Prehistoric Life During the Paleocene Epoch

Prehistoric Life During the Paleocene Epoch Although it didnt boast as wide an array of prehistoric mammals as the epochs that succeeded it, the Paleocene was notable for being the geologic stretch of time immediately following the extinction of the dinosaurswhich opened up vast ecological niches for surviving mammals, birds, reptiles and marine animals. The Paleocene was the first epoch of the Paleogene period (65-23 million years ago), the other two being the Eocene (56-34 million years ago) and Oligocene (34-23 million years ago); all these periods and epochs were themselves part of the Cenozoic Era (65 million years ago to the present). Climate and geography. The first few hundred years of the Paleocene epoch comprised the dark, frigid aftermath of the K/T Extinction, when an astronomical impact on the Yucatan peninsula raised enormous clouds of dust that obscured the sun worldwide. By the end of the Paleocene, however, the global climate had recovered, and was nearly as warm and muggy as it had been during the preceding Cretaceous period. The northern supercontinent of Laurasia had yet to completely break apart into North America and Eurasia, but the giant continent Gondwana in the south was already well on its way to separating into Africa, South America, Antarctica and Australia. Terrestrial Life During the Paleocene Epoch Mammals. Contrary to popular belief, mammals didnt suddenly appear on the planet after the dinosaurs went extinct; small, mouselike mammals coexisted with dinosaurs as far back as the Triassic period (at least one mammalian genus, Cimexomys, actually straddled the Cretaceous/Paleocene boundary). The mammals of the Paleocene epoch werent much larger than their predecessors, and only barely hinted at the forms they would later attain: for example, the distant elephant ancestor Phosphatherium only weighed about 100 pounds, and Plesidadapis was an extremely early, extremely small primate. Frustratingly, most mammals of the Paleocene epoch are known only by their teeth, rather than well-articulated fossils. Birds. If you were somehow transported back in time to the Paleocene epoch, you might be forgiven for concluding that birds, rather than mammals, were destined to inherit the earth. During the late Paleocene, the fearsome predator Gastornis (once known as Diatryma) terrorized the small mammals of Eurasia, while the very first terror birds, equipped with hatchet-like beaks, began to evolve in South America. Perhaps not surprisingly, these birds resembled small meat-eating dinosaurs, as they evolved to fill that suddenly vacant ecological niche. Reptiles. Paleontologists still arent sure why crocodiles managed to survive the K/T Extinction, while their closely related dinosaur brethren bit the dust. In any case, prehistoric crocodiles continued to flourish during the Paleocene epoch, as did snakesas evidenced by the truly enormous Titanoboa, which measured about 50 feet from head to tail and may have weighed more than a ton. Some turtles, too, attained giant sizes, as witness Titanoboas contemporary in the swamps of South America, the one-ton Carbonemys. Marine Life During the Paleocene Epoch Dinosaurs werent the only reptiles that went extinct at the end of the Cretaceous period. Mosasaurs, the fierce, sleek marine predators, also disappeared from the worlds oceans, along with the last straggling remnants of plesiosaurs and pliosaurs. Filling the niches vacated by these voracious reptilian predators were prehistoric sharks, which had existed for hundreds of millions of years but now had the room to evolve to truly impressive sizes. The teeth of the prehistoric shark Otodus, for example, are a common find in Paleocene and Eocene sediments. Plant Life During the Paleocene Epoch A huge number of plants, both terrestrial and aquatic, were destroyed in the K/T Extinction, victims of the enduring lack of sunlight (not only did these plants succumb to darkness, but so did the herbivorous animals that fed on the plants and the carnivorous animals that fed on the herbivorous animals). The Paleocene epoch witnessed the very first cactuses and palm trees, as well as a resurgence of ferns, which were no longer harassed by plant-munching dinosaurs. As in preceding epochs, much of the world was covered by thick, green jungles and forests, which thrived in the heat and humidity of the late Paleocene climate. Next: the Eocene Epoch

Formation and Types of Irregular Galaxies

Formation and Types of Irregular Galaxies The word galaxy brings to mind images of the  Milky Way  or perhaps  the Andromeda galaxy, with their spiral arms and central bulges. These  spiral galaxies  are what people commonly imagine all galaxies look like. Yet, there are many types of galaxies in the universe and theyre not all spirals. To be sure, we live in a spiral galaxy, but there are also elliptical (rounded without spiral arms) and lenticulars (sort of cigar-shaped). Theres another set of galaxies that are rather shapeless, dont necessarily have spiral arms, but do have a lot of sites where stars are forming. These odd, blobby ones are called irregular galaxies.  Sometimes they get lumped in with the so-called peculiar galaxies due to their unusual shapes or other characteristics. Hubble Space Telescopes deepest view of the cosmos. There are hundreds of galaxies of all shapes and sizes in this image. NASA/ESA/STScI As many as a quarter of known galaxies are irregular.  With no spiral arms or central bulge, they dont seem to visually share much in common with either spiral or elliptical galaxies. However, they have some characteristics in common with spirals, at least. For one thing, many have sites of active star formation. Some may even have black holes at their hearts. Formation of Irregular Galaxies So, how do irregulars form? It seems that they are typically formed through gravitational interactions and mergers of other galaxies. Most,  if not all of them began life as some other galaxy type. Then through interactions with each other, they became distorted and lost some, if not all of their shape and features. Hubble Space Telescope looked at a pair of colliding galaxies that are tangling as they interact. The shock of the collision has produced blue streamers that look like clouds. They are actually giant starburst regions, where clusters of hot, massive young stars are being born. In the future, this may end up being an irregular galaxy for a time. NASA/ESA/STScI Some may have been created simply by passing near another galaxy. The gravitational pull of the other galaxy would tug on it and warp its shape. This will happen particularly if they pass near larger galaxies. This is likely what happened to the Magellanic Clouds, the smaller companions to the Milky Way. It appears that they were once small barred spirals. Because of their close proximity to our galaxy, they were distorted by gravitational interactions into their current unusual shapes. The Large Magellanic Cloud (middle left) and Small Magellanic Cloud (upper center) over Paranal Observatory in Chile. European Southern Observatory Other irregular galaxies seem to have been created through mergers of galaxies. In a few billion years the Milky Way will merge with Andromeda galaxy. During the initial time of the collision, the newly formed galaxy (which is nicknamed Milkdromeda) may look to be irregular as the gravity of each galaxy pulls on the other and stretches them like taffy. Then, after billions of years, they may eventually form an elliptical galaxy. This NASA/ESA Hubble Space Telescope image shows the massive elliptical galaxy Messier 60 (also called M60, or NGC 4649). M60 is 120 million light-years across and contains an estimated 400 billion stars. Highlighted in the inset is the dwarf galaxy M60-UDC1 which orbits the giant elliptical.M60-UCD1 is a tiny galaxy with a diameter of 300 light-years - just 1/500th of the diameter of the Milky Way! Despite its size it is pretty crowded, containing some 140 million stars.The dwarf galaxy may actually be the stripped remnant of a larger galaxy that was torn apart during a close encounter with Messier 60. Circumstantial evidence for this comes from the recent discovery of a monster black hole, which is not visible in this image, at the centre of the dwarf. The black hole makes up 15 percent of the mass of the entire galaxy, making it much too big to have formed inside a dwarf galaxy. NASA/ESA/STScI Some researchers suspect that large irregular galaxies are an intermediate step between the merger of similarly sized spiral galaxies and their eventual final forms as elliptical galaxies. The most likely scenario is that two spirals either mingle together or simply pass very near each other, resulting in changes to both partners in the galactic dance.   There is also a small  population of irregulars that dont fit into other categories. These are called dwarf irregular galaxies. They also look a lot like some galaxies as they existed early in the history of the universe, without a definite shape and looking more like a shred of a galaxy. Does this mean that the irregulars that are observed today are more like early galaxies? Or is there some other evolutionary path that they take? The jury is still out on those questions as astronomers continue to study them and compare younger to the ones they see that existed many billions of years ago. Types of Irregular Galaxies Irregular galaxies come in all sorts of shapes and sizes. This is not surprising considering  they may have started out as either spiral or elliptical galaxies  and simply distorted through the merger of two or more galaxies, or perhaps by nearby gravitational distortion from another galaxy. However, irregular galaxies can still all into a number of sub-types. The distinctions are usually associated with their shape and features, or lack thereof, and by their size. Irregular galaxies, particularly the dwarfs, are still not well understood. As weve already discussed, their formation is at the heart of the issue, particularly as we compare old (distant) irregular galaxies to newer (nearer) ones. Irregular Sub-types Irregular I Galaxies (Irr I): The first sub-type of irregular galaxies are known as Irr-I galaxies (Irr I for short) and are characterized by having some structure, but not enough to classify it as a spiral or elliptical galaxies (or any other type). Some catalogs break this sub-type down even further into those that exhibit either spiral features (Sm) - or barred spiral features (SBm) - and those that have structure, but not structure associated with spiral galaxies such as a central bulge or arm features. These are therefore identified as Im irregular galaxies.   Irregular II Galaxies (Irr II): The second type of irregular galaxy does not have any feature what so ever. When they were formed through gravitational interaction, the tidal forces were strong enough to eliminate all identified structure of what galaxy type it may have been previously. Dwarf Irregular Galaxies: The final type of irregular galaxy is the dwarf irregular galaxy mentioned above. As the name suggests, these galaxies are smaller versions of the two sub-types listed above. Some of them contain structure (dIrrs I), while others have no trace of such features (dIrrs II). There is no official cut-off, size-wise, for what constitutes a normal irregular galaxy and what is a dwarf. However, the dwarf galaxies tend to have low metallicity (that means that they are mostly hydrogen, with low amounts of heavier elements). They may also form  in a different way than normal-sized irregular galaxies. However, some galaxies currently classified as dwarf Irregulars are simply small spiral galaxies that have been distorted by a much larger nearby galaxy. Edited and updated by Carolyn Collins Petersen.