Monday, 23 November 2020

Hurricane Eta exposes nineteenth century shipwreck on the coast of Florida.

Hurricane Eta swept across central Florida on 12 November 2020, causing eleven fatalities, in addition to the 178 it had already claimed in Central America, in addition to causing widespread flooding and causing about a billion dollars' worth of damage to property. After the storm had passed it was discovered that high tides brought on by the hurricane's storm surge had scoured the beach at the Fort Matanzas National Monument in St John's County on the east coast of Florida, exposing the remains of a ship that had been burried by the sand. 

 
Exposed timbers on the foreshore at Fort Matanzas National Monument, believed to have come from an nineteenth century shipwreck. CNN.

The site is now being investigated archaeologists from the St. Augustine Lighthouse Archaeological Maritime Program and students from Flagler College, who have determined that the remains probably belong to the Caroline Eddy, an American merchant ship which sank in the area on 29 August 1880, after becoming caught in a storm.

 
Chuck Meide of the St. Augustine Lighthouse Archaeological Maritime Program examaning timbers believed to have come from the Caroline Eddy, a merchant ship which sank off the coast of Florida in August 1880. St Augustine.

Tropical storms, known as hurricanes in the Caribbean, are caused by solar energy heating the air above the oceans, which causes the air to rise leading to an inrush of air. If this happens over a large enough area the inrushing air will start to circulate, as the rotation of the Earth causes the winds closer to the equator to move eastwards compared to those further away (the Coriolis Effect). This leads to tropical storms rotating clockwise in the southern hemisphere and anticlockwise in the northern hemisphere. These storms tend to grow in strength as they move across the ocean and lose it as they pass over land (this is not completely true: many tropical storms peter out without reaching land due to wider atmospheric patterns), since the land tends to absorb solar energy while the sea reflects it.

 
The formation of a tropical cyclone. Natural Disaster Management.

Despite the obvious danger of winds of this speed, which can physically blow people, and other large objects, away as well as damaging buildings and uprooting trees, the real danger from these storms comes from the flooding they bring. Each drop millibar drop in air-pressure leads to an approximate 1 cm rise in sea level, with big tropical storms capable of causing a storm surge of several meters. This is always accompanied by heavy rainfall, since warm air over the ocean leads to evaporation of sea water, which is then carried with the storm. These combined often lead to catastrophic flooding in areas hit by tropical storms. 

 
The formation and impact of a storm surge. eSchoolToday.

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Magnitude 6.1 Earthquake off the coast of Talca Province, central Chile.

The United States Geological Survey recorded a Magnitude 6.1 Earthquake at a depth of 19.7 km, off the coast of central Chile, roughly 99 km to the northwest of the city of Constitución in Talca Province, slightly before 10.55 pm local time on Saturday 21 November 2020 (slightly before 0.55 am on Sunday 22 November GMT). There are no reports of any injuries associated with this event, but peoplehave reported feeling it over a wide area.

 
The location of the 21 November 2020 Talca Province Earthquake. USGS.

Chile is located on the west coast of South America, which is also the convergent margin between the Nazca and South American Plates. The Nazca Plate is being subducted beneath the South American Plate and is sinking beneath the South American Plate. This is not a smooth process, the rocks of the two plates continuously stick together then, as the pressure builds up, break apart again, causing Earthquakes. As the Nazca Plate sinks deeper it is partially melted by the heat of the Earth's interior. Some of the melted material then rises up through the overlying South American Plate as magma, fuelling the volcanoes of the Chilean Andes.

 
The subduction of the Nazca Plate beneath the South American Plate, and how it causes Earthquakes and volcanoes. Pacific Earthquake Engineering Research Center.
 
Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. The international non-profit organisation Earthquake Report is interested in hearing from people who may have felt this event; if you felt this quake then you can report it to Earthquake Report here.
 
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Yaviichnus inyooensis: A new complex burrow system from the Oligocene of southern Mexico.

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.

 
Study area in southern Mexico. Guerrero-Arenas et al. (2020).

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.

 
Generalised stratigraphic column of the Oligocene sequence of Santiago Yolomecatl, Oaxaca, southern Mexico. Guerrero-Arenas et al. (2020).

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.

 
Yaviichnus iniyooensis morphology. (A), (C) Systems composed by a main chamber (red delineated in (B), (D)), secondary chamber (yellow delineated in (B), (D)) and tunnels (blue delineated in (B), (D)). (A), (C) Burrow systems are in situ, in 'middle beds' strata. Guerrero-Arenas et al. (2020).

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).

 
Yaviichnus iniyooensis cross-section of tunnels. (A) Horizontal tunnel cross-section in situ. From 'middle beds' strata. (B) Paratype UMPLIC- 386, elliptical cross-section of a tunnel. Scale bar is 7 mm. Guerrero-Arenas et al. (2020).

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.'

 
Yaviichnus iniyooensis bioglyphs. (A) Paratype UMPLIC-378, end of tunnel, covered with incisor marks. Scale bar is 1.3 mm. (B) Paratype UMPLIC-381, incisor marks on the cast surface of a tunnel segment. (C) Paratype UMPLIC-403, incisor marks on the cast surface of the end tunnel. Coin diameter is 14 mm. (D) Comparison of the marks, paratype UMPLIC-378 with incisor width of Gregorymys veloxikua specimen UMPE-671. Coin diameter is 16 mm. Guerrero-Arenas et al. (2020).

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.

 
General morphology of Yaviichnus iniyooensis; horizontal, straight segment tunnel. Scale is 34.3 cm. Guerrero-Arenas et al. (2020).

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.

 
General morphology of Yaviichnus iniyooensis; 'C' shape of a segment tunnel. Divisions of the rule are in centimetres; total length of the rule is 17 cm. Guerrero-Arenas et al. (2020).

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.

 
General morphology of Yaviichnus iniyooensis; 'H' shape of a segment tunnel. Scale is 34.3 cm. Guerrero-Arenas et al. (2020).

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.

 
General morphology of Yaviichnus iniyooensis; sinuous segment of a tunnel, probably descending from a secondary chamber. Divisions of the rule are in centimetres; total length of the rule is 17 cm. Guerrero-Arenas et al. (2020).

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.

 
General morphology of Yaviichnus iniyooensis; vertical and sub-vertical straight segments of tunnels delineated in black. Scale is 34.3 cm. Guerrero-Arenas et al. (2020).

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.

 
General morphology of Yaviichnus iniyooensis; Segment of a bifurcated tunnel. Divisions of the rule are in centimetres; total length of the rule is 17 cm. Guerrero-Arenas et al. (2020).

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.

 
Burrows showing the characteristic arrangement in packets of the sediment fillings, indicating of active burrowing. (A) Secondary chamber and segments of an almost straight tunnel. Length of the scale is 18 cm. (b) Isolated segment of sinuous tunnel and remains of a secondary chamber. Length of the scale is 17cm. (c) Remains of a horizontal tunnel, originally bifurcated, (D) Subhorizontal segment of a tunnel, bifurcated. All the specimens are in situ from 'middle beds' strata. Guerrero-Arenas et al. (2020).

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.

 
Remains inside the fillings of Yaviichnus iniyooensis. (A) A cast of Fictovichnus gobiensis inside the filling of a tunnel. (B) Remains of poscraneal remains of Gregorymys veloxikua inside the filling of paratype UMPLIC-398. Diameter of the coin is 4 mm. (C) Lithics inside the filling of paratype UMPLIC-393. Length of the scale is 20 mm. Guerrero-Arenas et al. (2020).

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.

Accordingly, large chambers, sub-vertical and long horizontal burrows in the same system could be convergent evidence of non-optimal conditions for life on the surface, which can be tested with other evidence recovered from the Yolomecatl strata.

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.

 
Insect trace fossils found in the same stratigraphic levels as Yaviichnus iniyooensis. (A) Specimen of Cellicalichnus? isp. from the 'lower beds'; (B) Celliforma isp. from the 'middle beds'. Diameter of the coin in both figures is 28 mm. Guerrero-Arenas et al. (2020).

It is well documented that a major climatic change occurred during the transition from the Eocene to the Oligocene. In response to this process, global aridification and the emergence of an open-habitat biota occurred at the beginning of the mid- to late Cainozoic. Climatic changes corresponded to a period of extensive diversification of subterranean taxa, and their tendency to occur in open, arid or semiarid habitats. Fossil Plants and pollen records show that a development and expansion of xeric vegetation in Mexico occured during the Oligocene (33–23 million years ago), when lowland forests and chaparrals were established in the central part of the country.
 
The presence of exclusively incisor traces in the walls of Yaviichnus iniyooensis could be also related to soil characteristics in arid or semiarid landscapes. Soil conditions (hardness, degree of compactation or dryness) have a significant effect on whether a digger adopts the tooth- or clawdigging strategy or combines both. There is a tendency in extant rodents such as Thomomys bursarius and Cynomys leucurus to use incisor-digging in particularly harsh and dry conditions. The chisel-tooth digging style is common in other Rodents, like Mole-rats, suggesting that incisors have been used as the main tools to enable exploitation of hard soils. The presence of incisor traces in the walls indicate that the soil in Yolomecatl was compact and dry; these conditions are related to the proposed environment.

Lithic fillings suggest that flooding events or gravity probably filled these burrows. Flooding events were related with intermittent currents of high energy recorded in the area due to seasonality in the area, related with the global tendency of climatic change during Oligocene. 

There are socioecological hypotheses to explain the evolution of sociality among Rodents. There are various causes and factors proposed to promote cooperative behaviors among individuals such as predation pressure, distribution of food resources and environmental and climatic conditions. One of the most cited proposals to explain sociality in rodents is the conceptual model of the Aridity-Food-Distribution Hypothesis. This hypothesis postulates that the abiotic factors that cause patchy distribution of food resources promote social interactions between individuals, and therefore the population’s survival. The energetic cost of burrowing through hard soil to locate patchily distributed but locally abundant food resources is the primary selective factor favoring group-living; by living together and working cooperatively to excavate tunnels, the Animals can locate enough food resources to survive. Subterranean foragers might be expected to modify their burrow architecture in different habitats, coinciding with different food resource idiosyncrasies. Burrowing Animals are limited to habitats where burrow excavation is energetically efficient, which is determined in part by the nature of the soil and the associated vegetation, among other factors.

In low productivity areas, more extensive exploration might be required to locate resources. The relative costs of social versus solitary diggings allow the presumption that complex structures were produced by more than one organism. The high branching of a burrow system is also expected to increase with the number of inhabitants. The complexity of the burrow system represented by Yaviichnus iniyooensis, composed of interconnected large and small chambers at different depths and vertical to horizontal burrows showing different morphologies and functions, plus the extension of these burrow systems, may suggest that the systems were constructed by more than one individual. A hypothesis for the construction of these burrows is that they were produced by more than one individual, in order to diminish burrowing costs in an environment with relatively scarce food resources. Arid or semiarid climate in Yolomecatl, evinced by paleosols, minerals and ichnofacies, probably produced a patchy and scarce distribution of vegetation sources for fossorial organisms; also, periodical flooding events could contribute to a patchy distribution of resources in the area.

Rodentia encompasses a vast array of social systems, which range from short-term seasonal aggregations to long-life social groups. If a non-solitary Gregorymys could be considered as the producer of Yaviichnus, it is implied that the Oligocene species had different habits from extant geomyids, which are reported as solitary during their entire life cycle, even under arid climates. However, it is possible to find solitary and social species within the same taxon in subterranean Rodents, e.g. Ctenomys sociabilis and Ctenomys haigi. Recent and relevant evidence of ecological genomics shows how genetics influence the evolution of complex behavioral differences of modern rodents in nature. Genetic changes contribute to the evolution of different architectures of burrows, even between closely related species (such as the Deermice, Peromyscus polionotus and Peromyscus maniculatus). Novel observations suggest that social organisation in Rodents is influenced by changing environmental conditions. Our hypothesis is that Gregorymys individuals in Yolomecatl could have some degree of social organization in environments triggered by aridity conditions.

The question of which degree of gregariousness was showed by Yolomecatl Geomyids remains open, but it is more probable that association of individuals was short and associated with cooperative behavior under certain climatic conditions, as it is observed with extant Bathyergids in arid landscapes. The fossil record offers an invaluable source of evidence of novel and unobserved combinations of behavioral characteristics never observed in living species. Yavichnus inyooensis could provide a new window to explore the evolution of social life in rodents during the Oligocene of southern Mexico.

The composition and importance of the burrowing herbivore guild changed during the Cainozoic. Convergent evolution of subterranean Mammals began across the planet during the global climatic transition from the Middle Eocene to the Early Oligocene (40–30 million years ago). As environments became more open in the Cainozoic, small herbivorous Mammals would opportunistically exploit small patches of open habitats and show rapid adaptations for life within new habitat types and subterranean life. During the Arikareean (30–18.5 million years ago) Entoptychine Geomyids were an important component of North American faunas. The extension of burrowed palaeosols and complexity of the burrows by the dominant Geomyids in Yolomecatl strata support both assertions.

Yaviichnus inyooensis is a new ichnotaxon for complex burrow systems composed of interconnected large and small chambers at different depths, as well as vertical to horizontal burrows, showing different morphologies and functions. A species of Gregorymys would be the most probable producer based on its fossorial habits, the presence of its remains inside the burrows and the paired grooves in the walls, which are compatible with Rodent incisors. The complexity of these burrows and underground life would have been triggered by semiarid to arid conditions shown by independent evidence such as paleosols, minerals and ichnofacies. The morphological complexity of burrows could be related with the action of more than one individual, indicating that the Oligocene Gregorymys of southern Mexico shows some degree of gregariousness influenced by environmental conditions.

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Sunday, 22 November 2020

Asteroid 2020 VT3 passes the Earth.

Asteroid 2020 VT3 passed by the Earth at a distance of about 470 800 km (1.23 times the average distance between the Earth and the Moon, or 0.33% of the distance between the Earth and the Sun), slightly after 7.40 pm GMT on Sunday 15 November 2020. There was no danger of the asteroid hitting us, though were it to do so it would not have presented a significant threat. 2020 VT3 has an estimated equivalent diameter of 3-10 m (i.e. it is estimated that a spherical object with the same volume would be 3-10 m in diameter), and an object of this size would be expected to explode in an airburst (an explosion caused by superheating from friction with the Earth's atmosphere, which is greater than that caused by simply falling, due to the orbital momentum of the asteroid) more than 31 km above the ground, with only fragmentary material reaching the Earth's  surface.

 
The closest approach of 2020 VT3 to the Earth on 15 November 2020. JPL Small Body Database.

2020 VT3 was discovered on 12 November 2020 (three days before its closest approach to the Earth) by the University of Arizona's Mt. Lemmon Survey at the Steward Observatory on Mount Lemmon in the Catalina Mountains north of Tucson. The designation 2020 VT3 implies that the asteroid was the 94th object (asteroid T3 - in numbering asteroids the letters A-Z excluding I, are assigned numbers from 1 to 25, with a number added to the end each time the alphabet is ended, so that A = 1, A1 = 26, A2 = 51, etc., which means that T3 = (25 x 3) + 19 = 94) discovered in the first half of November 2020 (period 2020 V - the year being split into 24 half-months represented by the letters A-Y, with I being excluded).

2020 VT3 has a 541 day (1.48 year) orbital period, with an elliptical orbit tilted at an angle of 0.31° to the plain of the Solar System which takes in to 0.83 AU from the Sun (83% of the distance at which the Earth orbits the Sun) and out to 1.76 AU (1.76% of the distance at which the Earth orbits the Sun, and more than the distance at which the planet Mars orbits the Sun). It is therefore classed as an Apollo Group Asteroid (an asteroid that is on average further from the Sun than the Earth, but which does get closer).

 
The orbit and current position of 2020 VT3. The Sky Live 3D Solar System Simulator.

This means that 2020 VT3 has occasional close encounters between the asteroid and Earth, with the last thought to have happened in January 2018 and the next predicted in May 2021. The asteroidalso has occasional close encounters with the planets Venus, which it last cam close to in May 1994 and is next predicted to pass in September 2041, and Mars, which it last came close to in September this year (2020) and is expected to pass again in November 2025. Asteroids which make close passes to multiple planets are considered to be in unstable orbits, and are often eventually knocked out of these orbits by these encounters, either being knocked onto a new, more stable orbit, dropped into the Sun, knocked out of the Solar System or occasionally colliding with a planet. 

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