Editing 3120: Geologic Periods

{{comic
| number    = 3120
| date      = July 25, 2025
| title     = Geologic Periods
| image     = geologic_periods_2x.png
| imagesize = 611x557px
| noexpand  = true
| titletext = Geologists claim it's because the earlier Cenozoic used to be called the Tertiary, but that's just a ruse to hide the secret third geologic period, between the Neogene and the Quaternary, that they won't tell us about.
}}

==Explanation==
{{incomplete|
*This explanation should focus more on explaining the jokes in the comic rather than the history of life on Earth. There are still no explanations of the "birds" and "raptors" joke, which are the main jokes in the comic.
*The title text explanation could be shortened a bit.}}

This comic depicts a table representing planet Earth's geological time scale. For each period, [[Randall]] highlights his rather idiosyncratic likes and dislikes among their characteristics, instead of accompanying each geological name with facts pertinent to it, such as the duration of the period represented, the state of the Earth (e.g., glaciated), or the flora and fauna most common.

{| class="wikitable sortable"
!Period and date ({{abbr|{{w|Million years ago|MYA}}|Millions of years ago}})
!Randall's comments
!Explanation
|-
! rowspan="2" | {{w|Precambrian}} (4500&#8288;&#8211;&#8288;539)
|{{Yes|Life develops}}
|The {{w|Precambrian}} (italicized in the comic since it's not a {{w|Period (geology)|geologic period}}) is the first 88% of Earth's history, including the time 4.1 to 3.4 billion years ago when life on Earth began.
|-
|{{No|Snowball Earth episodes}}
|According to the {{w|Snowball Earth}} hypothesis, during some time spans in the past Earth became nearly or entirely frozen, with no liquid water on the surface. This is similar to the {{w|Greenhouse and icehouse Earth#Icehouse Earth|Icehouse Earth}}, including now, when the planet fluctuates between glacial and interglacial periods.
|-
! rowspan="2" | {{w|Cambrian}} (539&#8288;&#8211;&#8288;487)
|{{Yes|Trilobites!}}
|{{w|Trilobite|Trilobites}} are related to present-day insects, crabs, and other arthropods, and appeared during the Cambrian.
|-
|{{No|Evolution could stand to calm down a little}}
|The {{w|Cambrian explosion}} was a sudden radiation of complex life forms when nearly all important animal phyla, or precursors to them, appeared. Randall apparently thinks it all happened a little too fast.
|-
! rowspan="2" | {{w|Ordovician}} (487&#8288;&#8211;&#8288;443)
|{{Yes|Earth might have had rings}}
|Due to the location of impact of one type of meteorite, they may have been part of a {{w|Rings of Earth|planetary ring system around Earth}}, formed by the destruction of a parent body that entered Eath's Roche limit, before colliding with it.
|-
|{{No|Scary volcanic eruption in North America}}
|The volcanic eruptions that deposited {{w|Deicke and Millbrig bentonite layers|layers of ash}} during the Late Ordovician were incredibly large. The volcanoes involved may have been formed during the mountain-building event in {{w|Taconic orogeny|what is now north-eastern North America}}.
|-
! rowspan="2" | {{w|Silurian}} (443&#8288;&#8211;&#8288;420)
|{{Yes|First land animals}}
|Green plants first became established on land during the Ordovician period, after having evolved ways to protect themselves from desiccation and ultraviolet light. During the Silurian, land animals (mostly arthropods resembling {{w|Kampecaris|millipedes}}) followed the plants.
|-
|{{No|Earth's newfound mold problem}}
| Mold soon evolved to attack them and decompose their remains. A "{{w|Indoor mold|mold problem}}" often refers to mold growing in damp places in a building, causing unpleasant odors and various {{w|Indoor_mold#Health_effects|negative health effects}}.
|-
! rowspan="2" | {{w|Devonian}} (420&#8288;&#8211;&#8288;359)
|{{Yes|Big mountains in Boston}}
|A section of the present-day Appalachian Range from the Canadian Maritimes to the Carolinas, including what is now the Boston area of Massachusetts, was created during this period. (At the time, Boston was in the tropics, just south of the equator.)
|-
|{{No|Yeah, sure, what those giant killer fish needed was ''armor''}}
|{{w|Placoderm}} fishes, which were common in the Devonian, had plates of {{w|dermal bone}} in the head and thoracic portions of the body. Not all placoderms were giants, or apex predators. These fishes likely had these bony plates because they helped protect them from predators.
|-
! rowspan="2" | {{w|Carboniferous}} (359&#8288;&#8211;&#8288;299)
|{{Yes|Cool forests}}
|Forests in the Carboniferous lacked the flowering plants and conifers that are common in present-day forests. Instead, forests were dominated by giant versions of today's {{w|Lycopodiaceae|club mosses}}, {{w|Equisetidae|horsetails}}, and {{w|Marattiaceae|ferns}}, as well as by several plant lineages that are now extinct. {{w|File:Meyers b15 s0272b.jpg|Artists' depictions}} of such forests are exotic-looking and considered "cool" by Randall.
|-
|{{No|Bugs too big}}
|Carboniferous '{{w|Carboniferous#Terrestrial invertebrates|bugs}}' included the largest-ever known land invertebrate, a {{w|Arthropleura|2.6 m millipede-like animal}}; the largest-ever known flying insect, resembling a {{w|Meganeura|dragonfly with a wingspan of 75 cm}}; and a {{w|Pulmonoscorpius|70 cm scorpion}}.
|-
! rowspan="2" | {{w|Permian}} (299&#8288;&#8211;&#8288;252)
|{{Yes|Pangea}}
|{{w|Pangaea}} was the most recent {{w|supercontinent}} containing nearly all of Earth's landmass.
|-
|{{No|Google "The Great Dying"}}
|{{w|Permian-Triassic extinction event|The Great Dying}} occurred at the end of the Permian and is the most severe of Earth's {{w|Extinction event#The "Big Five" mass extinctions|'Big Five' mass extinction events}}. In it, 80% of marine species and 70% of terrestrial vertebrate species were wiped out.
|-)
! rowspan="2" | {{w|Triassic}} (252&#8288;&#8211;&#8288;201)
|{{Yes|Tanystropheus}}
|The ''{{w|Tanystropheus}}'' was an {{w|Archosauromorpha|Archosauromorph}} with a proportionally unusually long neck (as depicted in the comic).
|-
|{{No|Damage to Canada still visible from space at Manicouagan}}
|{{w|Manicouagan Reservoir}} is a ring-shaped lake, the remains of the crater caused by a 5 km (3 mi) asteroid hitting {{w|Quebec}}.
|-
! rowspan="2" | {{w|Jurassic}} (201&#8288;&#8211;&#8288;143)
|{{Yes|Birds}}
|The {{w|Avialae|ancestors of modern birds}} emerged during the Jurassic. Randall has repeatedly found it cool that birds are modern-day dinosaurs.
|-
|{{No|Parasitoid wasps}}
|{{w|Parasitoid wasp}}s reproduce by implanting their eggs into still-living animals, whose bodies are then eaten from the inside out by the wasp's larva. It is such a grisly process that it caused a {{w|Parasitoid wasp#In culture|crisis of faith}} among 19th-century European scholars.
|-
! rowspan="2" | {{w|Cretaceous}} (143&#8288;&#8211;&#8288;66)
|{{Yes|Raptors}}
| rowspan="2" |{{w|Dromaeosauridae|Raptors}}, especially {{w|Velociraptors}}, are a [[:Category:Velociraptors|trope]] within xkcd, especially in its early years. They were popularized by their appearance in the ''[[:Category:Jurassic Park|Jurassic Park]]'' film series. In the films, velociraptors are depicted as small (shorter than adult humans) bipedal scaled dinosaurs which frequently attacked and killed humans. Velociraptors and the irrational fear of being attacked by them in the modern world are a subject of several [[xkcd]] strips. Thus, "raptors" appears in both the "Favorites" and "Complaints" columns of the table.
|-
|{{No|Raptors}}
|-
! rowspan="2" | {{w|Paleogene}} (66&#8288;&#8211;&#8288;23)
|{{Yes|Pretty horseys!!!}}
|Fossils of members of the {{w|Equidae|horse family}} first appear during this period. Horses, fossil and extant, are prime examples of {{w|charismatic megafauna}} ("Pretty horseys!"). The rapid diversification of horses from a presumed single common ancestor is an oft-cited example of mammalian adaptive radiation.
|-
|{{No|Paleocene-eocene thermal maximum}}
|The {{w|Paleocene–Eocene Thermal Maximum}} was a time when the global average temperature rose by 5-8 °C in a relatively short period of time. It can be viewed as a slower version of [[:Category:Climate change|climate change, which Randall has repeatedly complained about]].
|-
! rowspan="2" | {{w|Neogene}} (23&#8288;&#8211;&#8288;2.6)
|{{Yes|Forests of ''Dracaena'' dragonblood trees}}
|''{{w|Dracaena draco}}'' and ''{{w|Dracaena cinnabari}}'' trees are a source of {{w|dragon's blood}}, a naturally occurring bright red resin used as a varnish and a dye.
|-
|{{No|Zanclean flood}}
|The {{w|Zanclean flood}} is theorized to be the flood that refilled the Mediterranean Sea.
|-
! rowspan="2" | {{w|Quaternary}} (2.6&#8288;&#8211;&#8288;present)
|{{Yes|Burrito invented}}
|Randall jokes that, in the last 2.6 million years, his favorite moment was the invention of the {{w|burrito}}, rather than many other, much more significant discoveries. The precise origin of the burrito is not known, but the {{w|Maya civilization}} made food resembling burritos as early as 1500 BC.
|-
|{{No|Whoever picked the name for the third period of the Cenozoic}}
|The third period of the {{w|Cenozoic Era}} is the Quaternary ("Fourth"), named by Jules Desnoyers in 1829. Randall is riffing on the cognitive disconnect between "third" and "fourth", for which the current geological naming conventions offer no justification.
|}

The title text expands on the complaint about the "third" <em>v</em> "fourth" discrepancy in regard to the Quaternary Period by postulating the existence of an unnamed geologic period within the Cenozoic Era "that geologists won't tell us about". In fact, the use of "Quaternary" (and "Tertiary") in recent/current geological nomenclature is a relic of four centuries of the history of geological studies in Western Europe, complicated by the religiously-inspired acceptance until the beginning of the 19th century, among European scholars, of an Earth that was only 6000 years old. "Primary" rocks were those considered to have been present in mountains before the "Great Deluge" of Noah (the {{w|Genesis flood narrative}}), while "Secondary" rocks represented the rubble from the Flood. Igneous and metamorphic rocks came to be understood as "primary", and eldest (within the context of a 6000-year-old Earth), and sedimentary rocks as "secondary". More recent geological formations came to be known as "tertiary" (relatively newer) and "quaternary" (even more recent). As the idea of Earth being billions of years old gained acceptance and we invented tools for dating rocks, "primary" and "secondary" fell away as descriptors for both rock types and rock ages, replaced by terms that convey information about the rocks more accurately. "Tertiary" was applied to "Cenozoic minus Quaternary", and survived in formal nomenclature into the 21st century. No alternative for "Quaternary" has yet been accepted, so the name persists as fodder for cartoonists who wonder, not without cause, how a "third" element in Earth history could be labeled a "fourth" element.

==Transcript==
{{incomplete transcript|Don't remove this notice too soon.}}

:[A table with 3 columns, labelled: "Period", "My favorite part" and "My biggest complaint". There are 13 rows below the labels]
:[Row 1: Period:] ''Precambrian'' [My favorite part:] Life develops [My biggest complaint:] Snowball Earth episodes
:[Row 2: Period:] Cambrian [My favorite part:] Trilobites! [My biggest complaint:] Evolution could stand to calm down a little
:[Row 3: Period:] Ordovician [My favorite part:] Earth might have had rings [My biggest complaint:] Scary volcanic eruption in North America
:[Row 4: Period:] Silurian [My favorite part:] First land animals [My biggest complaint:] Earth's newfound mold problem
:[Row 5: Period:] Devonian [My favorite part:] Big mountains in Boston [My biggest complaint:] Yeah, sure, what those giant killer fish needed was '''''armor'''''
:[Row 6: Period:] Carboniferous [My favorite part:] Cool forests [My biggest complaint:] Bugs too big
:[Row 7: Period:] Permian [My favorite part:] Pangea [My biggest complaint:] Google "The Great Dying"
:[Row 8: Period:] Triassic [My favorite part:] Tanystropheus [accompanying the text in this cell is an image of a ''Tanystropheus'' and its characteristic elongated neck, with Cueball standing next to it for scale] [My biggest complaint:] Damage to Canada still visible from space at Manicouagan
:[Row 9: Period:] Jurassic [My favorite part:] Birds [My biggest complaint:] Parasitoid wasps
:[Row 10: Period:] Cretaceous [My favorite part:] Raptors [My biggest complaint:] Raptors
:[Row 11: Period:] Paleogene [My favorite part:] Pretty horseys!!! [My biggest complaint:] Paleocene-eocene thermal maximum
:[Row 12: Period:] Neogene [My favorite part:] Forests of ''Dracaena'' dragonblood trees [My biggest complaint:] Zanclean flood
:[Row 13: Period:] Quaternary [My favorite part:] Burrito invented [My biggest complaint:] Whoever picked the name for the third period of the Cenozoic

{{comic discussion}}<noinclude>
[[Category:Charts]]
[[Category:Geology]]
[[Category:Velociraptors]]
[[Category:Animals]]
[[Category:Food]]
[[Category:Comics featuring Cueball]]