Tag Archives: Dinosaur

A Journey Through Plant Evolution at the Lincoln Park Conservatory

I recently spent some time at the beautiful Lincoln Park Conservatory. It is very hot and humid in the greenhouse, which may not have been the most obvious choice on a day that was already 88º outside. Nevertheless, I thought it would be a good way to talk about the evolution of plants. My specialty in biology is animals, but I have always loved the story of plant evolution. Like all other major groups of life on Earth, plant life began in the water. Early plants were very reliant on water, but plants became less and less dependent on water as they evolved. In the water, life is easy. Dehydration is not a problem. Nutrients can be absorbed directly out of the water into the cells. The water will carry sperm for reproduction and disperse the offspring. Life in the water is good, but there was a lot of space to grow on land.

The Lincoln Park Conservatory is a great place to go. A cooler day is better.

The Lincoln Park Conservatory is a great place to go. A cooler day is better.

Liverworts

One of the first types of plants to live on land were the liverworts. They are very small, with leaves that lie almost flat on the ground. Liverworts have no ability to draw water up out of the ground, as later plants are able to do. As a result, they cannot grow very tall and must be damp all of the time. They cannot survive or reproduce without being wet.

Liverworts are one of the first plants to live on land.

My favorite room contains the primitive plants: ferns, moss, liverworts, and cycads.

My favorite room contains the primitive plants: ferns, moss, liverworts, and cycads.

Moss

Like liverworts, moss also cannot draw water up into their bodies. Are able to grow a little bit taller than the liverworts because they grow in dense mats that can trap water between individual plants. This allows them to grow up to about four inches tall.

Mosses do not dry out as easily as liverworts, but they do rely on water for reproduction. The male sperm must swim through the water to find a female plant.

Like an idiot, I forgot to take a picture of moss. This one form wikipedia.org will have to do.

Club Moss

Club moss are sometimes called “ground pines” because they can resemble pine trees, but they are neither pines nor moss. Modern club moss usually only grow to be a few inches tall, but during the Carboniferous period, when they were the dominant land plant, they grew as tall as modern trees.

Club moss are one of the first type of plants to have vascular tissue, which lets them draw water from the ground up into their bodies. This adaptation is of unparalleled importance for plants on land. For this reason, club moss were one of the first types of plants to be able to grow more than a couple of inches tall. Without vascular tissue, a plant more than an inch or two has no way of getting water to the upper part of the plant. Plants don’t need very much to live, but access to sufficient sunlight is one of their main requirements. When all of the plants around you are only two inches tall, a plant that can grow to be several feet tall or taller has an enormous advantage when it comes to getting sunlight. You can grow taller than your neighbors and spread out to get all the sun you want.

Club moss (not moss). Image from bio.sunyorange.edu

Coal is made of fossilized plants from the carboniferous period. The majority of coal is made up of ferns and club moss. Sometimes coal preserves the structures of the plants it was made from and we can use the coal fossils to learn about ancient plants.

A thin section of coal clearly shows the features of the stem of an ancient plant (in cross-section). Image from http://www.ucmp.berkeley.edu

Ferns

Evolutionarily speaking, ferns are slightly newer than the club moss. Like the club moss, ferns are have vascular tissue (so does everything else from here on).

Compared to most other plants, ferns grow sideways. The stem lies horizontally underground, and the fronds grow up out of the ground from it. When you see a cluster of fronds sticking up together, they are usually from the same plant.

Fern frond

Fern frond

Structures on the underside of the fronds, called “sori,” contain spores. These capsules break open, releasing the spores, and new ferns grow where the spores land.

Sori are clearly visible on the underside of fern fronds. These contain spores.

Sori are clearly visible on the underside of fern fronds. These contain spores.

Cycads

Cycads look superficially like palms, pineapples or yuccas, but is not closely related to any of them. They were one of the dominant types of plants during the mesozoic era — the age of the dinosaurs.

Cycads were among the first plants to use pollen in reproduction. Pollen is produced by the male structures on plants, and is responsible for carrying sperm to the ovule in the female structures on other plants. Unlike the earlier plants, which require water for the sperm to swim through, pollen is carried by the wind. This is great for plants that live away from water and want to be able to reproduce with individuals that are far away. The problem is that it is fairly inefficient. Plants whose pollen is carried by the wind need to produce vast quantities of the stuff in order for some of it to get to other plants. I grew up in New Hampshire, where there a lot of white pine trees (which are not cycads, but also reproduce with wind-borne pollen). I got up many a morning to find my car completely covered in yellow pollen. All of that pollen that didn’t end up on the right part of the female plants is wasted energy.

Cycads were also among the first plants to have seeds, instead of spores like the older plants. Spores are fine, but they cannot travel over long distances or lie dormant for a more opportune time to sprout. A seed contains the plant embryo as well as nutrients to keep it alive for months or years. If a spore happens to land on the back of a bird on its way to the other side of the country, the embryo inside may not survive the trip because its mother didn’t pack it lunch. An embryo inside a seed will survive the same journey because it is surrounded in an oil-rich substance called “endosperm.” When we eat nuts, it is the endosperm that we are after.

Cycads can superficially resemble pineapples, yuccas or palms, but they are not part of the same group.

Cycads can superficially resemble pineapples, yuccas or palms, but they are not part of the same group.

Flowering Plants

Flowering plants became the dominant plants of the world during the late mesozoic, and today account for the majority of plant species.There are over a quarter million living species of flowering plants, compared to only about 12,000 species of fern, and fewer than 10,000 species of liverwort.

The flowering plants have two evolutionary advancements that allowed them to be so successful: flowers and fruit. These allow plants to solve two big problems in the area of reproduction.

Flowers represent an exchange of goods and services between plants and animals. Big, colorful, aromatic flowers are nature’s equivalent of an “eat here” sign. Flowers produce sugar-rich nectar that animals like ants, butterflies, birds, and bats like to eat. While these “pollinators” are eating the nectar, they get covered in pollen. When they go to the next flower, they drop some pollen off and pick up some more. This results in animals carrying pollen directly from one plant to the next, with very little waste. Remember all that energy that earlier plants wasted trying to pollinate my car? Flowers allow plants to use their energy more effectively. The energy this saves over relying on wind pollination is part of why flowering plants are so successful evolutionarily.

Flowers attract certain animals, which carry pollen between flowers, helping the plants reproduce.

Flowers attract certain animals, which carry pollen between flowers, helping the plants reproduce.

Spreading seeds is another problem for plants. If seeds just fall off the parent plant and onto the ground, some will roll away or get kicked away. The others will sprout right next to their parent and compete for the same nutrients and light. This will reduce the success of both the parent and the offspring. Some (but not all) flowering plants produce fruit to solve this problem.

Fruit is a sugar-rich substance that is easy to get eat, which surrounds the seed, which contains an oil-rich substance that is protected by a hard shell. In human terms, the plant “wants” you to eat the fruit, but does not “want” you to eat the seed. If you eat the seed, you are eating the tree’s offspring. If you (or another animal) eat the fruit, there is a good chance that you will swallow the seeds by accident. The hard shell protects it from being broken in your mouth or digested in your stomach. After eating the fruit, you (or whatever animal) will walk or fly away and eventually deposit the seeds far away in a nutrient-rich pile of fertilizer.

Flowering plants are better at life on land than any other plants. They can draw water and nutrients out of the soil through their roots and vascular tissue, and they are very good at reproducing and spreading their offspring without the aid of water. This is why they are beating all of the other plants.

Cladogram showing the evolutionary relationships of the plant groups and major evolutionary advancements.

Cladogram showing the evolutionary relationships of the plant groups and major evolutionary advancements.

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The Evolution of Flight

Out of several thousand species of birds, almost all of them can fly. They all have the ability to fly because they evolved from a common ancestor that could fly. Bats can all fly because they evolved from a common ancestor that could fly. But why can both birds and bats fly? Did they evolve from a common ancestor that could fly? While they did evolve from a common ancestor, this ancestor could not fly. How, then, are both birds and bats able to fly?

In biology, there is a concept called “convergent evolution.” Some types of organisms have similar traits because they evolved from a common ancestor that had those traits. With only a few exceptions, all mammals, amphibians and reptiles (including birds) have four limbs — two arms/wings and two legs. This is because these three lineages all evolved from a common ancestor that had four limbs. Similar traits that are due to common ancestry are called “homologous traits.”

Other types of organisms have similar traits but did not evolve from a common ancestor that had those traits. Fish and whales are a classic example of convergent evolution. They both have a tail fin that propels them through the water, forward fins that help them steer, a fusiform body that makes them hydrodynamic, and a dorsal fin that keeps them stable.

The dolphin and fish share many traits that facilitate an aquatic life. Image from wikipedia.org

But there are more differences than similarities. Here are a few:

  • The tail fin of a fish is oriented vertically, whereas the tail fin of a whale is oriented horizontally
  • Fish lay eggs, whereas whales give live birth
  • Baby fish are fed by a yolk sack in their egg, whereas baby whales are fed from mammary milk
  • Fish use gills to extract oxygen from the water, whereas whales breathe atmospheric air
  • Fish are cold-blooded, whereas whales maintain a high body temperature.
  • Fish have scales covering their skin, but whales do not.
  • Whales have typical mammalian wrist and finger bones inside their pectoral fins, but fish do not.
  • Whales have hair, but fish do not.

Whales do, of course, share a common ancestor with fish, but this common ancestor is not the reason that whales have their aquatic adaptations. The ancestors of whales first evolved into a terrestrial life, then evolved back into the water, much later in life.

When two or more different types of organisms evolve a similar trait independently, these traits are called “analogous traits” and the process of evolving these analogous traits is called “convergent evolution.”

Off the top of my head, I can think of nine independent evolutionary origins of flight — that is, nine separate events of convergent evolution. There are probably more that I don’t know about. Let’s start with the three best fliers that are currently alive: Birds, bats and insects.

Birds and bats are both tetrapods, so they are stuck with four limbs. They both use primarily their front limbs for flight, but they do it differently. Bird hand and wrist bones are fused together to make a short, stumpy end bone. Feathers produce the area required to produce lift.

The bones of a bird wing. Image from wikipedia.org

When birds are in flight, they keep their legs and feet tucked out of the way so they do not interfere with flying.

Canada goose in flight. Note that the legs are not used in flight. Image from wikipedia.org

Bats have a membrane of skin that stretches between their arms and legs that help produce lift. The legs and feet of bats are very important for flight.

Bat in flight. The legs are important in forming the wings. Image from wikipedia.org

Bats have elongated fingers that make up most of the wings. They use skin that is stretched between their fingers to create the area required to produce lift.

The arm bones in the bats and birds are homologous to one another, but their wings are the result of convergent evolution.

Insects have six legs and two pairs of wings. Insect wings are inflexible, except for where the connect to the body; a little bit like the oars on a boat. There are no bones or muscles inside the wings. Birds and bats have aerodynamic bodies that allow them to pass through the air efficiently. Some insects, like the dragonflies, have aerodynamic bodies, but bees and beetles do not.

Dragonfly. Image from wikipedia.org

The pterosaurs were not technically dinosaurs, but they were close relatives. Modern birds, which are dinosaurs, are not direct descendants of the pterosaurs, but birds are more closely related to the pterosaurs than they are to bats. Despite the closer genetic relatedness, the pterosaurs flight ability resembles bats more than birds in a variety of ways. First, they did not appear to have had feathers. Instead, they probably used a membrane of skin to form their wings much the way bats do.

Bats use fingers 2-4 (index through pinkie) for flight, and finger 1 (the thumb) for limited gripping. Pterosaurs only had four fingers, and only finger 4 was used for flight, whereas fingers 1-3 were used for gripping.

Pterosaur wing. Image from http://www.geol.umd.edu

 

Other, lesser fliers:

These are animals that fly sort of like a paper airplane. They cannot propel themselves once they are in the air — they have to jump to get their initial momentum. But once they are in the air, they can control their direction and create an air foil to slow their falling. Humans can do this with the aid of a wingsuit:

Flying squirrels: A little bit like bats, flying squirrels have a membrane of skin that stretches between their front and rear legs. This allows them to glide over longer distances than they would otherwise be able to jump.

Flying squirrel in flight. Image from wikipedia.org

 

Flying lizards: Although the word “dinosaur” literally means “terrible lizard,” lizards and dinosaurs are completely different types of reptile. Flying lizards in the genus “Draco” are not very closely related to the flying dinosaurs. The flying lizards are very unusual because they do not use any of their four limbs for flying. Instead, they are able to spread out their ribs to form fairly immobile wings which allow them to glide for short distances.

 

Flying dragon. Image from wikipedia.org

Flying fish: Flying fish are much better at flying than you would expect. They use their tail to get out of the water and get speed. Once they are in the air they can glide for fairly long distances. If they want to increase their speed, they can put their tail back into the water and give themselves another push. This makes them the only glider that I know of that can add energy to their glide without landing.

Flying Fish. Image from wikipedia.org

 

Flying frogs: Like bats, flying frogs create “wings” by stretching skin between long fingers. Unlike bats, the “wings” of the flying frogs are limited to their feet, and do not include any skin on the arms or legs.

Flying frog in flight. Image from http://endangeredliving.files.wordpress.com/

Flying snakes: To people who are afraid of snakes, nothing sounds more horrifying than snakes that can fly. But don’t worry — the flying snakes are the worst flyers of the group. They are able to flatten out their bodies to create a very minimal air foil. Their “flight” looks a lot like jumping or falling, but research has shown that they are able to steer themselves in the air. It may not seem like I should have included these in a list of things that evolved to fly, but remember that everything that evolved to fly had to go through many stages of flying ability. In the first stages, the animals would have just been jumping. In later stages, they would have a rudimentary ability to glide and navigate. For this reason, I firmly consider these snakes to be an example of incipient flight.

 

 

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Dinosaurs are not Extinct

Let me get this out of the way up front: Birds are dinosaurs. Birds are not LIKE dinosaurs. They are not BASICALLY dinosaurs. They are not RELATED to dinosaurs. They are not dinosaurs *nudge*nudge*wink*wink*. They are literally dinosaurs. Period. To this end, scientists use the term “non-avian dinosaurs” when they want to describe the things we called dinosaurs when we were kids but not birds. With a little bit of genetic manipulation, you can actually turn a bird into a more recognizable dinosaur

Scientists have known this for some time, now, and the evidence is still mounting. Birds have skeletal features that clearly place them within the dinosaur lineage, but there is more exciting evidence. Within the last 20 years, scientists have discovered that many non-avian dinosaurs had feathers and were warm-blooded, just like modern birds. Although birds are the only living lineage of reptiles that are bipedal, they belong to a lineage of dinosaurs that was also bipedal — the “Theropods.”  This lineage also includes fan favorites such as the Tyrannosaurus rex and Velociraptor, as well as the Dilophosaurus, Allisaurus, Ornithomimus, and Compsognathus. More specifically, modern birds belong to the “Maniraptora” lineage, which makes them particularly close relatives of the Velociraptor and Deinonychus. As you can see, birds are not classified as dinosaurs by a mere technicality. Birds are buried so deeply within the dinosaur lineage that it is basically impossible to classify anything that we like to call dinosaurs as dinosaurs without including birds. Although birds superficially resemble the pterosaurs, pterosaurs are not the direct ancestors of birds, nor are pterosaurs true dinosaurs. The following cladogram shows the evolutionary relatedness of birds, non-avian dinosaurs, and other reptiles (for help in interpreting cladograms, go here):

A cladogram showing the relatedness among birds, non-avian dinosaurs, and other reptiles.

A cladogram showing the relatedness among birds, non-avian dinosaurs, and other reptiles.

Pterosaurs were flying reptiles that superficially resembled birds. The two are not very closely related. Image from wikipedia.org

The Archaeopteryx is a fantastic transition fossil between birds and their ancestors, possessing traits of both modern birds and the ancient Maniraptors. Like modern birds, Archaeopteryx had large wings with large feathers, and a beak. Like the ancient Maniraptors, the Archaeopteryx had small teeth, and lacked the ridged or “keeled” sternum that serves as an attachment point for the flight muscles of modern birds. The hand bones in the front limbs of Archaeopteryx were not fused together like modern birds, but separated into long fingers just like the Velociraptor.

Archaeopteryx fossil. Feathers are clearly visible. Image from ucmp.berkeley.edu

Modern discussions of the relatedness among organisms typically involve mention of DNA evidence. For example, part of the reason that we know that humans are more closely related to chimpanzees than we are to gorillas is that our DNA is more similar to that of chimpanzees than it is to that of gorillas. The reason that I haven’t mentioned this yet is because we don’t have any. DNA is a fairly stable molecule, but it will break down over time. Under the best conditions, we can sequence DNA that is tens or hundreds of thousands of years old. In warm, humid climates, however, DNA doesn’t last more than a few thousand years — this is why scientists were unable to sequence the genome of the recently discovered “hobbit” fossils (Homo floresiensis), even though they are less than 10,000 years old. It may be possible in the future, but with modern technology no one has ever extracted DNA from a sample 65 million years old.

But DNA is not the only type of molecular analysis. In 2005, a group of paleontologists discovered, wait for it, soft tissue from a T. rex that was still soft and flexible. I’m going to pause for a second to let that sink in. Take all the time you need.

Soft tissue from a T. rex. Image from nbcnews.com

By some fluke of nature, the conditions were perfect for this tissue to remain soft, flexible, moist, and mostly undamaged for the last 68 million years, which is the estimated age of the fossil. My own excitement over this cannot come close to that of the scientists who made this discovery.

Although the scientists were unable to recover DNA from the sample, the proteins were still intact. DNA works by telling what amino acids to assemble and in what order to form a protein. Similarity in this strand of amino acids can be used to determine relatedness among organisms in the same way that is done with strands of DNA. Sure enough, they compared this sequence to that of modern birds and it was a close enough match to place birds within the dinosaur lineage.

Consider this: most Americans eat dinosaurs on Thanksgiving.

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