Nest cells of Leafcutter Bees from the Rancho La Brea Tar Pits.

Leafcutter Bees (Megachilidae) cut their name from their habit of cutting disk-shaped segments from leaves, from which they build their nests. Each female Bee digs, tunnels, or co-opts a burrow in soft soil or plant material, within which several small nests cells, each comprising an egg and a supply of food wrapped in a parcel made from leaf material, are deposited. The oldest traces of this activity, fossil leaves with cutouts thought highly likely to come from Leafcutter Bee activity, date from the Early-to-Middle Eocene of Europe and North America.

In a paper published in the journal PLoS One on 9 April 2014, Anna Holden of the Entomology Section at the Natural History Museum of Los Angeles County, Jonathan Koch of the Department of Biology and Ecology Center at Utah State University, Terry Griswold of the USDA-ARS Pollinating Insect Research Unit at Utah State University, Diane Erwin of the Museum of Paleontology at the University of California, Berkeley and Justin Hall of the Dinosaur Institute at the Natural History Museum of Los Angeles County, discuss the discovery of two Leafcutter Bee nest cells from the Rancho La Brea Tar Pits in Los Angeles County, California.

The nest cells come from Pit 91 at Rancho La Brea, and are thought to be between 23 000 and 40 000 years old, and thought likely to belong to the extant species Megachile gentilis. Each is cylindrical, about 10.5 mm in length and 4.9 mm in diameter.

(A) Nest cell containing male pupa showing cylindrical shape, tapered, rounded bottom at left typical of the first constructed cell, and remains of oblong leaf disc with Type 1 venation. (B) Bottom of first constructed cell (containing male pupa) with possible portion of bottom circular leaf disk visible and outlined with arrows. (C) Cap of nest cell containing male pupa. (D) Nest cell containing female pupa. Arrow shows margin of oblong leaf disc with Type 2 venation. (E) Circular, bottom disc of nest cell (containing female pupa). In life, this end abutted the anterior end of the next cell. (F) Cap of nest cell containing female pupa. (G) Nest cell of female pupa showing oblong side wall leaf cutout which does not reach bottom of cell and is instead supported by circular bottom disc. (H) Remains of oblong leaf disc with relatively smooth-cut margins and Type 3 venation. (I) View showing five overlapping oblong disks (1–5) comprising the sidewalls and circular bottom discs. (J) Nest cell containing female pupa showing Type 4 venation on upper, right corner. (K) Nest cell of modern Megachile gentilis, showing circular disc bottom and oblong, sidewall leaf. Holden et al. (2014).

Micro-CT scans of the cells revealed the presence of two immature Bees, one male and one female, apparently preserved when pupating. The pupae of Leafcutter Bees probably have better preservational potential than other stages, due to the tough secretions produced by the larva prior to pupating, which from a hard protective case around the insect during this vulnerable period.

Micro-CT scans of the male pupa and its position within the nest cell. (A) Dorsal view of pupa within nest. (B) Dorsal view of pupa. (C) Lateral view of pupa within nest. (D) Lateral view of pupa. (E) Cross-section of nest and pupae. (F) Ventral view of pupa. Holden et al. (2014).

Megachile gentilis is today found in California and southwest Arizona, in dry areas where the temperature seldom falls below 0°C. The temperature in the area where the nests were found is likely to have varied considerably between 23 000 and 40 000 years ago, though all tar pit fossils are, by their nature, likely to have been laid down in warmer periods (tar pits are formed when crude oil deposits become exposed at the surface, when this happens in a warm climate the lighter fractions evaporate leaving a thick tar in which animals can become trapped and preserved). 

Micro-CT scans of the female pupa and its position within the nest cell. (A) Dorsal view of pupa within nest. (B) Dorsal view of pupa. (C) Lateral view of pupa within nest. (D) Lateral view of pupa. (E) Cross-section of nest and pupae. (F) Ventral view of pupa. Holden et al. (2014).

See also…

 Two new species of Orchid Bee from Columbia and Brazil.

The Orchid Bees (Euglossini) of Central and South America have an unusual relationship with the Orchids they pollenate, in that the Orchids do not produce a food substance with which to attract the Bees, but rather a variety of aromatic chemicals used by the males Bees to attract a mate. Each male must visit a number of flowers in order to collect enough...

 A new species of Leafcutter Bee from Mexico.

Leafcutter Bees (Megachilidae) get their name from their habit of cutting segments from leaves with which to line their nests. They are solitary Bees, each female Bee building her own nest in a wood...

 The diversification of Bees.

Bees (Anthophila) are generally accepted to have arisen during the Mesozoic, but estimates of exactly when vary considerably. The group are not well known in the fossil record (at least in part because many early palaeontologists tended to ignore Insect fossils in Mesozoic beds, happily destroying them in the quest for Dinosaurs and other large, glamorous Vertebrates), with the earliest putative Bee being Melittosphex burmensis...

Follow Sciency Thoughts on Facebook.

Insect borings in bones from the Rancho La Brea Tar Pits.

The Rancho La Brea Tar Pits are located in what is now central Los Angeles, California. They are essentially oil deposits identical to those worked by oil drills in other parts of the world, but exposed at the surface. When oil deposits are exposed in this way the lighter fractions (crude oil is made up of a mixture of different oils, known as 'fractions' due to the process used to separate them, fractional distillation) such as petroleum evaporate off, leaving the heavier fractions, known as tar, or asphalt, behind. These form oily pools in which animals can become trapped. The La Brea Tar Pits appear to have been exposed at least intermittently at the surface for around 40 000 years, during which time a great number of animals have fallen into them.

In a paper published in the journal PLoS One on 3 July 2013, Anna Holden of the Department of Entomology at the Natural History Museum of Los Angeles County, John Harris of the George C. Page Museum and Robert Timm of the Department of Ecology and Evolutionary Biology and Biodiversity Institute at University of Kansas discuss the results of a study of insect damage to bones from the La Brea Tar Pits, and the implications of these results for the climate and conditions under which the deposits were formed.

Holden et al. compared damage on preserved bones from the Tar Pits to damage created in the lab by allowing Beetle larvae to feed on Pig and Chicken bones. They found that the damage to the ancient bones matched closely with that created by Dermestid and Tenebrionid Beetle larvae.

Damage caused to bones by Dermestid Beetle larvae. (A) Damage caused to a Pig bone in the laboratory. (B & C) Damage to the bones of Bison from the Rancho La Brea Tar Pits. Holden et al. (2013).

This has implications for the climate at the time when the bones became trapped in the tar. The types of Beetles that Holden et al. think were causing damage to bones in the La Brea Tar Pits are associated with far warmer climates than those suggested by pollen inclusions in the tar. Holden et al. suggest that this actually helps to clear up a mystery surrounding the Tar Pits; based upon pollen analysis, the temperatures at the times when the deposits were forming would have been to cool for the tar to melt, leaving a solid asphalt which animals would simply have been able to walk across.

Damage to bones from Dermestid and Tenebrionid Beetle larvae. (A) Chicken bone damaged by Dermestid Beetle larvae in the lab. (B) Camel bone from the La Brae Tar Pits showing damage by both Dermestid and Tenebrionid Beetle larvae. Holden et al. (2013).

The warmer temperatures implied by the Beetle damage suggest that the temperatures were warm enough for the tar to melt at least periodically, suggesting a fluctuating climate. Most pollen grains used in climate studies come from trees, long lived plants which can survive fairly long periods outside their optimum temperature range. A tree that lives for a hundred years or more will not die out because there is the occasional decade in which it is too warm for it to reproduce (most organisms can survive outside the conditions which they need to reproduce successfully). Thus a fluctuating climate which occasionally produced summers, or even strings of summers, would be all that was needed to produce the fossils of the La Brea Tar Pits.

Bones showing damage from Tenebrionid Beetle larvae. (A & B) A Chicken bones damaged by Tenebionid larvae in the lab. (C) Horse bone showing signs of Tenebrionid damage from the La Brae Tar Pits. (D) Bison bone from the Tar Pits showing signs of Tenebrionid damage. Holden et al. (2013).

The extent of damage can also be used to estimate how long the bones were available at the surface. Unlike a mammalian scavenger, Beetles do not simply turn up, gnaw a bone then leave, but rather lay eggs on the bone, which have to have time to hatch, and then the Beetle larvae must spend some considerable time on the bone to make holes of any size - something which can be measuered by observing Beetles in the lab.

Bones showing damage from Tenebrionid Beetle larvae. (A) Chicken bone damaged in the lab. (B) Horse bone from the La Brea Tar Pits. Holden et al. (2013).

Based upon these calculations, Holden et al. conclude that the bones examined were available at the surface for around five months, suggesting that quite long periods of warm weather occurring, i.e. that the deposits were formed in times of long warm summers, something which supports the theory that the area was subject to climatic cycles rather than occasional freak warm weather.

See also A new species of Skin Beetle from Yunnan Province, China, Insect borings in Triasic wood and Fossil Owls from the La Brea Tar Pits.

Follow Sciency Thoughts on Facebook.

Fossil Owls from the La Brea Tar Pits.

The La Brea Tar Pits are located in what is now central Los Angeles, California. They are essentially oil deposits identical to those worked by oil drills in other parts of the world, but exposed at the surface. When oil deposits are exposed in this way the lighter fractions (crude oil is made up of a mixture of different oils, known as 'fractions' due to the process used to separate them, fractional distillation) such as petroleum evaporate off, leaving the heavier fractions, known as tar, or asphalt, behind. These form oily pools in which animals can become trapped. The La Brea Tar Pits appear to have been exposed at least intermittently at the surface for around 40 000 years, during which time a great number of animals have fallen into them.

Owls are not well represented in the La Brea Tar Pit deposits for a number of reasons, though they are some of the most abundant predators in the sort of open woodland terrain in which the deposits formed. Firstly, like all birds, owls have lightweight, fragile skeletons, ideal for flight, but giving them poor preservational potential. Secondly there is collection bias; the deposits have been heavily excavated for fossils since their discovery in 1875, however for much of that time collectors concentrated on big, prestigious, fossils such as mammoths and saber-toothed cats; many small fossils were simply discarded and lost. Finally there is the behavior of owls themselves. Predators are well represented in the Tar Pits deposits, it is thought because many became trapped while scavenging on the bodies of other trapped animals. Owls, however do not forage for carrion, they hunt living prey, mostly insects and small mammals. Such animals, if court in tar, will expire rapidly, and owls will ignore non-moving prey items. In addition owls live primarily in trees and spend little time on the ground, making them less likely to become trapped in tar pits.

Despite all this a number of owls have been described from the La Brea Tar Pit deposits. Prior to now these have been divided into nine species, eight still extant and one unique to the Tar Pits. In a review of the known owl fossils from the La Brea deposits, Kenneth Campbell of Vertebrate Paleontology at the Natural History Museum of Los Angeles County, and Zbigniew Bocheński of the Institute of Systematics and Evolution of Animals at the Polish Academy of Sciences, to be published in a forthcoming paper in the journal Acta Palaeontologica Polonica, conclude that two previously undescribed species of miniature owls have are present in the collections, and formerly describe those species.

Both the new species are described from their leg bones; not unusual with bird fossils, since the leg bones are generally those that survive as fossils best, which makes it easy to see why the fossils were overlooked by non-owl specialists.

The first new owl was assigned to the genus Glaucidium (pygmy owls) and named as Glaucidium kurochkini; Kurochkin's Pygmy Owl, in honor of the late Russian palaeornothologist Evgeny Kurochkin. Campbell and Bocheńska calculate the living owl would have weighed about 71.4 g, and assign 12 specimens to the new species.

Tarsometatarsals from (A) Glaucidium kurochkini and (B) the extant Glaucidium californicum. From Campbell and Bocheńska (2012).

The second owl was placed in the new genus Asphaltoglaux (Asphalt Owls, in reference to the Tar Pits) and named Asphaltoglaux cecileae, or Cécile's Asphalt Owl, in honor of the French palaeornithologist Cécile Mourer-Chauviré, of the Université Claude Bernard in Lyon. Campbell and Bocheńska estimate Asphaltoglaux cecileae would have been a robust bird, weighing about 78.2 g.

Tarsometatarsals from (A) Asphaltoglaux cecileae and (B) the extant Aegolius acadicus. From Campbell and Bocheńska (2012).

See also The Penguins of Africa, Was Archaeopteryx black? A new fossil bird from the Palaeocene of Brazil, How did raptors use their claws? (and did it help them learn to fly?) and Birds on Sciency Thoughts YouTube.