Paleozoic Calendars

by Charles Dixon

Charles Dixon was at the time this article was written, Editor of East Texas Mensa's "SpectruM" newsletter, chemist, math and science teacher and member of the Mensa Calendar SIG.

"Paleozoic Calendars" is a somewhat fanciful description of possible calendars which could have been used by dinosaurs, trilobites and others. The basis for this article is the known changes in the number of days per year. Evidence comes from fossil corals (algal stromatolites) and from calculations.

Six hundred million years ago the moon was closer to the earth, the tides were higher, and the days were shorter. Paleozoic years were just as long, but there were more days in the year. Coral reefs had (then and now) a tiny growth ring for each day of the year. The dominant animals were Trilobites, the earliest known members of Arthropoda, the group which now includes lobsters and insects.

Fossil Trilobite Calendar
(two of the 16 months)

A fossil Trilobite calendar from the late Precambrian period has 424 days in a 16 month year. Two of the alternating 26 and 27 day months are pictured. The months are divided into two "weeks."

As you can see, the Trilobites counted by 4's, probably indicating that a single nerve served two pairs of appendages. Since the atmospheric oxygen content was only about 2%, instead of the present 21%, Trilobites probably had a little trouble remembering what the date was. Another Trilobite calendar from the late Cambrian can be dated to about 500 million years ago. It has 412 days, divided into two 27 day months and fourteen 26 day months.

Gondwana Amphibian Calendar
(13 identical 30 day months)

The first calendar we know of with 30 day months is a 390 day amphibian calendar from late Carboniferous times 280 million years ago. The amphibians had a six day week, which means they either had very short weekends or they only worked four days a week. With my luck I would have to work 5 1/2 days a week.

Utah Fossil Dinosaur Calendar
(13 identical 30 day months)

A dinosaur calendar from the Jurassic Period about 170 million years ago has been found in Utah. It has 380 days and shows the dinosaurs had a five day week. The biggest problem with their calendar is the nineteen months, each named after one of those five-syllable monsters!

In late marsupial times (65 million years ago) early in the Paleocene epoch there were 371 days per year. Instead of having 53 seven day weeks, marsupials followed the Trilobite example and alternated 26 and 27 day months - 14 of them. Marsupials didn't scratch in the mud like the Trilobites. They left their calendars in the form of coprolite mounds, which we can't show in this family publication.

The 366 day year could have ushered in the age of man, but we weren't ready yet. There were 366 days in the year ten million years ago when primates were moving into the expanding grasslands of Africa. You have probably heard of the Julian Calendar. Old Julius Neanderthal could have invented the Julian Calendar, because there were 365 1/4 days in the year 100,000 years ago. Julius Caesar was even late for the Gregorian correction. They should have started dropping three leap year days each 400 years back about 1900 B.C. In 28,500 years the Gregorian II Calendar will delete leap year day every century year. Leap year will disappear completely in about three million years when there will be exactly 365 days per year.


Million years    Age      Days per year        Calendar
700        Precambrian        424       Trilobite I;  8x27 & 8x26 days
500 (BC)   Cambrian           412       Trilobite II; 2x27 & 14x26 days
280        Late Carboniferous 390       Gondwana Amphibian; 13x3O days
170        Jurassic           380       Utah Dinosaur; 19x2O days
 65        Early Paleocene    371       Marsupial; 7x26 & 7x27
 10        Miocene            366       Primates; 6x3O & 6x31
  -        Gregorian I        365.2425  1900 BC to 28,500 AD
  3.1 (AD) Saharan            365       Excess CO2 causes extensive deserts
 55        Antarctic          360       Antarctica begins to move north
110        Penguin I          355       Penguin diversification; 5x71 days
165        Penguin II         350       Penguins dominate planet, 14x25 or 10x35
230        Penguin III        345       Extraterrestrial contact; 23x15 days
                                              Intergalactic Period

The projections in the calendar chart show that a basic 12 month, 30 day calendar could be used far into the future just by adding or subtracting a few days a year. By 110 million years from now, the penguins will probably be ready for something different. A 355 day year could have 5 seasons with 71 days in each season. The 71 days could be grouped in 5 twelve day weeks and 1 eleven day week. The more sophisticated civilizations in the Penguin II age will probably have several calendars. 350 days could be divided into 14 months of 25 days or 10 months of 35 days. Some penguins will probably demand a compromise of 12 months of 30 days, which is 360 days! Others will romantically cling to lunar calendars.

By Penguin III times citizens will be ready for the three day work week and five days per week. If they use a three week month, they will need 23 months each year. They could name them after all their predecessors on the plant, but they will probably name them after generals, popes and numbers like one, two etc. Will the extraterrestrials laugh at a 23 month calendar? I doubt it - they've probably seen all these and more!

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