Paleohistology (WHY TEETH EXIST) with Yara Haridy

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• The earliest mineralized structures resembling teeth, called odontodes, appeared on the outside of fish scales and dermal bone around 455 million years ago, long before they were located inside the mouth.  Copied to clipboard!
• Teeth are composed of hard tissues like enamel and dentine, and their study via paleohistology allows scientists to infer details about an ancient animal's growth, health, and environment.  Copied to clipboard!
• Mammals, unlike most other vertebrates like reptiles and salamanders, have evolved to have a highly reduced tooth replacement strategy, typically only having two sets of teeth (baby and adult), possibly due to the need for precise occlusion in chewing.  Copied to clipboard!
• Mammals, unlike many other animals like sharks and crocodiles, generally only have two sets of teeth (milk and permanent) because precise occlusion is evolutionarily favored, though research is ongoing to regenerate teeth using stem cells.  Copied to clipboard!
• Snails' rasping structures (radulas) are made of keratin and are not considered true teeth because they lack odontodes, which are structures exclusive to vertebrates.  Copied to clipboard!
• New synchrotron scanning techniques, including the use of particle accelerators, revealed that a previously identified earliest vertebrate fossil from the late Cambrian was actually an arthropod covered in sensory structures called syncillae, pushing back the confirmed origin of mineralizing vertebrates to the middle Ordovician period.  Copied to clipboard!
• The discovery that ancient arthropod structures resembled vertebrate teeth and the finding that modern catfish odontodes are innervated suggests that the original function of vertebrate teeth and odontodes may have been sensory, which could explain the sensitivity of modern human teeth.  Copied to clipboard!

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Introduction to Paleohistology

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(00:01:00)

• Key Takeaway: The episode will explore ancient animal anatomy, specifically focusing on little weird teeth, through the lens of paleohistology.
• Summary: Alie Ward introduces the topic of ancient animal anatomy and the field of paleohistology (the study of fossilized prehistoric tissue samples) with her guest, Dr. Yara Haridy.

Introducing Dr. Yara Haridy

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(00:04:10)

• Key Takeaway: Dr. Haridy is a paleontologist, evolutionary biologist, and science communicator whose research focuses on ancient tissues.
• Summary: Dr. Haridy introduces herself, confirms her pronouns (she/her), and Alie provides background on her impressive academic journey, including her PhD and influential Nature paper.

Defining Paleohistology and Teeth

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(00:06:06)

• Key Takeaway: Paleohistology is the study of ancient tissues, and teeth are composed of tissues like dentine and enamel.
• Summary: Dr. Haridy clarifies that her main method is paleohistology (study of ancient tissues). They discuss that a tooth is a structure made up of tissues like dentine and enamel.

Path to Paleontology

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(00:06:42)

• Key Takeaway: Dr. Haridy’s interest in paleontology was sparked by studying anatomy in pre-med and then discovering the depth of tissue preservation in ancient fossils.
• Summary: Dr. Haridy shares her background, growing up in the Middle East, studying pre-med in Canada, and how volunteering in a paleo lab led her to fall in love with paleontology, especially after learning about paleohistology.

Fossil Preservation and Tissue Survival

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(00:10:26)

• Key Takeaway: Teeth, due to enamel being the hardest vertebrate tissue, are the most commonly fossilized structures, but soft tissues can also be preserved through mineralization.
• Summary: They discuss that 99.9% of fossils are teeth because enamel is so hard. Dr. Haridy explains that soft tissues like feathers or skin can sometimes be preserved when they mineralize or are replaced one-to-one by stable chemicals.

Origin of Mineralized Tissues

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(00:12:06)

• Key Takeaway: Mineralized tissues, like bone and teeth, trace back to the middle Ordovician period, around 455 million years ago, through a process called biomineralization.
• Summary: The discussion moves to when things started getting teeth, defining biomineralization, and establishing that mineralized tissues have existed for a very long time, predating dinosaurs by hundreds of millions of years.

Odontodes: Teeth Outside the Mouth

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(00:13:52)

• Key Takeaway: The very first tooth-like structures, called odontodes, appeared on the outside of ancient fish scales, made of enamel and dentine, possibly for protection or hydrodynamics.
• Summary: Dr. Haridy introduces ‘odontodes’—tooth-like structures on the exterior of early fish. They discuss hypotheses for their function (armor or locomotion efficiency) and note that modern sharks still have them on their skin.

Transition to Teeth in the Mouth

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(00:19:37)

• Key Takeaway: Teeth likely moved into the mouth during the Devonian period (the age of fishes) either through gradual migration or genetic re-expression of odontode-making tools inside the jaw.
• Summary: They explore how odontodes moved into the mouth, noting the evolution of jaws during the Devonian period. Two main hypotheses for this transition are discussed: structural migration or genetic re-expression.

Tusks vs. Teeth

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(00:21:52)

• Key Takeaway: Tusks are essentially teeth (made of dentine, often losing their enamel cap) that are continuously growing and non-replacing, sometimes serving sensory functions.
• Summary: The difference between a tusk and a tooth is explored, focusing on elephants. Tusks are primarily dentine, continuously growing, and may function as sensory organs.

Tooth Sensitivity and Ancestry

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(00:22:56)

• Key Takeaway: Human tooth sensitivity (to cold, pressure, etc.) is due to dentine tubules transferring signals to the pulp nerve, a trait traceable to our fish ancestors.
• Summary: They discuss how dentine transmits sensations to the nerve. Alie complains about post-dental work sensitivity, and Dr. Haridy suggests blaming their fish ancestors for this inherited trait.

Groundbreaking Research on Early Vertebrates

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(00:25:17)

• Key Takeaway: Dr. Haridy’s recent Nature paper investigates the origin of vertebrate teeth by studying tiny, fragmentary fossils from the latest Cambrian period.
• Summary: Dr. Haridy details the winding path of her research, which involved studying tiny, 300-400 million-year-old fossil fragments to understand the origin of mineralized exoskeletons and teeth.

The Tiny Fossils of Paleohistology

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(00:27:21)

• Key Takeaway: The earliest relevant fossils are extremely small and fragmentary, often looking like gravel or ‘grape nuts,’ requiring meticulous work to identify the individual odontodes.
• Summary: Dr. Haridy shows Alie vials of these microscopic fossils, which are dermal bone fragments covered in tiny odontodes, highlighting the challenge of working with such small specimens.

Field Work and Funding Challenges

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(00:32:34)

• Key Takeaway: Fieldwork involves opportunistic collection, such as digging in areas slated for destruction (like a road cut), and securing funding for preparation is often harder than the initial fieldwork.
• Summary: Dr. Haridy describes recent fieldwork in Colorado on a site about to be dynamited. They discuss how cheap fieldwork is compared to the high cost of skilled fossil preparation, making funding difficult.

Charitable Donation and Sponsors

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(00:35:24)

• Key Takeaway: Dr. Haridy selected the Samir project, which aids families returning to Gaza, for the podcast’s charitable donation.
• Summary: Alie announces the donation to the Samir project and thanks the show’s sponsors: Squarespace and Ritual.

Baleen vs. Teeth

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(00:40:05)

• Key Takeaway: Whale baleen is not considered teeth; it is a keratinized structure, completely different from the mineralized tissues of teeth (enamel/dentine).
• Summary: Patron questions lead to a discussion on baleen. Dr. Haridy confirms it is keratin, not mineralized tissue, and notes that baleen whales evolved from toothed ancestors, with the transition being a topic of research.

Bird Beaks and Tooth Loss

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(00:43:05)

• Key Takeaway: Bird beaks are keratin structures, not teeth, and modern birds have completely lost their teeth, relying on crops and grit for processing food.
• Summary: They confirm bird beaks are not teeth. Birds lost their teeth as their beaks (rhamphotheca) fully developed, leading to a grab-and-swallow eating style.

Hormones and Bone Density

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(00:46:34)

• Key Takeaway: Hormones significantly affect bone density maintenance (e.g., estrogen in women), leading to conditions like osteoporosis when levels drop.
• Summary: Addressing a patron question, Dr. Haridy confirms hormones affect bone density by regulating bone-building (osteoblasts) and bone-destroying (osteoclasts) cells. This is also relevant to bone loss in space travel.

Tooth Adaptation vs. Bone Adaptation

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(00:50:07)

• Key Takeaway: Unlike bone, teeth do not adapt to pressure in real-time because they lack the necessary cells (like osteocytes); their defense against damage is secondary dentine deposition.
• Summary: They contrast bone’s adaptability with teeth’s static nature. Teeth try to block cavities with secondary dentine but cannot actively build up due to wear like bone can.

Mammalian Tooth Replacement Limits

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(00:52:35)

• Key Takeaway: Constant tooth replacement is the ancestral state for vertebrates, but mammals drastically reduced this to only two sets (baby and adult teeth), possibly due to the need for precise occlusion (chewing fit).
• Summary: Patrons ask why humans only get two sets of teeth. Dr. Haridy explains that continuous replacement is the norm for most animals, and mammals sacrificed this for specialized, perfectly fitting molars.

Mammalian Teeth vs. Constant Replacement

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(00:55:17)

• Key Takeaway: Mammals have fixed teeth requiring complex implants, unlike animals like sharks that constantly replace theirs.
• Summary: Discussion on precise occlusion in mammals and the difficulty of constant tooth change. Mentions ongoing research into growing new teeth using stem cells in mice.

Salamander Teeth Function

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(00:56:25)

• Key Takeaway: Salamander teeth are tiny ‘velcro’ structures used to guide food down, compensating for the lack of a secondary palate.
• Summary: The host asks about salamander teeth, which are described as tiny structures helping direct worms and bugs down the throat, preventing them from going the wrong way due to lacking a secondary palate.

Defining True Teeth vs. Radula

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(00:56:51)

• Key Takeaway: Snail rasps (radulas) are not true teeth because they are made of keratin, unlike vertebrate teeth/odontodes.
• Summary: Clarification that snail radulas are not teeth because they are keratinous structures, establishing that true teeth (odontodes) are a vertebrate-only feature.

Tooth Replacement Costs and Enamel

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(00:57:29)

• Key Takeaway: High tooth replacement rates in sharks and reptiles suggest it’s worth the mineral cost, possibly because their enamel is thinner than that of mammals.
• Summary: Exploring why animals like sharks replace teeth so often, suggesting chipping is common. Contrasts this with thick mammalian enamel.

Dinosaur and Reptile Tooth Loss

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(00:58:18)

• Key Takeaway: Crocodiles, alligators, and dinosaurs constantly replaced their teeth, leading to many fossilized teeth being found.
• Summary: Confirmation that crocodiles, alligators, and dinosaurs constantly replaced their teeth, resulting in abundant fossilized teeth in the record.

Fossil Ethics and Hadrosaur Batteries

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(00:58:43)

• Key Takeaway: Fossil context is vital; people should contact paleontologists if they find teeth, especially since some dinosaur teeth (like hadrosaur batteries) fall apart easily.
• Summary: Discussion on finding dinosaur teeth and the importance of leaving them in situ for context. Describes hadrosaurs using dental batteries made of fused, leaf-shaped teeth.

Herbivore Teeth and Cementum

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(00:59:45)

• Key Takeaway: Herbivores like deer use cementum and complex wavy cusps/pits to create a ‘grater’ for abrasive plant matter.
• Summary: Analysis of herbivore teeth structure, focusing on how they manage abrasive diets using cementum and differential tissue hardness to maintain grinding surfaces.

Ever-Growing Rodent Incisors

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(01:00:23)

• Key Takeaway: Rabbits and rodents only have ever-growing incisors; if not worn down, they can curl into the palate.
• Summary: Explaining that lagomorphs and rodents only have continuously growing front teeth, which requires constant chewing to maintain length and shape.

Yellow Rodent Teeth Composition

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(01:01:14)

• Key Takeaway: Rodent teeth are yellow due to iron in the enamel, which makes them much stronger and creates a self-sharpening bevel.
• Summary: The reason for the yellow color in rodent teeth is iron content in the enamel, which aids in chewing through tough materials like wood.

Marsupial Teeth and Human Overcrowding

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(01:02:35)

• Key Takeaway: Marsupials have diverse dentition (40-50 teeth); human overcrowding may stem from softened diets reducing jaw growth selection pressure.
• Summary: Questions about marsupial teeth diversity and why humans experience overcrowding, potentially linked to evolutionary dietary softening.

Fossil Preservation and Diet Analysis

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(01:05:08)

• Key Takeaway: Fossilized bones exposed to sun fragment like wood; isotope analysis of enamel reveals specific dietary intake (e.g., C4 plants or seafood).
• Summary: Discussion on how bone degradation in the sun affects fossils, followed by an explanation of how enamel isotopes reveal detailed dietary history.

Revisiting the Earliest Vertebrate Fossil

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(01:08:25)

• Key Takeaway: Synchrotron scanning revealed that a supposed earliest vertebrate fossil was actually an arthropod with sensory structures, not a tooth.
• Summary: Dr. Haridy details using a particle accelerator (synchrotron) to scan early fossils, leading to the surprising conclusion that a key specimen was misidentified.

Teeth as Sensory Organs

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(01:13:06)

• Key Takeaway: Teeth and odontodes may have originally evolved as sensory organs, explaining why dentine (the sensitive tissue) is central to them.
• Summary: The realization that arthropod sensory structures (syncillae) look like odontodes leads to the hypothesis that vertebrate teeth/odontodes were sensory, supported by examples like the narwhal tusk.

Catfish Odontodes and Innervation

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(01:15:18)

• Key Takeaway: Catfish odontodes, found all over their bodies, are innervated (have nerves), confirming that external odontodes can be sensory structures.
• Summary: Dr. Haridy describes breeding plecos to use ‘clarity’ staining to trace nerves to their external odontodes, proving they are sensory, linking back to the ancient fish findings.

Impact of New Findings on Vertebrate Origins

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(01:19:06)

• Key Takeaway: The Cambrian fossil was kicked out of the vertebrate record, pushing the origin of mineralizing vertebrates back to the Middle Ordovician, with sensation as a key function.
• Summary: Summarizing the paper’s conclusion: the earliest mineralizing vertebrates are now dated later, and the original function of odontodes was likely sensation, inherited by our sensitive teeth.

Challenges and Joys of Paleontology

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(01:20:20)

• Key Takeaway: The hardest part is balancing the focus on deep time with current global crises; the greatest joy is the unique feeling of discovering a fossil untouched for eons.
• Summary: Dr. Haridy discusses the emotional difficulty of focusing on ancient bones amidst modern problems, contrasting it with the profound feeling of being the first person to see a fossil.