Module 10 Flashcards
The Rodent Model
Explore the ecological and morphological adaptations in rodent, multituberculate, and lagomorph evolution, and instances of convergent evolution.
Comprehend the specialized masticatory apparatus of rodents, focusing on their distinctive incisors and how this trait evolved in various mammalian groups and why.
Rodentia is the most species order of mammals, containing over 2500 extant species, all possessing a highly specialized and distinctive masticatory apparatus including a pair of elongated, curved and continuously growing incisors.
This condition known as diprotodonty, is not, however, unique to rodents, having evolved independently in a number of other mammalian groups including multituberculates, lagomorphs, hyraxes, marsupials, and primates (aye-ayes).
The internal incisor appears to play a small role in resisting bending forces and may act to strengthen the mandible. Highly elongated incisors may be driven by other factors such as rapid incisor wear as well.
Understand the distinction between multituberculates and rodents, including an overview of the Palaeocene multituberculates Ptilodus and the differences in their evolutionary relationships to modern rodents.
Multituberculates: Nemegibataar, Ptilodus, and Lambdopsalis.
Multituberculates vs. Rodents:
Multituberculates: Extinct mammalian order (†Multituberculata) dominant in the Mesozoic and Paleocene; not closely related to rodents.
Rodents: Modern order (Rodentia) arising in the Paleocene/Eocene; evolved independently from multituberculates.
Key Differences:
Dentition: Multituberculates had unique blade-like premolars and multicusped molars; rodents possess ever-growing incisors and simplified molars.
Jaw Motion: Multituberculates had a palinal (backward) jaw stroke; rodents use a propalinal (forward-backward) or gnawing motion.
Evolutionary Lineage: Multituberculates were part of the early mammalian group Allotheria; rodents are Therians (placental mammals).
Ptilodus (Palaeocene Multituberculate):
A squirrel-like arboreal herbivore with specialized teeth for slicing vegetation.
Exhibited convergent evolution with rodents in niche occupation but lacked direct ancestry.
Evolutionary Relationships:
Multituberculates went extinct in the Eocene, leaving no descendants.
Rodents evolved later from a separate therian lineage, becoming highly successful due to their dentition and adaptability.
Conclusion: Despite superficial similarities, multituberculates and rodents represent distinct, unrelated mammalian radiations.
Analyze the diversification of rodents through time, examining the phylogenetic tree and the shifts in diversification rates across different epochs.
🐭 Rodent Evolution – Time & Clade Breakdown
Paleocene (~66–56 million years ago)
Early rodent families start to branch out but no major diversification yet.
Eocene (~56–34 million years ago)
1st SDR (Significant Diversification Rate) appears in Castorimorpha (blue) – includes beavers and relatives.
1st SDR also in Ctenohystrica (red) – includes porcupines, guinea pigs, etc.
Sciuromorpha (green) begins to diverge as well – squirrels and relatives.
Oligocene (~34–23 million years ago)
2nd SDR in Sciuromorpha – more squirrel diversification.
2nd SDR in Ctenohystrica, specifically the Hystricognathi (like caviomorph rodents).
1st SDR in Anomaluromorpha (grey) – gliding and springhare-like rodents.
1st SDR in Myodonta (red) – includes mice, rats, hamsters, voles.
Miocene (~23–5 million years ago)
Big explosion of diversification, especially in Myodonta:
Murinae (orange): 5 SDRs – includes mice.
Sigmodontinae (red-orange): 9 SDRs – South American rats and mice.
Arvicolinae (red): 2 SDRs – voles, lemmings.
Cricetinae (red): 3 SDRs – hamsters and relatives.
Sciuromorpha now at 4 SDRs total.
Castorimorpha gets a 2nd SDR.
Ctenohystrica continues with multiple SDRs (up to 5 shown).
Pliocene to Present (~5 million years ago to now)
Diversification slows down but lineages continue to expand.
Understand the evolutionary context of Lagomorpha, including the early appearance of lagomorphs such as Gomphos, and their relation to modern rodent species.
Early Origins:
Lagomorphs (rabbits, hares, pikas) first appeared in the Paleocene (~60–65 Ma).
One of the oldest known lagomorphs, Gomphos (from early Eocene, ~55 Ma), shows transitional traits between ancestral Glires and modern forms.
Relationship to Rodents
Shared Ancestry: Both belong to Glires, a clade of gnawing mammals that split from other Euarchontoglires ~80–100 Ma.
Divergence: Lagomorphs and rodents split by the Late Cretaceous (~65–70 Ma), evolving independently.
Key Differences:
Lagomorphs: Two pairs of upper incisors (one small “peg tooth” behind the main pair).
Rodents: Single pair of ever-growing incisors.
Fossil Evidence & Adaptations
Gomphos had long hind limbs, suggesting early hopping adaptations.
Lagomorphs diversified in the Eocene, filling herbivorous niches before true rodents dominated.
Modern Relatives
Closest living relatives: Rodents (but not direct ancestors).
Both groups evolved high-crowned teeth for herbivory, a case of convergent evolution.
Conclusion
Lagomorphs represent an ancient lineage within Glires, with early forms like Gomphos showing key adaptations that predate modern rodent diversification.
Define diprotodonty.
When the incisors are elongated, curved, and continually growing.
Define masticatory apparatus.
The masticatory apparatus refers to the anatomical structures involved in the process of chewing and breaking down food. It includes the teeth, jaws, muscles of mastication, and temporomandibular joints.
True or false? Multituberculates are close relatives to rodents.
False!
What are Multituberculates?
- an extinct order of rodent-like mammals with a fossil record spanning over 130 million years
- first appeared in the Middle Jurassic, and reached a peak diversity during the Late Cretaceous and Paleocene.
- eventually declined from the mid-Paleocene onwards, disappearing from the known fossil record in the late Eocene.
How is diprotodonty a great example of convergent evolution?
Because it exists in several separate mammalian lineages.
Marsupials: diprotodontia (kangaroos, wallabies, possums, koala, wombats, and many others)
Therians: lagamorphs, rodents
Multituberculates: Nemegibataar, Ptilodus, and Lambdopsalis.
Hyracoidea: hyraxes
What the ** is a an aye-aye?
The aye-aye is a long-fingered lemur, a strepsirrhine primate native to Madagascar with rodent-like teeth that perpetually grow and a special thin middle finger that they can use to catch grubs and larvae out of tree trunks. It is the world’s largest nocturnal primate.
Analyze the diversification of lagomorphs through time, examining the phylogenetic tree and the shifts in diversification rates across different epochs.
- Lagomorpha (includes Lepus, Oryctolagus, Ochotona) is shown as a sister group to Rodentia.
- The lagomorph clade shows limited branching, suggesting low diversification compared to rodents.
- Lagomorphs appear to have diverged early but remained relatively stable through time.
- Rodents show much higher diversification, especially in Muridae and Hystricognathi.
Appreciate current rodent and lagomorph diversity and success of this group.
Order Rodentia (Rodents)
Families: ~34–40 (depending on classification)
Approx. Species: 2,600+ (largest mammalian order)
Major Families:
Muridae (rats, mice, gerbils) – ~834 spp.
Cricetidae (hamsters, voles, New World rats/mice) – ~768 spp.
Sciuridae (squirrels, chipmunks, marmots) – ~307 spp.
Echimyidae (spiny rats) – ~93 spp.
Caviidae (guinea pigs, capybaras) – ~21 spp.
Heteromyidae (kangaroo rats, pocket mice) – ~66 spp.
Dipodidae (jerboas, jumping mice) – ~51 spp.
Order Lagomorpha (Lagomorphs)
Families: 2 (sometimes 3, with Prolagidae extinct)
Approx. Species: 110–130
Major Families:
Leporidae (rabbits, hares) – ~70 spp.
Ochotonidae (pikas) – ~30–40 spp.
†Prolagidae (Sardinian pika – extinct)
Key Differences
Rodents: Much more diverse (~2,600 spp. vs. ~110 in lagomorphs).
Lagomorphs: Only 2 living families (Leporidae, Ochotonidae).
Dentition:
Rodents have one pair of ever-growing incisors.
Lagomorphs have two pairs of upper incisors (second pair small, peg-like).
Both orders belong to Glires, a clade of gnawing mammals, but diverged ~65–80 million years ago.
Compare the four different arrangements of the middle and deep layers of the masseter muscles in rodents: protrogomorph, hystricomorph, sciuromorph, and myomorph conditions.
** More in-depth imagery in slides
Who is she?
Mountain Beaver (Aplodontia rufa):
- Only living species in genus
- Aplodontia and family Aplodontiidae.
- Not related to true beavers (closer to squirrels).
- Less efficient kidneys suggest evolutionary antiquity.
- Seven subspecies:
-Six in California, three endemic to the state.
Tell me about the only known rodent to have horns.
Horned gophers are rodents from the genus Ceratogaulus, a member of the extinct fossorial rodent family.
Ceratogaulus is the only known rodent genus with horns, and is the smallest known horned mammal.
Ceratogaulus lived from the late Miocene to the early Pleistocene era.
Give four examples of rodents in the Sciuromorpha infraorder.
squirrels, chipmunks, marmots, and prairie dogs
Including flying squirrels
What species are in infraorder Castorimorpha?
beavers (2 species and extinct relatives)
Give four examples of rodents in the Geomyoidea infraorder.
Pocket gophers, kangaroo rats, kangaroo mice, pocket mice
Define Sciurognathi and Hystricognathi
Sciurognathi is a suborder of rodents that includes squirrels, chipmunks, beavers, and many types of mice. The group is characterized by a specific shape to the lower jaw. In sciurognaths, the angular process of the jaw is in the same plane as the root of the incisors.
This is in contrast to the suborder Hystricognathi where the angular process is outside the plane formed at the root of the incisor due to the presence of a shelf for muscle attachment.
What six rodents are included in infraorder Muroidea?
mice, rats, hamsters, gerbils, voles, and lemmings
What are Bathyergomorphs?
African mole rats
Give two cool facts about mole rats.
(1) Immune to cancer, barely age, etc.
(2) Eusocial (queen and sterile workers)
What are Hystricomorphs?
Old World porcupines
Name six rodents in Caviomorpha infraorder.
New World porcupines, chinchillas, guinea pigs, agoutis, nutria, cavies, etc.
What is unique about the niche of the capybara?
These are large rodents that filled this niche as a herbivorous mammal.
No longer feed like rodents but like large herbivores, however maintained their rodent physiological structures.
There were even giant ones the size of rhinos (now extinct).
What rodent shows convergent evolution with deer?
Patagonian Mara
- deer niche left open and filled by rodents
Name five examples of lagomorphs.
Jack rabbit, European hare, Snowshoe hare, bush rabbit, pika
