Hundreds of ancient mummified bees found dead in their cocoons

That is, hundreds of bees met their demise within their cocoons, becoming preserved in meticulous anatomical detail.

What's truly remarkable about this find is its exceptional preservation, defying the usual fate of insect skeletons that decompose due to their chitinous composition. 

"The degree of preservation of these bees is so exceptional," said Carlos Neto de Carvalho, the scientific coordinator at the Faculty of Sciences of the University of Lisbon, said in a press release.

"We were able to identify not only the anatomical details that determine the type of bee but also its sex and even the supply of monofloral pollen left by the mother when she built the cocoon."

This discovery results from a comprehensive project that identified four paleontological sites with a high density of bee cocoon fossils. Carbon 14 dating was employed, revealing that these ancient cocoons originated almost three thousand years ago.

The cocoons appear to encapsulate the young adults of the Eucera bee. These species still thrive, with 700 others in mainland Portugal today. 

What's even more captivating is the intricate thread that lines the interior of these cocoons, woven by the mother bee from an organic polymer. This thread sometimes cradles remnants of monofloral pollen – the sustenance for the larvae in their earliest days of life.

Micro-computed tomography unveiled a striking three-dimensional view of these mummified bees in their sealed cocoons. 

What caused the mass death of ancient bees? 

Bees, with over twenty thousand species globally, are crucial in pollination. Sadly, their populations have dwindled due to human activity and climate change. 

Understanding the ecological factors behind the large-scale death and mummification of bee populations millennia ago could provide invaluable insights into resilience strategies against climate change.

In this specific coastal region, the climatic conditions of nearly three thousand years ago were characterized by colder and wetter winters than what we experience today. 

Carlos Neto de Carvalho suggests that a rapid drop in nocturnal temperatures at winter's end or prolonged flooding during the off-season rainy period might have contributed to these tiny bees' demise and subsequent mummification.

This discovery is not just a fascinating glimpse into the past but a crucial piece of the puzzle in understanding the intricate relationship between ancient ecosystems, climate shifts, and the creatures that inhabited them.

The complete study was published in  Papers in Palaeontology and can be found here. 

Study abstract: 

The c. 100 myr extensive fossil record of bee brood nests and cells (calichnia) in siliciclastic sedimentary deposits, or palaeosols, is virtually devoid of the presence of their producers. The absence of a more specific assignment to a producer of the different ichnogenera of the ichnofamily Celliformidae precludes their use in phylogenetic and palaeobiogeographic studies. Omission surfaces developed in incipient carbonate palaeosols during the late Holocene (middle Neoglacial), c. 2975 yr cal BP, on the southwest coast of mainland Portugal show insect calichnia in dense ichnofabrics dominated by shallow discrete cells (Palmiraichnus castellanosi) and cells at the terminus of vertical shafts. At Carreira Brava, one of the studied sites, bees ready to abandon their cells were found in an exceptional state of preservation inside the sealed brood chambers. The chambers also preserve the inner cell hydrophobic polymerized membrane and remains of the monospecific Brassicaceae-type pollen provision. Although the cause of mass mortality remains a mystery, oxygen depletion due to sudden flooding of the nesting substrate and consequent or overnight temperature drop, just before emergence, are plausible causes. The anaerobic conditions and later rapid carbonate diagenetic lithification are the likely causes of the preservation of the bees and the inner cell organic membrane. The favourable climate conditions for the development of successive, dense ichnofabrics from an omission suite dominated by bee brood cells may be the result of slightly colder and higher-precipitation winters during the Neoglacial interval.