Evidence of a Hopping, Bipedal Mammaliform From Cretaceous Korea

Korean trackway of a hopping, mammaliform trackmaker is global first from the Cretaceous

Authors:

Kim et al

Abstract:

A trackway of a small hopping mammaliform trackmaker from the Jinju Formation (Lower Cretaceous) of the Jinju City area, Korea, is the first of this type reported from the Mesozoic of Asia, and globally. The animal left a narrow trackway (∼20 mm wide) with small tetradactyl footprints averaging less than 10 mm in diameter. Only two footprints registered with each hop (mean length 41 mm) thus indicating a bipedal gait. All trackway evidence suggests a small “mouse-like” trackmaker. Previous reports of trackways made by hopping tetrapods from the Mesozoic are rare and presently restricted to ichnogenus Ameghinichnus isp. indet., from the Jurassic of Argentina. Ichnogenus Musaltipes from the Cenozoic of North America, is the only other ichnogenus representing a hopping mammal. The Korean specimen here named Koreasaltipes jinjuensis, is different from these aforementioned ichnogenera in digit count, digit proportion and trackway configuration, as well as lack of tail trace. Koreasaltipes jinjuensis is from a lake shore paleoenvironment associated with nematode tracks (Cochlichnus isp.), other small tetrapod tracks attributed to avian theropods (birds), pterosaurs and crocodylomorphs, as well as larger saurischian dinosaurs.

Did the Permian Extinction Trigger a Shrinkage of Red Blood Cells in the Ancestors of Dinosaurs and Mammals?

Many mammals and birds are remarkable athletes; mice work hard to dig burrows for protection and sparrows fight gravity with each flap of their wings. In order to have the energy to sustain vigorous exercise, the body's tissues need a steady supply of oxygen, and red blood cells (RBCs) are the center of the oxygen delivery system. Size matters, too; athletic mammals and birds have much smaller RBCs than other vertebrates with lesser capacities for exercise. Biologists have long been puzzled over the evolutionary origins of RBC size. Were predecessors of mammals and birds -- including dinosaurs -- athletes and did they have tiny red blood cells? How do you measure the blood of extinct animals?

Now, biologists at the University of Utah and the Natural History Museum of Utah have established a 'fossilizable' indicator of athleticism in the bones of extinct vertebrates.

The study, which published online in Current Biology on Dec. 22, is the first to draw a link between RBC size and the microscopic traces of blood vessels and bone cells inside the bone. The researchers measured the bony channels that deliver oxygen to bone tissue to pinpoint when our mammal ancestors, bird and dinosaur predecessors evolved small RBCs. They found that extinct mammal relatives, or cynodonts, and extinct bird relatives had smaller RBCs and were likely better athletes than earlier terrestrial vertebrates. The timing of RBC-size reduction coincided with the greatest mass extinction event on Earth 252 million years ago, an event that paved the way for the age of the dinosaurs.

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Dietary Ecology of Pleistocene Quaternary Camels

Dietary ecology of Pleistocene camelids: Influences of climate, environment, and sympatric taxa

Authors:

Yann et al

Abstract:

Wild members of Camelidae live in some of the most arid environments, including North Africa, Arabia, the Gobi Desert of China and Mongolia and high elevation environments in the Andes Mountains. A better understanding of the paleoecology of the three most abundant Pleistocene camelids (Camelops, Hemiauchenia, and Palaeolama) may clarify modern adaptations to arid environments. Mammalian tooth enamel δ13C values were used to compare diets of co-occurring species in California, Texas, and Florida and δ18O values were used to investigate climate. Carbon isotope analysis suggests Camelops was likely an opportunistic browser that consumed both C3 and C4 browse/CAM plants, potentially consuming C4 browse (e.g., saltbush). Hemiauchenia had an opportunistic and highly generalized diet, while Palaeolama was a specialized forest browser. Stable oxygen isotopes and aridity index values suggest that Ingleside was warmer than McKittrick Brea, but there are no significant differences in aridity between the two sites. Co-occurrence data from the Paleobiology Database suggest that Palaeolama was restricted to forested environments as it occurred with two browsers, Tapirus and Odocoileus, at 90.5% of all sites. Camelops and Hemiauchenia both co-occurred with a broader range of taxa, further suggesting these camelids lived in diverse habitats. The generalized diet of Hemiauchenia, the likely ancestor of modern South American camelids, allowed for the adaptations of extant Lama and Vicugna to survive in the arid environments of the Andes Mountains. Collectively, these data clarify the dietary ecology of extinct camelids and provide insight into the potential importance of generalist diets for increased resilience to changing environments and/or climates.

A new spalacolestine mammal from the Early Cretaceous Jehol Biota

A new spalacolestine mammal from the Early Cretaceous Jehol Biota and implications for the morphology, phylogeny, and palaeobiology of Laurasian ‘symmetrodontans’

Authors:

Han et al

Abstract:

‘Symmetrodontans’ are extinct mammals characterized by having a reversed-triangle molar pattern in which three main cusps define a triangular molar crown. This dental morpholgy has been regarded as being intermediate between the ‘triconodont’ tooth and the tribosphenic pattern characterizing therians; it is a key feature in taxonomy of Mesozoic mammals and one to understand mammalian evolution and palaeobiology. Here we report a new genus and species of ‘symmetrodontan’ mammal, Lactodens sheni, from the Early Cretaceous Jehol Biota, represented by a partial skeleton with dentary and upper and lower teeth with dental morphologies well-preserved. The new species has a dental formula of three upper incisors, one canine, three premolars, and six molars/three lower incisors, one canine, five premolars and six lower molars, double-rooted canines, extremely low-crowned and transversely thin premolars, and acute angled molars. The dental morphologies of molars and peculiar deciduous premolars are similar to those of Spalacolestes from North America. The associated upper and lower dentitions from one individual animal helped to clarify tooth identification of some spalacotheriids represented only by fragmentary material. Phylogenetic analyses indicate a close relationship of the new species to North American spalacolestines and faunal interchanges between Eurasia and North America, thus supporting the notion that small-bodied spalacotheriids were diverse and had a pan-Laurasian distribution during the Early Cretaceous. Absence of the Meckelian groove suggests acquisition of the definitive mammalian middle ear in spalacolestines, and deciduous canines and premolars in the slim and extremely long dentary imply a faunivorous diet.

Microleo attenboroughi: a Fossil Marsupial Carnivore From Miocene Neogene Australia

It weighed a little more than that 20-ounce bottle of Mountain Dew on your desk and is named after one of the giants in nature broadcasting: Meet the pocket-sized marsupial lion that you wish you could have met for real 19 million years ago.

Microleo attenboroughi is the latest fascinating fossil to emerge from northern Australia, which earlier this year gave us the hypercarnivore Whollydooleya tomnpatrichorum (go on, just say that name aloud. You know you want to). Researchers unearthed M. attenboroughi at the fossiliferous Riversleigh UNESCO World Heritage Site and dated the animal to about 19 million years ago, when the area was a lush rainforest.

Named in honor of beloved broadcaster Sir David Attenborough (also arguably the world’s most famous naturalist and, come on, the guy everyone wishes would turn up at a dinner party), this mini-marsupial got his academic debut this week in the online, open-access Palaeontologia Electronica.

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Pleistocene Giant Sloths Found in Brazil

New Pleistocene remains of megalonychid ground sloths (Xenarthra: Pilosa) from the intertropical Brazilian region

Authors:

De Iuliisa et al

Abstract:

The Pleistocene fossil sloth Australonyx aquae De Iuliis, Cartelle, and Pujos, 2009 (Mammalia, Xenarthra, Megalonychidae) was described from the intertropical region of Brazil. However, its mandible was not known and only cursory descriptions of the ear ossicles were included. The mandible was subsequently recognized among the remains originally collected from the type locality, and belongs to the holotype individual. As a particularly important skeletal element for specific recognition, it requires description to complement our understanding of this species. The ossicles, usually poorly represented in the fossil record, require further description to allow differentiation from those of other sloths. Comparisons of the mandible and ossicles are conducted with homologous elements of the contemporaneous and sympatric Ahytherium aureum Cartelle, De Iuliis, and Pujos, 2008, the only other megalonychid sloth known from intertropical Brazil, and reinforce the distinction between these two species detailed in their initial descriptions. Comparisons with other sloths (e.g., Acratocnus, Megalonyx, Neocnus) also reveal differences with Au. aquae in such features as form and size of the caniniform tooth, angular process, and mandibular condyle. Differences among the malleus and incus of Au. aquae and several species of other sloth clades reveal clade level distinctions among Megatheriidae, Nothrotheriidae, and Megalonychidae. A well-preserved skull from the Brazilian state of Rondônia is noted as probably belonging to Au. aquae. This skull cannot be assigned formally to this species because it is not deposited in a recognized institution, but it does extend considerably the known range of the species.

New Euprimate Fossils From Eocene Paleogene India

New euprimate postcrania from the early Eocene of Gujarat, India, and the strepsirrhine–haplorhine divergence

Authors:

Dunn et al

Abstract:

The oldest primates of modern aspect (euprimates) appear abruptly on the Holarctic continents during a brief episode of global warming known as the Paleocene-Eocene Thermal Maximum, at the beginning of the Eocene (∼56 Ma). When they first appear in the fossil record, they are already divided into two distinct clades, Adapoidea (basal members of Strepsirrhini, which includes extant lemurs, lorises, and bushbabies) and Omomyidae (basal Haplorhini, which comprises living tarsiers, monkeys, and apes). Both groups have recently been discovered in the early Eocene Cambay Shale Formation of Vastan lignite mine, Gujarat, India, where they are known mainly from teeth and jaws. The Vastan fossils are dated at ∼54.5 Myr based on associated dinoflagellates and isotope stratigraphy. Here, we describe new, exquisitely preserved limb bones of these Indian primates that reveal more primitive postcranial characteristics than have been previously documented for either clade, and differences between them are so minor that in many cases we cannot be certain to which group they belong. Nevertheless, the small distinctions observed in some elements foreshadow postcranial traits that distinguish the groups by the middle Eocene, suggesting that the Vastan primates—though slightly younger than the oldest known euprimates—may represent the most primitive known remnants of the divergence between the two great primate clades.

Echovenator sandersi: a Toothed Whale Fossil Shows High Frequency Echo Location Hunting Evolved 10 Million Years Earlier Than Previously Thought

A newly-named fossil whale species had superior high-frequency hearing ability, helped in part by the unique shape of inner ear features that have given scientists new clues about the evolution of this specialized sense.

In a study published August 4 in Current Biology, researchers from New York Institute of Technology and colleagues from the National Museum of Natural History in France describe a new species of whale, Echovenator sandersi ("Echo Hunter"), an ancient relative of the modern dolphin, and its ability to hear frequencies well above the range of hearing in humans.

The research pushes the origin of high frequency hearing in whales farther back in time -- about 10-million years than previous studies have indicated.

"Previous studies have looked at hearing in whales but our study incorporates data from an animal with a very complete skull," says Morgan Churchill, a postdoctoral fellow at NYIT College of Osteopathic Medicine and the paper's lead author. "The data we gathered enabled us to conclude that it could hear at very high frequencies, and we can also say with a great degree of certainty where it fits in the tree of life for whales."

"This was a small, toothed whale that probably used its remarkable sense of hearing to find and pursue fish with echoes only," says Associate Professor Jonathan Geisler, a study co-author. "This would allow it to hunt at night, but more importantly, it could hunt at great depths in darkness, or in very sediment-choked environments."

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Kollikodon ritchiei: a Second Specimen of the Rare Australosphenidan Mammal Found, Sister Group to Monotremes

The upper dentition and relationships of the enigmatic Australian Cretaceous mammal Kollikodon ritchiei

Authors:

Pian et al

Abstract:

Mesozoic mammals from Australia are rare, so far only known from the Early Cretaceous, and most are poorly represented in terms of dentitions much less cranial material. No upper molars of any have been described. Kollikodon ritchiei is perhaps the most bizarre of these, originally described on the basis of a dentary fragment with three molars. Here we describe a second specimen of this extremely rare taxon, one that retains extraordinarily specialised upper cheekteeth (last premolar and all four molars). Each molar supports rows of bladeless, rounded cuspules many of which exhibit apical pits that may be the result of masticating hard items such as shells or chitin. Reanalysis of the phylogenetic position of this taxon suggests, based on a limited number of apparent synapomorphies, that it is an australosphenidan mammal and probably the sister group to Monotremata. This reanalysis also supports the view that within Monotremata, tachyglossids and ornithorhynchids diverged in the early to middle Cenozoic.

How can we Tell if Fossil Mammals Were Aquatic or Terrestrial?

Despite the extensive fossil record of mammals, it is often difficult to use fossil data to reconstruct the lifestyles and habitats of extinct species. The fact that some species spent all or part of their time underwater, respectively similar to modern-day whales and seals, further complicates this.

Konami Ando and Shin-chi Fujiwara, researchers at Nagoya University, addressed this by developing a new index for predicting if a species lived its entire life in the water. The index is based on how the ribs must be relatively strong for an animal to walk or crawl over land, but not for it to swim. After establishing the index via measurements of living terrestrial, semiaquatic, and exclusively aquatic species, Ando and Fujiwara used it to predict that some extinct species could not have supported themselves on land.

Although mammals originally evolved as terrestrial organisms, cladistics shows that some returned to aquatic lives, and that this sometimes occurred independently. Examples include whales, dolphins, and manatees, which never leave the water, and seals and hippopotamuses, which split time between land and water. Studies of fossils of extinct species also suggest some species spent all or some of their time in the water. However, inability to use fossil records alone to determine a species' lifestyle has made this hard to confirm.

In their study, reported in the Journal of Anatomy, Ando and Fujiwara analyzed rib cages and their resistance to vertical compression in a range of mammalian species. This important factor represents an animal's ability to support its body weight against gravity while walking or crawling; a trait aquatic organisms do not need. The researchers investigated 26 modern-day terrestrial, semiaquatic, and exclusively aquatic species, including the killer whale, polar bear, dugong, giraffe, and hippopotamus. They used their data to establish an index for differentiating between groups with different habitats. They then applied the index to four extinct mammalian species, all of which had retained their four limbs but showed signs of having been partially or completely aquatic, to shed light on their potential lifestyles.

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Thylacoleo carnifex (Marsupial Lion) Appears to Have had a Semi Opposable Claw-Thumb

Ecomorphological determinations in the absence of living analogues: the predatory behavior of the marsupial lion (Thylacoleo carnifex) as revealed by elbow joint morphology

Authors:

Figueirido et al

Abstract:

Thylacoleo carnifex, or the “pouched lion” (Mammalia: Marsupialia: Diprotodontia: Thylacoleonidae), was a carnivorous marsupial that inhabited Australia during the Pleistocene. Although all present-day researchers agree that Thylacoleo had a hypercarnivorous diet, the way in which it killed its prey remains uncertain. Here we use geometric morphometrics to capture the shape of the elbow joint (i.e., the anterior articular surface of the distal humerus) in a wide sample of extant mammals of known behavior to determine how elbow anatomy reflects forearm use. We then employ this information to investigate the predatory behavior of Thylacoleo. A principal components analysis indicates that Thylacoleo is the only carnivorous mammal to cluster with extant taxa that have an extreme degree of forearm maneuverability, such as primates and arboreal xenarthrans (pilosans). A canonical variates analysis confirms that Thylacoleo had forearm maneuverability intermediate between wombats (terrestrial) and arboreal mammals and a much greater degree of maneuverability than any living carnivoran placental. A linear discriminant analysis computed to separate the elbow morphology of arboreal mammals from terrestrial ones shows that Thylacoleo was primarily terrestrial but with some climbing abilities. We infer from our results that Thylacoleo used its forelimbs for grasping or manipulating prey to a much higher degree than its supposed extant placental counterpart, the African lion (Panthera leo). The use of the large and retractable claw on the semiopposable thumb of Thylacoleo for potentially slashing and disemboweling prey is discussed in the light of this new information.

Studying Mouse Lemur Genetics Reveals Ancient Forest Mosaic Environment

Today, Madagascar is home to a mosaic of different habitats--a lush rainforest in the east and a dry deciduous forest in the west, separated by largely open highlands. But the island off the southeast coast of Africa hasn't always been like that--a new study published in the Proceedings of the National Academy of Sciences announces that these two ecologically different portions of the island were once linked by a patchwork of forested areas. And to figure it out, the scientists analyzed the DNA of some of the cutest animals on earth--mouse lemurs.

"For a long time, scientists weren't sure how or why Madagascar's biogeography changed in very recent geological time, specifically at the key period around when humans arrived on the island a few thousand years ago. It has been proposed they heavily impacted the Central Highland forests," says Steve Goodman, MacArthur Field Biologist at The Field Museum in Chicago, who co-authored the study and has been studying Malagasy animals for thirty years. "This study shows the landscape was changing thousands of years before humans arrived."

So scientists wanted to learn about the history of Madagascar's landscape--why study mouse lemurs? The tiny primates are the perfect combination of fast-breeding, hardy, and unique to the island. "They reach reproductive maturity within a year, and that means that a lot of generations are produced very quickly," explains Goodman. "That enables us to see evolution at work faster than we would in an animal that took, say, five years to first reproduce." The lemurs, which are found only on Madagascar, live across much of the island, even forested areas that have been damaged by humans. That means that for scientists studying how the island changed over time, mouse lemurs are a jackpot. "The mouse lemurs are forest dependent--as the forest changes, they change. By studying how mouse lemurs evolved in different areas of the island, we're able to glimpse how the island itself changed and learn whether those changes were caused by humans," says Goodman.

By analyzing DNA from five different mouse lemur species, the scientists were able to tell when the different kinds of lemurs branched out from each other. "We were able to characterize tens of thousands of changes in the genomes of mouse lemurs that are now isolated and form separate species. By analyzing these DNA changes, we were able to understand when the species diverged from each other, and by inference, identify the ecological forces that might have driven them apart," says Anne Yoder, Director of the Duke University Lemur Center and lead author on the paper.

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The Differences in how Mammals and Cynodonts Grew

The evolution of growth patterns in mammalian versus nonmammalian cynodonts

Authors:

O’Meara et al

Abstract:

One of the major evolutionary transitions of the mammaliaform lineage was the origin of a typically mammalian pattern of growth. This is characterized by rapid juvenile growth followed by abrupt cessation of growth at adult size and may be linked with other important mammaliaform apomorphies of dental replacement and morphology. Investigation of growth patterns in the tritylodontid cynodont Oligokyphus and the basal mammaliaform Morganucodon provides insight into this crucial transition. We collected mandibular depth measurements from large samples of Morganucodon and Oligokyphus and constructed distributions of mandibular depth versus frequency for each species. These were compared with distributions from species from three different growth classes of extant amniote: testudines + crocodilians, mammals + birds, and lepidosaurs. Discriminant function analysis was used to differentiate between known growth classes by using different combinations of three measures of mandibular depth distribution shape (skew, kurtosis, and coefficient of variation) as proxies for different juvenile and adult growth patterns. Classification of the fossil species showed that Morganucodon closely resembled extant placental mammals in having rapid juvenile growth followed by truncated, determinate adult growth. Oligokyphus showed intermediate growth patterns, with more extended adult growth patterns than Morganucodon and slightly slower juvenile growth. This suggests a gradual evolution of mammalian growth patterns across the cynodont to mammaliaform transition, possibly with the origin of rapid juvenile growth preceding that of truncated, determinate adult growth. In turn, acquisition of both these aspects of mammalian growth was likely necessary for the evolution of diphyodont tooth replacement in the mammaliaform lineage.

Eutherian Mammals (Placentals) Evolved Faster Right After Dinosaurs Went Extinct

Eutherians experienced elevated evolutionary rates in the immediate aftermath of the Cretaceous–Palaeogene mass extinction

Authors:

Halliday et al

Abstract:

The effect of the Cretaceous–Palaeogene (K–Pg) mass extinction on the evolution of many groups, including placental mammals, has been hotly debated. The fossil record suggests a sudden adaptive radiation of placentals immediately after the event, but several recent quantitative analyses have reconstructed no significant increase in either clade origination rates or rates of character evolution in the Palaeocene. Here we use stochastic methods to date a recent phylogenetic analysis of Cretaceous and Palaeocene mammals and show that Placentalia likely originated in the Late Cretaceous, but that most intraordinal diversification occurred during the earliest Palaeocene. This analysis reconstructs fewer than 10 placental mammal lineages crossing the K–Pg boundary. Moreover, we show that rates of morphological evolution in the 5 Myr interval immediately after the K–Pg mass extinction are three times higher than background rates during the Cretaceous. These results suggest that the K–Pg mass extinction had a marked impact on placental mammal diversification, supporting the view that an evolutionary radiation occurred as placental lineages invaded new ecological niches during the Early Palaeocene.

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How Mammals Adapt to the Sea

Though mammals adapted on land, a new study by Maria Chikina and Nathan Clark has shown that during three major independent evolutionary events, a number of mammals harkened back to the sea.

For the manatee, walrus, dolphin, and killer whale, the return to the sea involved many evolutionary trade-offs amongst hundreds of genes: a general loss of the number of sensory genes for smell and taste, new functions for genes forming skin and connective tissue, and genes involved in muscle structure and metabolism.

In a massive undertaking, they used 59 placental mammal genomes to calculate the relative rates of evolution for all branches in 18,049 gene trees. To hone in identifying key genes, they used a new approach: rather than catalog single genetic mutations from genome-wide studies, or look for candidate genes in key pathways, or catalog single amino acid changes at the protein level, they calculated a genome-wide average rate of evolution across all species. Then, they determined whether sea mammals had put the evolutionary gas pedal or brake on compared to the average rate. In the next step, they employed these relative rates to identify those genes that may have independently shifted to higher (or lower) rates for five marine species - bottlenose dolphin, orca, walrus, Weddell seal, and West Indian manatee. "By exploiting the diversity within mammals, we stand to learn much about our own genetics and physiology," said Clark. "Our new approach is poised to reveal the evolutionary adaptations for countless other environments, from the sea, high altitudes, the desert, or even underground."

They identified hundreds of genes that revealed three main evolutionary themes - a burst of adaptation, followed by relaxation, and additional constraint -in response to the marine environment.

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Nabotherium: a new Anthracothere From Oligocene Paleogene Egypt With Interesting Dental Adaptations

A new anthracothere (Artiodactyla) from the early Oligocene, Fayum, Egypt, and the mystery of African ‘Rhagatherium’ solved

Authors:

Sileem et al

Abstract:

Recent work on new anthracothere (Mammalia, Artiodactyla) specimens from the Jebel Qatrani Formation, early Oligocene, Fayum, Egypt, has revealed the presence of a new genus. Nabotherium new genus is described on the basis of a partial skull, several mandibular and maxillary specimens, and isolated teeth. The new genus exhibits a distinctive combination of features not seen in other Paleogene anthracotheres. The most noticeable characteristics of the new genus include the presence of large and well-developed upper and lower canines, caniniform third incisors, the presence of only a short diastema between the canine and first premolar, and broad, bunodont cheek teeth. This is in contrast to other contemporary anthracotheres, including other forms from the Fayum, which show a spatulate third incisor, a reduced canine, a much longer canine-premolar diastema, and more narrow, bunoselenodont cheek teeth. The presence of a relatively short rostrum with closely packed incisors, low-crowned and simple premolars, and low-crowned, bunodont molars indicates that members of the new genus would have been more efficient at crushing foods than slicing vegetation, and suggests a more varied herbivorous and frugivorous diet than was favored by other, more bunoselenodont Fayum anthracotheres.

Ground Sloths Spread Into North America Multiple Times

The manus of Mylodon darwinii Owen (Tardigrada, Mylodontidae) and its phylogenetic implications

Authors:

Haro et al

Abstract:

The first nearly complete and articulated manus of the ground sloth Mylodon darwinii, from the upper Pleistocene of Argentina, is described. It shares similarities with Mylodonopsis ibseni from Brazil, including a cuneiform with a distinct pisiform facet, an obliquely concave ulnar facet, and a prominent distolateral process, as well as a gracile metacarpal III. It shares a flattened pisiform with Glossotherium robustum. The trapezoid is unique in the obliquely elongate proportions of its dorsal surface. Shapes of the articular facets indicate different functions in digits II and III, with the former having a greater range of motion and the latter greater stability at the joints. Clear arboreal or fossorial adaptations are absent. A phylogenetic analysis recovered M. darwinii as closely related to M. ibseni and agrees with larger phylogenetic analyses of sloths based on craniomandibular evidence. Our data support more than two mylodontine dispersal events to North America.

Mammals Developed Night Vision During Jurassic to Survive Dinosaurs

Early mammals evolved in a burst during the Jurassic period, adapting a nocturnal lifestyle when dinosaurs were the dominant daytime predator. How these early mammals evolved night vision to find food and survive has been a mystery, but a new study publishing June 20 in Developmental Cell suggests that rods in the mammalian eye, extremely sensitive to light, developed from color-detecting cone cells during this time to give mammals an edge in low-light conditions.

Cone cells are specialized for certain wavelengths of light to help animals detect color, while rods can detect even a single photon and are specialized for low-light vision. "The majority of mammals have rod-dominant retinas, but if you look at fish, frogs, or birds, the vast majority are cone-dominated--so the evolutionary question has always been, 'What happened?'" says Anand Swaroop, a retina biologist at the National Eye Institute, part of the National Institutes of Health. "We've been working for a long time to understand the fundamental mechanisms behind rod and cone development."

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90% of North American Mammals Estimated to Have Died out Because of the Dinosaur Killer

Over 90 per cent of mammal species were wiped out by the same asteroid that killed the dinosaurs in the Cretaceous period 66 million years ago, significantly more than previously thought.

A study by researchers at the Milner Centre for Evolution at the University of Bath and published in the Journal of Evolutionary Biology, reviewed all mammal species known from the end of the Cretaceous period in North America. Their results showed that over 93 per cent became extinct across the Cretaceous-Paleogene (K-Pg) boundary, but that they also recovered far more quickly than previously thought.

The scientists analysed the published fossil record from western North America from two million years before the Cretaceous-Paleogene boundary, until 300,000 years after the asteroid hit. They compared species diversity before and after this extinction event to estimate the severity of the event and how quickly the mammals recovered. The extinction rates were much higher than previous estimates based on more limited data sets.

Dr Nick Longrich from the Milner Centre for Evolution, in the University of Bath's Department for Biology & Biochemistry, explained: "The species that are most vulnerable to extinction are the rare ones, and because they are rare, their fossils are less likely to be found. The species that tend to survive are more common, so we tend to find them.

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Bramble Cay melomy: First Mammal Species Recognized to Have Gone Extinct due to Global Warming

Climate change is by definition a global phenomenon, but there are few locations feeling its immediate impacts like Australia's Great Barrier Reef. Just a month after local scientists were left reeling as warming seas triggered the worst coral bleaching event in its history, researchers are now reporting the reef's only endemic mammal species has become extinct as a result of rising sea levels, describing its demise as the first mammal species to be killed off by human-induced global warming.

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