebrate paleontologists, such as Leonardi and Carvalho [7, 8, and references therein].For this particular case, the code is composed of the letters ATRM, which indicate the Municipio de Atexcal (AT) and the locality name known as Río Magdalena (RM, Magdalena River).Following the code, a letter was assigned to each of the trackways, and to complete the designation a consecutive number for each of the tracks composing a particular trackway.This final number (without a trackway-designating letter) is also used to designate isolated tracks that do not form part of a recognizable trackway sequence.Measurements such as pace, stride, and pace angulation were taken; in addition to these, the measurements for each track include the maximum length, measured across digit III; maximum width, measured from the lateral margin to the medial margin of the track; total angle of divarication between digits II-IV, as well as each angle between digits II-III and III-IV.

Results
A 9 m thick local stratigraphic section was measured in the site, within this column at least six ichnofossiliferous levels were identified (Figures 2 and 3).The older one is herein named Level I and includes three trackways (ATRM-A, ATRM-B, and ATRM-C) plus associated isolated tracks (Figures 4 and 5).The remaining five levels were named II to VI, in consecutive ascendant order, and they preserve isolated tracks that are relatively well preserved.

ATRM-A.
Represents a trackway of a theropod dinosaur.It starts with a single impression of digit III followed by two tridactyl and mesaxonic tracks.Digits are stout and narrow and end in subacute tips.Digits II and IV are shorter than digit III, with digit II being the shortest in the track.
ATRM-A-1 is well defined, and it represents just the impression of digit III; it is elongate, 22 cm long, and 8 cm in maximum width.At the most elevated portion, the impression has fusiform ends and it is 4 cm wide.The digital impression preserves a well-defined mud rim around it.
ATRM-A-2 represents a left track; it is 32 cm long, 25.5 cm wide, and has a width/length ratio of 0.79.The ATRM-A-3 represents a right track however shallow impressed; it is 28 cm long and 26 cm wide, the width/length ratio is 0.93.The interdigital angulations are of 34 • and 37 • for digits II-III and III-IV, respectively, so the total angle of divarication is of 71 • .
This trackway (ATRM-A) has a mean step of 93.8 cm, a stride of 185 cm, and a pace angulation of 172 • .The tracks show a light inner rotation of 9 • with respect to the trackway midline.5).Represents a theropod dinosaur trackway; it is composed of three tridactyl and mesaxonic tracks.The first of these tracks shows a morphological similarity with the tracks of iguanodontid dinosaurs; however, the next tracks that compose this trackway show clearly the morphological traits of theropod tracks, such as welldefined claw impressions.The extramorphological variation exhibited is related to substrate conditions.ATRM-B-1 corresponds to a left impression; it is 34 cm long and 33.5 cm wide; its width/length ratio is thus 0.98.Digits are in appearance robust and wide; however, in each of the digits, it is recognized as a longitudinal central border that represents the real toe impressions, which are slender and stout.The plantar area is relatively as wide as the heel area.The interdigital angulation is in both cases for digits II-III and III-IV of 33 • , in this way the total angle of divarication is of 66 • .

ATRM-B (Figure
ATRM-B-2 represents a right track; it is 26 cm long and 25.5 cm wide; its width/length ratio is 0.98.The interdigital angulation is 46 • and 22 • for digits II-III and III-IV, respectively; in this way the total angle of divarication is of 68 • .In this track, digits are well defined, and they are robust with well-defined, subtriangular claw impressions. ATRM-B-3 represents a left track (Figure 4(b)), it is 28 cm long and 26 cm wide, and its width/length ratio is 0.93.The interdigital angulation is of 34 • and 37 • for digits II-III and III-IV, respectively; thus, the total angle of divarication is of 71 • .All the digits have well-defined, acute distal ends.In addition, digit II has a subtriangular claw impressions isolated somewhat from the digital impression (Figure 4(b)).
ATRM-B has a mean pace of 90 cm, a stride of 179 cm, and a pace angulation of 169 • .The tracks show a slight inward rotation of 13.3 • with respect to the trackway midline. 2 and 4(a)).Represents a short trackway composed of a single step of a theropod dinosaur.ATRM-C-1 corresponds to a left tridactyl and mesaxonic track; it is 33 cm long, 25 cm wide, and has a width/length ratio of 0.76.The interdigital angulation is 26 • between digits II-III and III-IV, so the total angle of divarication is of 52 • .This is one of the best preserved tracks on the outcrop, the digits preserve well-defined distal ends, and digit II is slightly separated from digits III and IV by a shallow indentation.

ATRM-C (Figures
ATRM-C-2 represents part of a right track, preserving only the impressions of digits II and III and part of the medial area of the heel.The estimated length of this track is 31 cm, and the angle between digits II and III is 33 • .

Isolated
Tracks.The first level (Level I) preserves five isolated tracks; there were initially six tracks, but one was vandalized.These tracks were labeled ATRM-1 to 5.
ATRM-1 (Figures 4(c) and 5) is characterized by wide digits with rounded distal apices and a wide plantar zone.This track is 25 cm long, 19.5 cm wide, and the width/length ratio is 0.78.The angle of divergence among digits II-III is 22 • , while the III-IV is 25.5 • ; thus, the total angle of divarication among digits II-IV is 47.5 • .The heel is bulbous and bears a shallow concavity in a posterior position with respect to one of the digits (II or IV); in addition, it preserves a well-defined mud rim approximately 4 cm wide.Most of the features of this track agree with those of ornithopod dinosaurs, in particular iguanodontids [9,10].
ATRM-2 has a configuration similar to that of ATRM-1, so it is attributed as well to an iguanodontid ornithopod (Figure 5).This track is 20.5 cm long, 17.5 cm wide, and has a width/length ratio of 0.85.The interdigital angulation is 27 • and 33 • between digits II-III and III-IV, respectively, so the total angle of divarication is 60 • .ATRM-3 (Figure 5) is characterized by narrow digital impressions ending in subacute apices.This track is 27 cm long, 23 cm wide, and the width/length ratio is 0.85.The interdigital angulation between digits II-III is 29 • , between digits III-IV is 31 • , so the total angle of divarication is 60 • .A shallow indentation is observed posteromedially, just behind digit II; digit III is arched toward the track medial side.In addition to these observations, the impression of digit III of this track partially overlaps the track ATRM-C-1; this suggests that the producer of ATRM-3 crossed the area later that produced the ATRM-C trackway.Most of the features observed in this track agree with those seen in theropod dinosaurs [9,10].ATRM-4 and ATRM-5 are partially preserved; however, they share their general morphology and sizes with ATRM-3 and are equally assigned to Theropoda.
Level II preserves poorly preserved ornithopod tracks similar to those observed in Level I.However, in the next levels (III-V), a change in the composition of track makers is observed, where the tracks of sauropod dinosaurs are recognized.
These sauropod tracks are preserved as convex hyporeliefs (Figure 6); however, Level V preserves tracks as concave epireliefs over thin sandstone (Figure 6(c)).These tracks are of subcylindrical form; they have a mean diameter of 33 cm and depth of 10-15 cm.One of the sauropod tracks from Level IV preserves a prominent, "V"-shaped (as seen in cross section), mud rim; a possible manus tracks were recognized in association with this pes track (Figure 6(b)).
Tracks on Level V are preserved in a brownish sandstone bed 22 cm thick; however, this bed thins to 10 cm thick in some areas.These tracks are observed in cross section.At least one of these tracks seems to be made by a member of Sauropoda; features such as large size and rounded shape support this assignment.
Level VI corresponds to the youngest ichnofossiliferous level (Figure 3); at this level it is possible to recognize tracks attributable to Ornithopoda and perhaps to Sauropoda.One of the preserved tracks, ATRM-6, preserves features that undoubtedly permit its assignment to Ornithopoda (Figures 7 and 8); the general morphology of ATRM-6 strongly recalls those of iguanodontids reported from the Dakota group of western USA [11].
ATRM-6 is tridactyl and mesaxonic, 24.7 cm long and 21 cm wide, so the width/length ratio is 0.85.This track preserves a well-defined, rounded plantar pad and teardropshaped digital pads with the acuminate ends pointing distally.This track bears concave, nearly circular, interdigital areas and a rounded heel.Digits are separated from the plantar pad by skin folds (Figure 7(b)).
The digits of ATRM-6 have a total angle of divarication of 99 • ; this is 55 • between digits II-III and 44 • for digits III-IV.Another feature of this track is that the distal ends of the digits are displaced from their original positions due to a small fracture that affected this ichnofossiliferous level (Figure 7(a)).

Discussion
The inferred paleoenvironment of the San Martín Atexcal track site (at least that represented in track-bearing Level I) corresponds to that of a tidal (marginal marine) mudflat.In this way, the tracks and trackways from the San Juan Raya Formation support the evidence of continental facies in such a formation.The tracks on Level I show differential preservation; this is due to different conditions of the substrate (e.g., variations in humidity, composition, and/or texture), which result in shallower or deeper tracks within the same trackbearing layer or even within the same trackway.Thus, these conditions help to explain the extramorphological variation observed in the tracks that compose the trackways ATRM-B and ATRM-C; these tracks show a morphological similarity to those of iguanodontid dinosaurs; however, among other features, the presence of well-defined claw impressions, arching of some digits, indentation between digits II and III-IV, and a long step, supports their assignment to Theropoda.The theropod tracks from this locality (Level I) suggest medium-sized individuals, with an estimated hip-height of 1.50 m, using the method of Thulborn [9].Based solely on size and age of the locality, it is probably that the track maker was a member of the Allosauroidea, a group of dinosaurs that includes carnivorous forms that are widely distributed and have a time span ranging from the Late Jurassic to the Late Cretaceous [13].
Most of the theropod tracks reported from the Lower Cretaceous of North America represent small forms such as coelurosaurian dinosaurs [14].There exists evidence of theropod tracks made by larger forms such as those attributed to the allosauroid Acrocanthosaurus [15]; however, these traces differ in size and configuration from those found in the San Martín Atexcal track site.
ATRM-1 and ATRM-2 can be clearly attributed to iguanodontid dinosaurs.In particular, ATRM-1 strongly recalls the iguanodontid tracks found in some Cleveland-Lloyd outcrops, in eastern Utah; in both cases, the tracks are  characterized by having a rounded heel and a strong concave indentation located posteromedially (cf.[14] Figure 5.2).Within this dinosaur ichnofauna, it is possible to recognize at least another group of ornithopod dinosaurs.One of the best preserved tracks from Level VI (ATRM-6) shows similarities with Dinehichnus; however, its general morphology strongly recalls the ornithopod tracks reported from the Dakota group of the western USA (Figure 8) [11,12,16,17].
The dinosaur tracks from San Martin Atexcal permit an appreciation of an interesting ichnofaunal succession.Levels I and II preserve iguanodontid tracks; however, the first level preserves theropod tracks as well; levels III to V preserve, among other undifferentiated tracks, those of sauropod dinosaurs; finally, level VI preserves a track from an ornithopod dinosaur, morphologically different, from the iguanodontids present in lower levels.
Sauropod body fossils are known to occur in sediments of diverse terrestrial environments; however, the tracks of sauropod dinosaurs are conspicuously present in coastal marine facies [18,19].Thus, the track distribution in the San Martin Atexcal section may reflect facies change, from less coastal environments (levels I, II, and VI) to more coastal environments (levels III to V).
The dinosaur ichnofauna from Atexcal represents the second report of dinosaur tracks from the State of Puebla (cf.[20]) and the second record of dinosaur tracks from the Lower Cretaceous of Mexico (cf.[21]).Recently, two other tracksites have been recognized within the San Juan Raya Formation; however, these require further study [22].
Lower Cretaceous dinosaur tracks are known from many deposits in Europe, eastern Asia, and South America [14,[23][24][25][26].In most cases, the dominant track morphotypes are attributed to iguanodontid dinosaurs and related taxa.
In North America, most of the record of Lower Cretaceous dinosaur tracks is restricted to the last 19 millions of years during the Aptian and Albian [14,27].However, Berriasian dinosaur footprints are known from the Mist Mountain Formation in Canada [28].Most of these are records from a few geological formations such as the Gates in Alberta (Canada) and in the United States such as the Cedar Mountain (Utah) and the Dakota Group in Colorado, New Mexico, Oklahoma, Kansas, and Utah [14,27].
Thus, the dinosaur tracks from San Martín Atexcal, Puebla, establish the geographical continuity of the Dinosauria from Alaska to Central Mexico during Early Cretaceous times and obviously place this record as the most austral in the North American subcontinent during that time.This record adds importantly to the scarce paleoichnological knowledge about dinosaurs during epochs such as the early Cretaceous in Mexico and other areas in North America.

Figure 1 :
Figure 1: Geographic location of the State of Puebla (a) in central Mexico and the Lower Cretaceous dinosaur track site near San Martin Atexcal, Puebla (b).The locality is indicated by the asterisk.

Figure 2 :
Figure 2: Panoramic view of the San Martin Atexcal track site.Numbers are placed at the six ichnofossiliferous levels identified; white arrows point to a couple of observable footprints on ichnofossiliferous levels III and VI.

Figure 3 :
Figure 3: Stratigraphic local column of the San Martin Atexcal track site, with the ichnofossiliferous levels indicated by Roman numerals I to VI; dinosaur silhouettes indicate the particular dinosaur taxon in each ichnofossiliferous level, being Allosauroidea in level I, Iguanodontidae in levels I and II, Sauropoda in levels III to V, and Ornithopoda in level VI.

Figure 4 :
Figure 4: General view of dinosaur tracks and trackways from ichnofossiliferous level I (a), compare with Figure 5. (b), track referred herein to a member of Allosauroidea (ATRM-B-3) and (c), Iguanodontidae (ATRM-1).Two poorly preserved theropod footprints are not shown in this photograph; however, these are illustrated in Figure 5.

Figure 5 :
Figure 5: Map of the distribution of dinosaur tracks and trackways in level I. Dashed area indicates the place where one of the tracks was vandalized.Scale bar equals 1 m.

Figure 6 :
Figure 6: Sauropod tracks preserved as natural casts (a, b) and as seen in cross section (c).These tracks correspond to levels III, IV, and V, respectively; m, manus; p, pes; mr, mud rim.Geological hammer is 28 cm in length.

Figure 7 :
Figure 7: Ornithopod footprint from level VI.The white arrows in (a) point to a fracture that displaced the digit apexes from their original sites; (b), digital reconstruction of the footprint with the digit apices on their site.

Figure 8 :
Figure 8: Comparison between track ATRM-6 from level VI (a) with Dinehichnus from the Late Jurassic Morrison Formation (b) and an ornithopod track from the Dakota group of the western USA (c).Note the morphological similarities between ATRM-6 and the track from the late Albian Dakota group.(b) from[12], (c) from[11].Scale bar equals 30 cm.