4.1. Material

The fallen rock block bears at least three ankylosaurian tracks preserved as natural casts (convex hyporeliefs), as well as plant leaves (Figure 6A, B). In the case of the ankylosaurian tracks, a manus-pes set and a partial pes were recognized (Figure 6C); the block is composed of fine-grained sandstone; however, as mentioned above, the tracks were originally preserved in a carbonaceous shale layer. The catalogue codes are CFZ-Zt 305 for the ankylosaur manus and pes, and CFZ-Zt 306 for the ankylosaur manus natural sandstone cast.

4.2. Description

Pes impression is tetradactyl (Figure 6C); this impression presents a well-defined rounded heel, short and stout digits I and II, digits III, and IV slightly elongated, with faintly acuminated tips; these are curved and show an outward orientation. Additionally, digits III and IV preserve ventrally curved toe tips, suggesting that these were deeper impressed in the sediment. Pes impression measures 33 cm in maximum antero-posterior length and 26 cm in maximum medio-lateral width. The interdigital angles are of 22o for digits I - II, 41.5o for digits II - III, and of 25.5o for digits III - IV; thus, the digit total divarication is of 89o (digits I - IV). The distance between the toe tips is 9 cm for digits I - II, 17.5 cm for digits II - III, and 10.2 cm for digits III - IV. The anterior portion of a second pes impression is preserved in one of the block ridges, thus it is possible that it forms a single step, 63.5 cm in length, with the pes impression previously described (Figure 6C).

Manus impression is pentadactyl and fits the outline of a rough pentagon (Figure 6C); it is clear that a manual pad is present; it is wider than long, being 21 cm and 16 cm, respectively; digit V is observed as a small bump located posterolaterally, while the other digits are short and blunt (Figure 6C). The angles between digits is 55.5o between digits I - II, 33o between digits II - III, 55.5o between digits III - IV, and 55o between digits I - V. Thus, the digit total divarication is 199o (digits I - V).

A curved structure is preserved together with these footprints. It is slightly elongate, forming an arch, with an acute posterior end and a wider anterior portion; it is here presumed to be a tail drag (Figure 6A).

4.3. Comments

In spite of these tracks representing a manus-pes set, the manus track represents a right one and pes track represents a left one (Figure 7C). It implies that the manus-pes set belongs to a single individual organism and/or that the footprints had a distribution due to a particular mode of locomotion. A third possibility is that these tracks represent overstepping of two individuals traveling in the same direction.

Figure 7 CFZ-Zt 306, Tetrapodosaurus manus. It is a right manus natural sandstone cast. A. Palmar view; B. Dorsal view; C. Distal view; D. Palmar view of sandstone cast and superimposed skeletal manus elements of the nodosaur Edmontonia (in white color), the black color ones are modified to fit the digit position as suggested by the natural cast; E. Possible skin impressions. Scale in E equals 1cm. 

The original level containing the ankylosaurian tracks was identified through petrographic slides through the track-bearing rock block and associated rock layer (Figure 3), both being litarenite. In addition to this, two other ankylosaurian tracks were identified in situ (Figure 8). These are preserved as natural sandstone casts; although weathered, it is possible to note one hand impression (Figure 8).

Figure 8 Location of provenance of the rock block bearing Tetrapodosaurus footprints. A. The site is marked with a white asterisk in Figure 3; B. Fossilized log; C. In situ manus sandstone cast in dorsal view (scale 10 cm). 

Additionally, an isolated sandstone cast of a hand was found, as an eroded footprint-bearing layer (Figure 7). This natural cast presents the same morphological features as the hand impression described above; it suggests the presence of a palmar pad in these ankylosaur tracks, in having small and rounded digital impressions (Figure 7A, B). It is wider than long, being 26.3 cm and 16 cm, respectively; digit V is observed as a small bump located posterolaterally, while the other digits are short and blunt (Figure 7A, B; compare with Figure 6C). The angles between digits is 55o between digits I - II, 42o between digits II - III, 30o between digits III - IV, and 58o between digits IV - V. Thus, the total digit divarication is 185o (digits I - V). An additional interesting feature of this natural cast is that it preserves a palmar surface with a medium-lateral concavity (Figure 7C); suggesting that the original footprint was not homogeneously impressed in the sediment; this feature is, perhaps, indicative of the functional mechanics of the ankylosaurian manus (Figure 7C, D). Small patches of faint skin impressions are preserved in some areas of this hand natural cast (Figure 7E). A photograph of the palmar view of the nodosaurid Edmontonia hand was digitally superimposed to this cast, the fit was remarkable (Figure 7D).

5.1. Producer of the tracks

The Tlaxiaco tracks contain obvious features such as tetradactyl pes and pentadactyl manus; the general features of these tracks allow the comparison with those of ankylosaurs and ceratopsians (^{McCrea et al., 2001}). ^{McCrea et al. (2001)}, refer to some morphological criteria that help to discriminate between ceratopsian and ankylosaur footprints. For this purpose, the authors compared the general morphology of two ichnogenera, Tetrapodosaurus and Ceratopsipes, which represent the tracks of ankylosaurs and ceratopsians, respectively (^{Carpenter, 1984}; ^{Lockley and Hunt, 1995}; ^{McCrea et al., 2001}). Although Tetrapodosaurus and Ceratopsipes have a pentadactyl manus, it is small in proportion to the pes size in Ceratopsipes and larger in proportion to the pes size in Tetrapodosaurus (^{McCrea et al., 2001}). One of the most conspicuous differences is that digits are better defined in Tetrapodosaurus than in Ceratopsipes (^{McCrea et al., 2001}). The pes in both ichnogenera are tetradactyl; however, Ceratopsipes pes is roughly symmetrical in outline (^{Lockley and Hunt, 1995}), it has shorter toe impressions (i.e., with more enclosing flesh), and the inner digit (digit I) is larger than in Tetrapodosaurus (^{McCrea et al., 2001}). Tetrapodosaurs bears an asymmetrical pes and longer toe impressions (i.e., with less enclosing flesh) and a shorter inner digit or digit I (^{McCrea et al., 2001}). Based on these features, the Tlaxiaco tracks are undisputable, ankylosaurian in origin. ^{McCrea et al. (2001)} reviewed the global record of ankylosaur tracks; these authors summarized the ichnogenera considered as produced from ankylosaurian dinosaurs and included Tetrapodosaurus ^{Sternberg, 1932}; Deltapodus ^{Whyte and Romano, 1994}; Macropodosaurus ^{Zakharov, 1964}; Metatetrapous ^{Nopcsa, 1923}, and Ligabueichnium ^{Leonardi, 1984}.

Initially, the ichnogenus Macropodosaurus was considered as produced from a theropod dinosaur (^{Zakharov, 1964}); however, ^{McCrea et al. (2001)}, based the similarity of the pes impressions and compared this ichnogenus to Metatetrapous, originally considered as produced by an ankylosaur by ^{Haubold (1971)}. However, some authors still relate the ichnogenus Macropodosaurus to segnosaurian theropods (^{Sennikov, 2006}).

Ligabueichnium, was established by ^{Leonardi (1984)} for a trackway from the El Molino Formation (Potosí Group), in Bolivia; the author suggested a possible relationship with a ceratopsid or an ankylosaur; however, in a subsequent work, he mentioned that “Ligabueichnium bolivianum may be attributed to an unusually large Ankylosauria” (^{Leonardi, 1994: 39}) and it is now considered as ankylosaurian in origin by several authors (^{McCrea et al., 2001}). In regard to Deltapodus, ^{Whyte and Romano (1994)} reported this ichnotaxon from the Middle Jurassic of Yorkshire, England; they interpreted it, as made by a sauropod dinosaur. ^{Lockley et al. (1994)} subsequently reinterpreted these tracks as produced by a thyreophoran dinosaur; currently, Deltapodus is widely considered as an ichnotaxon made by stegosaurian dinosaurs (e.g., ^{Milàn and Chiappe, 2009}; ^{Mateus et al., 2011}).

The ichnogenus Tetrapodosaurus was established by ^{Sternberg (1932)}, based on a trackway found in sediments of the Lower Cretaceous Gething Formation in British Columbia, Canada. Although, it was initially interpreted as ceratopsian in origin, it is now attributed with confidence to nodosaurid ankylosaurs (^{McCrea et al., 2001}). In addition to a tetradactyl pes with well-developed heel and pentadactyl manus, the general morphology of the Tlaxiaco tracks strongly resembles that of Tetrapodosaurus.

Regarding a possible ceratopsian origin for the Tlaxiaco tracks, it is necessary to mention that the earliest record of ceratopsians are teeth collected in late Early Cretaceous sediments of the Arundel and Cedar Mountain formations in U.S.A. (^{Chinnery et al., 1998}). However, the earliest record of large ceratopsians is known from Turonian deposits of Moreno Hill Formation in New Mexico (^{Wolfe and Kirkland, 1998}). In this way, being of Middle Jurassic age, ceratopsians are discarded as the trackmakers of the Tlaxiaco tracks. Ankylosaurian dinosaurs are known to occur since the Middle Jurassic in Europe and Asia (^{Galton, 1983}; ^{Zhiming, 1993}; ^{Carpenter et al., 1998}; ^{Kirkland et al., 1998}). In North America, the earliest record of ankylosaurs comes from the Late Jurassic Morrison Formation (^{Kirkland and Carpenter, 1994}; ^{Carpenter et al., 1998}); in fact, the sediments of the Morrison Formation of Western Colorado have yielded the only ankylosaur track known from the Late Jurassic (^{Hups et al., 2008}). This track resembles the manus of Tetrapodosaurus and has been illustrated by ^{Lockley et al. (2014)}.

The Tlaxiaco ankylosaur pes preserves the toe tips, digits III and IV, in such a form that indicates these were deeply impressed in the sediment. A similar situation has been mentioned by ^{McCrea et al. (2001)}; these authors noted that some Cenomanian Tetrapodosaurus tracks from the Dunvegan Formation of British Columbia, Canada, preserve deeply impressed tip digits, and were impressed in rather dried sediment. In addition, ^{McCrea et al. (2001)} mention two ankylosaur trackways, from the Maastrichtian El Molino Formation of Bolivia; where the trackways “seem to suggest that the tracks were perhaps made on an overlying layer of sediment and represent the penetration of the distal portion of the toes into an under layer” (^{McCrea et al., 2001: 442}).

Findings from Canada and Bolivia together with the Tlaxiaco ankylosaur footprints, could yield insights into the mechanical function of the anklylosaur pes, at least suggesting a good subjection to the ground; perhaps for the use of the tail. Indeed, a similar condition could be suggested by the isolated natural cast of the hand from Tlaxiaco (Figure 7) that bears a well-developed palmar pad with a ventral medio-lateral concavity (Figure 7C). This suggests that the original footprint was not homogeneously impressed in the sediment; a feature suggesting, as well, part of the functional mechanics of the ankylosaurian manus. Some authors have suggested that a palmar pad is an unexpected feature for ankylosaurs (^{Senter, 2011}); this based on track morphology of the hand impressions, as well as the ankylosaurian metacarpal configuration (^{Senter, 2011}). However, the features of the three ankylosaur hand impressions from Tlaxiaco support the presence of a palmar pad.

Together with the tracks, a supposed tail drag is preserved; no tail impressions are known to occur associated to ankylosaur footprints. However, in regard to this, nodosaurid dinosaurs had a longer tail than that present in ankylosaurids (^{Carpenter, 1982}, ¹⁹⁸⁴). For example, the ankylosaurid Euoplocephalus had 21 caudal vertebrae; while it has been estimated 40 to 50 caudal vertebrae for nodosaurids, such as Sauropelta (^{Carpenter, 1982}, ¹⁹⁸⁴). It is thus possible that some nodosaurids left an occasional tail drag.

Most of the early members of Ankylosauria were small; forms such as Mymoorapelta and Gargoyleosaurus in North America, Tianchisaurus in Asia, and Cryptodraco and Dracopelta in Europe, were not longer than 3 m (^{Galton, 1983}; ^{Carpenter, 1984}; ^{Zhiming, 1993}; ^{Kirkland and Carpenter, 1994}; Carpenter et al., 1998). However, the nodosaur Sauropelta, which has been regarded as the trackmaker of the Tetrapodosaurus tracks (^{Carpenter, 1984}; ^{McCrea et al., 2001}), was an organism with a calculated length and weight of approximately 5.2 m and 1500 kg, respectively (^{Carpenter, 1984}). When compared with the data raised from Sauropelta and Tetrapodosaurus footprint size (^{Carpenter, 1984}; ^{McCrea et al., 2001}; ^{Sternberg, 1932}), the Tlaxiaco Tetrapodosaurus tracks suggest an organism ca. 3.8 m in length and nearly 1 t in weight.

5.2. Paleoenvironmental conditions

The plant macrofossils associated with the Tlaxiaco dinosaur footprints include Bennettitales, the most diverse order with 17 of the 34 species identified in the plant community; among these, Zamites lucerensis (Wieland) ^{Person and Delevoryas, 1982} and Otozamites hespera ^{Wieland, 1914}, are abundant, while Anomozamites triangularis (Nathorst) ^{Pott and McLoughlin, 2009}; Anomozamites sp. cf. intermedium ^{Antevs, 1919} as well as Zamites sp., are rather rare. Conifers are represented by Podozamites sp. cf. kidstonii Etheridge, in ^{Jack and Etheridge, 1892}. Mexiglossa varia ^{Delevoryas and Person, 1975} and Trigonocarpus oaxacensis ^{Wieland, 1914} are common and frequent respectively, however, both are of uncertain taxonomic position (^{Lozano-Carmona, 2012}; ^{Lozano-Carmona and Velasco-de León, 2016}). The order Ginkgoales is represented by several new species within the genera Ginkgoidium and Sphenobaiera (^{Velasco-de León et al., 2015}). Ferns are important herbaceous elements and, together with Equisetales, have been associated to a climate with humid seasons (^{Van Konijnenburg-Van Citter, 2002}; ^{Rojas-Chávez, 2010}); while the genera of conifers identified in the Jurassic of Mexico have been associated to a warm climate (^{Person and Delevoryas, 1982}). However, the presence of Ginkgoales and Coniferales in the study area suggests a rough mountain relief, temperate and mesic conditions, as well as little variations in the local climate (^{Rees et al., 2000}; ^{Wang et al., 2005}; ^{Zhou, 2009}).

The identified flora bears a high percentage of small leaves; in this case, 72% of the identified plants had microphyl leaves with an area between 0.08241 to 1.3660 cm² (^{Lozano-Carmona, 2012}; microphyl 1 type sensu ^{Ortiz-Martínez et al., 2013}). This feature indicates the dominance of a subwarm, sub-humid climate, and with periods of low water circulation in the floodplain allowed the swamps establishment; these swamps accumulate a great amount of organic matter that formed the coal layers, 1.5 m thick, present in the Zorrillo-Taberna Indiferenciadas Formation. The paleobasin was subject to a continuous subsidence that permitted the homogeneous depositional and environmental conditions during its formation (^{Corro-Ortiz and Ruíz-González, 2011}). The plant remains preserved in the Zorrillo-Taberna Indiferenciadas Formation surely were part of the diet of the Middle Jurassic herbivorous dinosaur communities present at the Tlaxiaco area, included is the nodosaurid ankylosaur, represented by these Tetrapodosaurus tracks.

In regard to the paleoecological conditions, the Tlaxiaco nodosaurid ankylosaur tracks were found in a coal-bearing formation, such as the Zorrillo-Taberna Indiferenciadas Formation. This observation agrees with that of ^{McCrea et al. (2001)}; these authors mention that ankylosaur footprints are concentrated in coal-bearing and floodplain facies. Also, ankylosaurs seem to have a preference for well-vegetated and well-watered lowlands (^{McCrea et al., 2001}). This environmental condition is supported by the great amount of coal and plant remains found in the Zorrillo-Taberna Indiferenciadas Formation (^{Lozano-Carmona, 2012}; ^{Velasco-de Léon et al., 2013}). Even more, well preserved Zamites lucerensis leaves were found preserved in the same surface of the Tetrapodosaurus footprints (Figure 6B).

The fact that the Tlaxiaco nodosaurid ankylosaur tracks are preserved in a coal-bearing formation, such as the Zorrillo-Taberna Indiferenciadas Formation, suggest that at least the North American Ankylosauria was adapted to similar paleoecological conditions since Middle Jurassic times.