The behavior of burrowing has probably been present in Mammals since their early origins. Soil provides physical protection; it also supports Plants and Animals that many fossorial Mammals use. Underground shelter provides two main services: protection from predators and from environmental fluctuations or extreme conditions predominating above the ground. It is assumed that subterranean Mammals exploited the underground ecotope during the global climatic transition from the middle Eocene to the early Oligocene. There are several early Oligocene localities in temperate North America, but the only reported Oligocene Mammalian locality from tropical North America is Santiago Yolomecatl in southern Mexico. It includes a few fossorial taxa, such as the Amphisbaenian, Rhineura, as well as three Rodents, the Gophers, Gregorymys veloxikua, Gregorymys sp. and a member of the extinct Florentiamyidae. Several specimens of Gregorymys veloxikua and Gregorymys sp. had been collected inside burrows, which were tentatively identified as belonging to the ichnogenus Alezichnos; however, further detailed study on these burrows suggested that these structures were much more complex than Alezichnos.
In a paper published in the journal PLoS One on 12 March 2020, Rosalía Guerrero-Arenas and Eduardo Jiménez-Hidalgo of the Laboratorio de Paleobiología at the Universidad del Mar, and Jorge Fernando Genise of the División Icnología at the Museo Argentino de Ciencias Naturales, describe the new complex burrow system and include it in an ichnotaxonomical frame; test the hypothesis that a species of Gregorymys was the system’s producer; discuss the factors that promoted the development of such complex burrow systems; and analyse the possibilities that gregarious Geomyids were present during the Oligocene in southern Mexico.
The study area is in northwestern Oaxaca state, in southern Mexico. Fossiliferous outcrops are within the municipality of Santiago Yolomecatl. Lithological units represent a fluviolacustrine succession with several palaeosol horizons. Stratigraphical description of the study zone had previously been reported in detail.
The fossiliferous deposit was originally regarded as late Eocene given the presence of Miohippus assiniboiensis and previously published radiometric dates from overlying andesites outside of the study area. Before it was known that the Yolomecatl sequence was fossiliferous, it was considered part of the late Eocene–early Oligocene Chilapa Formation. Years later, it was considered a new geologic formation of middle Eocene age, based on an argon⁴⁰-argon³⁹ age of 40.3±1.0 million years. Guerrero-Arenas et al. could not locate the dated tuff at the reported location, so to obtain a more precise age estimation of the fossiliferous beds of Yolomecatl, uranium-lead detrital zircon geochronology was used to determine the maximum depositional age of a conglomeratic sandstone bed that is within the fossiliferous beds. Its maximum depositional age was estimated at 30.6 million years, placing the age of the Yolomecatl deposits, and their fossils (Iniyoo Local Fauna) in the early Oligocene. Some newly collected Mammalian taxa (Oreodontoides, Mammacyon, Cormocyon) also indicate an early Oligocene age (Arikareean 1 North American Land Mammal Age) from their sedimentary sequence. This new age agrees with the previously reported age of deposition for the Chilapa Formation, which was considered to be 35.6 to 29-million-years-old. Additionally, new regional stratigraphic relationships, as well as petrographic and mineralogical data indicate that the fossiliferous beds of Yolomecatl represent the marginal facies of the Chilapa Formation.
Strata from the Yolomecatl succession can be informally subdivided in three sections: the 'lower beds' are a sequence of limestone of freshwater origin and shale strata, with siltstone, sandstone and conglomerate intercalations; the 'middle beds' consist of a sequence of clayey silt and silty sandstone with sandstone and conglomerate interbedding; the 'upper beds' are a diverse sequence consisting of clayey siltstone, silty sandstone, silcretes, sandstone and conglomerate strata. Fossils and trace fossils are present along the whole sequence; vertebrate burrows appear in the 'lower beds' where they are isolated and scarce. The burrows are particularly abundant in some levels of the 'middle beds' where they compose complex systems or are isolated. Burrow abundance decreases in the 'upper beds' where only a few isolated specimens occur. Vertebrate burrows occur in palaeosols developed in floodplains and lake shores.
Most of the large casts of chambers and tunnels are preserved in full relief in the field. Systems cover approximately 100 m² in the best-preserved exposures. Since the systems crop out in ravines and uncovered soils, weathering by several agents (wind, water and Cattle) is a permanent menace for the trace fossils’ preservation.
Diagnosis, descriptions and surface morphology of walls were based on the best-exposed or -preserved specimens. Chamber shape may be more deformed by carbonate cementation in some stratigraphic levels than in others, so Guerrero-Arenas et al. only included measurements of chambers from the 'middle beds', which preserve the best examples. There is no evidence of sediment compaction in the burrowing fossiliferous levels. When the burrows were accessible, measurements were taken in the field. When they were located in inaccessible vertical exposures, digital photographs and Image-J software were used for measurements.
The burrows are described as a new ichnogenus and species, Yaviichnus iniyooensis, where 'Yaviichnus' is derived from 'Yavi' from the Mixteco language (typical of the region of Oaxaca where the study zone is placed), meaning 'rodent cave', and 'ichnus' from the Greek Ikhnos, meaning 'trace', and 'iniyooensis' is derived from 'Iniyoo' the Mixteco name of Yolomecatl, where the fossiliferous strata crop out.
These are interconnected burrow system composed of shafts, tunnels and two types of chambers. Large- to medium-sized superior chambers are connected to descending, radiating and inclined shafts, or to horizontal tunnels. Smaller secondary chambers are present at the end of these burrows or lateral to them. Horizontal burrows are straight, sinuous or show 'C' or 'H' paths. Vertical to sub-vertical burrows are straight, curved, sinuous or show consecutive arches resembling roughly a helical design. Tunnels and shafts are branched or simple. Horizontal burrows are wider than they are tall, whereas vertical ones are almost circular in cross section. Surface morphology of some burrows includes short, straight, paired marks on the external surface of the burrow fill.
Main chambers are roughly ellipsoidal and flattened in shape, but some of them are deformed by carbonate deposition and weathering, therefore looking more distorted. The best exposed main chambers measure 35–90 cm wide and 47–100 cm long (based upon 5 specimens), with a height of 18–90 cm (based upon 2 specimens). They are located near the palaeosol top. The entrance tunnel was not preserved. Main chambers show lateral and horizontal burrows, as well as vertical or sub-vertical shafts radiating from the lower part of the chamber. Horizontal to sub-horizontal tunnels range from almost straight to sinuous, or they show a 'C-' or 'H-' path. The longest fragment of a horizontal tunnel measured in situ is 135 cm. The number of vertical to sub-vertical, radiating burrows are variable, but mostly they are five or six. Vertical and sub-vertical shafts are straight, sinuous, curved or showing successive arches resembling a roughly helical design; they are simple or bifurcated. They extend 8 to 10 m below the chambers. Some vertical shafts (9 specimens) are completely straight and almost circular in cross section; minor diameter ranges from 5.1 to 8.7 cm; whereas major diameter ranges from 5.5 to 8.9 cm. Secondary chambers are smaller than main chambers and located at the end of shafts or are lateral to them. They are 20–32 cm wide, 24–47 cm long height (11 specimens). Cross-sections of horizontal, sub-vertical and some vertical burrows are elliptical. The width of burrows ranges from 6.5 to 14.4 cm, whereas height ranges from 5.4 to 14 cm (71 specimens).
Some fillings are arranged in clumps, which were more observable in weathered specimens. Packets were 1.2–10 cm long (44 specimens). They were found in the 'lower beds' and in some paleosols of the 'middle beds.'
Several casts with fine- to medium-sized sediment fillings show paired grooves on the external surface (0.29–1.11 cm wide, 1.25–8.5 cm long; based upon 33 specimens). Most of these traces are oriented with their long axes parallel to the long axis of the burrow, but others have an almost perpendicular orientation. They are distributed mainly in the ceiling and the lateral sides of the tunnels. Bioglyphs were not detected in the walls of chambers. Casts with bioglyphs are especially abundant in the upper horizons of the 'upper beds'.
Some isolated horizontal burrows have coarser sediments (pebbles and cobble-sized grains) inside the filling.
Iniyoo Local Fauna contains only four fossorial representatives: the Squamate Rhineura (Amphisbaenidae) and three taxa of rodents (Gregorymys veloxikua, Gregorymys sp. and Florentiamyidae indet.). Yaviichnus is composed of chambers and burrow systems different and much larger from those produced by Amphisbaenians. Burrows of Amphisbaenians (like Rhineura) consist of complex, interconnected networks, with multiple branches per junction, composed of cylindrical, sinuous or straight tunnels. The surface morphology consists of triangular impressions on the top and sides of tunnels.
It is probable that the producer of Yaviichnus iniyooensis was a Rodent, not just because the only other fossorial components of the Iniyoo Local Fauna were Rodents, but also because of the presence of paired traces in the walls of the burrows, which are usually produced by Rodent incisors from gnawing and breaking the soil. The presence of incisor traces may have two explanations: producers were probably juvenile organisms that preferred to use their incisors instead of their weaker forelimbs; or soil was so hard that the producer used their incisors to loosen the soil more effectively. The former hypothesis is unlikely considering that no forelimb traces were found in any burrows, where adults had to have been present as well.
Compared to other fossilised chamber and tunnel systems probably produced by Geomyidae, Yaviichnus is different with its unique arrangement of chambers, associated tunnels, and bioglyphs.
Only two fossil Rodent burrow systems have been associated directly to Geoymids: Alezichnos and Daemonelix. Alezichnos trogodont has been attributed to Geomyids. Alezichnos consists of primary tunnels, which occasionally branch into secondary ones. It lacks main and secondary chambers, as well as the diversity of tunnel morphologies and orientations observed in Yaviichnus. Morphology also differs in Alezichnos and Yaviichnus. Alezichnos trogodont has sinuous, tubular morphology with varying directionality and bilobated chamber; Yaviichnus is a system composed by two types of chambers, and horizontal and vertical tunnels. Yaviichnus iniyooensis burrows have paired grooves related exclusively with incisors; A. trogodont has small scratches and grooves on the surface of the ceiling and upper halves of burrows, produced by a combination of incisors and claws; the entire surface of the chamber is covered with regularly spaced claw marks; this combination was not observed in Yaviichnus, and bioglyphs are probably related to soil type. The other fossil burrow associated with Gregorymys is Daemonelix, due to the presence of remains of this Geomyid inside these helical burrows that are attributed to Paleocastor. Architecture of Daemonelix is clearly different from Yaviichnus, since the first is a vertical, helical shaft with an inclined chamber at the base.
Two main arguments could be considered to argue that Gregorymys is not the potential producer of Yaviichnus. The complexity of Yaviichnus iniyooensis is not similar to any extant Geomyid burrow systems. They consist of a less complex architecture: a main burrow, generally 10–46 cm below and parallel to the ground surface, with a variable number of lateral burrows branching from the main one; there are also deeper branches that are used as nests and food stores. These simple burrow systems of Geomyids are related to the reported solitary habits of all known species. Yaviichnus iniyooensis differs notably from this pattern. However, neither does the configuration of modern system match with other fossil burrows attributed to Gregorymys spp.
The presence of remains of Gregorymys inside Yaviichnus iniyooensis might be also explained by passive transport: 30% of the cranial and postcranial remains of Gregorymys collected in Yolomecatl were recovered from casts of Yaviichnus iniyooensis, whereas the remaining 70% was collected in the rock matrix and is apparently not associated with the burrows. An alternative hypothesis to explain the low percentage of remains inside the fills is that Gregorymys were secondary occupants of the burrows, as is interpreted in other burrows produced by large Mammals.
Guerrero-Arenas et al. consider Gregorymys spp. as the most probable producer of Yaviichnus in Yolomecatl localities by several reasons. Gregorymys veloxikua and Gregorymys sp. are the only taxa of fossorial Rodents in Iniyoo Local Fauna, where their remains are as abundant as the burrows. No other fossorial Vertebrate species was identified in the Yolomecatl outcrops to be considered as the main inhabitant and excavator of the burrow systems. Some remains were found in burrows with active fillings, resulted from the behavior of backfilling; active fillings could be identified because they have the same lithology of the rock matrix, and in some cases are structureless. Low percentage of remains of potential producers inside the fills could be explained because Animals could scape from floods, before they entered into the burrows.
So, evidence reveals that Oligocene Geomyidae in southern Mexico produced different burrow systems that appear to be more complex than any other extant or fossil representatives of this family.
A critical piece of evidence to relate Yaviichnus iniyooensis to Gregorymys is that the paired traces on the external surface of casts match with the width of the incisors of this Geomyid. These bioglyphs indicate chisel-tooth digging. Even though the primary digging mode among Geomyids is scratch digging, to some extent they also use their procumbent incisors to break up soil as a secondary digging mode, especially in hard soils. Florentiamyidae individuals can be disregarded as producers because their incisors are much smaller and thinner (approximately 40–50%) than the traces recorded in the burrows.
The functions of the different components of the system represented by Yaviichnus iniyooensis may be interpreted according to the knowledge of similar morphologies for burrows of extant species.
In recent Geomyids, chambers could be used as nests, latrines or food storages. In Yaviichnus iniyooensis the original fillings of chambers were replaced, so it was not possible to distinguish between those chamber types. Large chambers in other Rodent burrows may have a nesting and/or a thermoregulatory function, or can be used as latrines or for food storage. Burrows tend to become more complex in Mammals whose entire existence occurs underground because they display different functions, such as shelter, protection from external conditions and provision for the development of juveniles.
Yaviichnus iniyooensis has vertical and horizontal burrows. Descending sub-vertical and inclined shafts could be used for thermoregulation, since it is observed that burrowing Mammals dig deeper when environmental conditions become more severe by an increase in temperature and/or lack of humidity. Completely straight vertical tunnels could be also used as drainage canalisation. Horizontal burrows could be used when searching for food resources underground. In modern systems they may run across different vegetated areas and through soils of different types. It has been observed that fossorial Rodents (like Ctenomys) construct longer and more complex burrow systems in an environment with lower food cover and availability.
Trace fossils produced by insects, such as Fictovichnus gobiensis, Teisseirei barattinia and Celliforma ispp reported previously, are representative of the Celliforma Ichnofacies, indicative of scrub and woodland of arid to semiarid environments, or of palustrine vegetation or bare soils due to frequent flooding. The palaeolandscape was inferred as a scrubland or a woodland with a low vegetation cover. This was recently confirmed by the presence of interbedded calcrete layers among the sequence and by dolomite, zeolite and attapulgite minerals, which are also indicative of aridity.
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