Descoberta revela que o espinossauro era realmente um ‘monstro do rio’

27/09/2020

 

A descoberta de mais de mil dentes fossilizados do dinossauro espinossauro no leito de um rio pré-histórico acaba com nossa definição atual de dinossauros.

Embora já soubéssemos que o espinossauro fosse um dinossauro semiaquático, geralmente os paleontólogos consideram esse grupo extinto de répteis como exclusivamente terrestres.

Mas essa espécie definitivamente não permanecia seca.

Dinossauro nadador

A espécie Spinosaurus aegyptiacus tinha uma cauda gigante em forma de barbatana.

Alguns cientistas têm certeza de que o dinossauro espinossauro era nadador — o primeiro exemplo conhecido entre os dinossauros.

MAIS INFORMAÇÕES

Agora, centenas de dentes dessa criatura e quase metade do enorme estoque foram encontrados no Marrocos, o que convence ainda mais esses cientistas disso.

O paleobiólogo David Martill, da Universidade de Portsmouth, disse a partir dessa pesquisa que podemos confirmar este local como o lugar onde este gigantesco dinossauro viveu até morrer.

Ainda mais, os resultados são totalmente consistentes com a ideia de um verdadeiro ‘monstro do rio’ que vive na água.

Essa enorme quantidade de dentes pertence tanto aos dinossauros quanto a alguns animais aquáticos. Contando mais de 1.200 fósseis, os pesquisadores descobriram que pouco menos da metade eram do espinossauro.

Estilo de vida aquático

A grande abundância de dentes de espinossauro no leito do rio é um reflexo de seu estilo de vida aquático, em relação a outros dinossauros, como argumenta a equipe.

Porque a probabilidade de ter dentes no depósito do rio para um animal que vive grande parte de sua vida na água é maior do que para aqueles dinossauros que talvez apenas visitassem o rio para beber e se alimentar ao longo de suas margens, é o que escrevem os pesquisadores.

Em 2014, o paleontólogo Nizar Ibrahim defendeu pela primeira vez o espinossauro semiaquático.

Outros que examinaram o fóssil, na época, discordaram argumentando que o dinossauro era apenas um flutuador impelido pela água pela fome de peixes, na melhor das hipóteses.

Então, no início deste ano, Ibrahim e seus colegas encontraram uma cauda fossilizada de espinossauro.

A descoberta acrescentou muito mais peso à ideia de que esse predador gigante passou pelo menos algum tempo nadando. Tanto que a equipe declarou sua cauda a primeira “evidência inequívoca de uma estrutura propulsora aquática em um dinossauro”.

A nova descoberta em Marrocos só confirma essa ideia. O argumento é que o dinossauro espinossauro não era apenas semiaquático, mas “em grande aquático” e passou grande parte de sua vida na água.

VEJA TAMBÉM: Divulgadas novas descobertas sobre dinossauro brasileiro

Embora muitos fósseis de espinossauros tenham sido obtidos comercialmente e com origens desconhecidas, esses dentes vêm do sistema pré-histórico do rio Kem Kem, que antes fluía do Marrocos até a Argélia.

Esse era o lar de peixes-serra, crocodilos, répteis voadores e, ao longo das margens, dinossauros.

Durante o trabalho de campo de reconhecimento no sudeste do Marrocos, os cientistas descobriram um leito ósseo de arenito repleto de fósseis do Cretáceo.

Os autores concluem que, com tamanha abundância de dentes do espinossauro, é altamente provável que este animal vivesse principalmente dentro do rio, e não ao longo de suas margens, argumenta o paleontólogo Thomas Beevor da Universidade de Portsmouth.

Estudo publicado na Cretaceous Research.

 Fonte: https://socientifica.com.br/descoberta-revela-que-o-espinossauro-era-realmente-um-monstro-do-rio/?fbclid=IwAR3-daS05NxBmLr2rvlJy6RpuYrSY5YHm7tMQfk4bIKuNr4vcZBvHVBjsy4 

 

Plesiadapis: Habitat, Behavior, and Diet

•   

Name:

Plesiadapis (Greek for "almost Adapis"); pronounced PLESS-ee-ah-DAP-iss

Habitat:

Woodlands of North America and Eurasia

Historical Period:

Late Paleocene (60-55 million years ago)

Size and Weight:

About two feet long and 5 pounds

Diet:

Fruits and seeds

Distinguishing Characteristics:

Lemur-like body; rodent-like head; gnawing teeth

About Plesiadapis

Um dos primeiros primatas pré-históricos já descobertos, Plesiadapis viveu durante a época do Paleoceno, meros cinco milhões de anos ou mais após a extinção dos dinossauros - o que explica muito seu tamanho pequeno (os mamíferos do Paleoceno ainda não tinham atingido os tamanhos grandes típicos de a megafauna de mamíferos do final da Era Cenozóica). O Plesiadapis, semelhante ao lêmure, não se parecia em nada com um humano moderno, ou mesmo com os macacos posteriores dos quais os humanos evoluíram; em vez disso, esse pequeno mamífero era notável pela forma e disposição de seus dentes, que já eram semi-adequados para uma dieta onívora. Ao longo de dezenas de milhões de anos, a evolução enviaria os descendentes de Plesiadapis das árvores para as planícies abertas, onde comeriam oportunisticamente qualquer coisa que rastejasse, pulasse ou escorregasse em seu caminho, ao mesmo tempo desenvolvendo cérebros cada vez maiores.

Levou um tempo surpreendentemente longo para que os paleontólogos entendessem o Plesiadapis. Este mamífero foi descoberto na França em 1877, apenas 15 anos depois que Charles Darwin publicou seu tratado sobre a evolução, On the Origin of Species, e numa época em que a ideia de humanos evoluindo de macacos e símios era extremamente controversa. Seu nome, grego para "quase Adapis", faz referência a outro fóssil de primata descoberto cerca de 50 anos antes. Podemos agora inferir a partir da evidência fóssil que os ancestrais de Plesiadapis viveram na América do Norte, possivelmente coexistindo com dinossauros, e então gradualmente cruzaram para a Europa ocidental por meio da Groenlândia.

 

Three New Ancient Primates Identified From 46 Million Years Ago - In San Diego

 

By News Staff | August 28th 2018 11:03 AM

Three new species of fossil primates have been named, residents of San Diego County from a time when southern California was filled with lush tropical forests.

The three previously unknown omomyoid primates lived 42 million to 46 million years ago. The researchers named these new species Ekwiiyemakius walshi, Gunnelltarsius randalli and Brontomomys cerutti. 

Enamel is the hardest tissue in the body. And as a result, teeth are more likely to be preserved in the fossil record. Studying the teeth, researchers concluded the three new genera, which represent the bulk of the undescribed Friars Formation omomyoid sample at SDNHM, range in size from 113 to 796 grams and are most likely related to a group of extinct species comprising the primate subfamily Omomyinae.


Anthropology professor Chris Kirk's father and Austin-based artist Randy Kirk produced his own rendering of what the species might have looked like. Painting on marble by Randwulph. 

Ekwiiyemakius walshi, the smallest of the three new species, was estimated to weigh between 113 and 125 grams — comparable in size to some modern bushbabies. It was named for Walsh, who collected and prepared many of the specimens, and also derives from the Native American Kumeyaay tribe’s place name, Ekwiiyemak — meaning “behind the clouds” — for the location of the headwaters of the San Diego and Sweetwater Rivers.

Gunnelltarsius randalli was named for Gregg Gunnell, the researchers’ late colleague and expert on Eocene mammals, and for SDNHM fossil collections manager Kesler Randall. It was estimated to weigh between 275 and 303 grams, about the size of today’s fat-tailed dwarf lemur.

Brontomomys cerutti was large compared with most other omomyoids and was estimated to weigh between 719 and 796 grams — about the size of a living sportive lemur. Due to its large size, its name derives from the Greek word brontē, or “thunder,” as well as for Richard Cerutti, the retired SDNHM paleontologist responsible for collecting many of the Brontomomys specimens.

3. Evolução dos Primatas

Figure 3.1 Anthropoid Evolution by Keenan Taylor.

While we have no primate fossil material prior to the Eocene Epoch, the first primates are thought to have evolved prior to the Paleocene Epoch (66–56 mya), possibly as far back as 90 mya, during the Late Cretaceous Period. With the extinction of the dinosaurs at the end of the Cretaceous, many terrestrial niches became available and predation pressures were somewhat relaxed. In addition, temperatures were higher than in the recent past (see Figure 3.2) and the angiosperms (flowering plants) were undergoing an adaptive radiation, i.e. relatively rapid speciation, and spreading globally. The spread of flowering plants resulted in an adaptive radiation of insect pollinators and herbivores (plant-eaters), as well as insectivorous and herbivorous arboreal vertebrates.

Figure 3.2 Temperature change over time. “65 Myr Climate Change” by Robert A. Rohde is licensed under CC BY-SA 3.0. Notes: Pal = Paleocene, Eo = Eocene, Ol = Oligocene, Mio = Miocene, Pli = Pliocene, and Plt = Pleistocene

Figure 3.3 Primate phylogeny. “Primate phylogeny” from “Strepsirrhini” in Wikipedia is licensed CC-BY-SA

The earliest primates likely descended from a small, nocturnal, insectivorous mammal. The tree shrews and colugos (also known as flying lemurs) are the closest living relatives to primates. The tree shrew is used as a living model for what the earliest primates, or primate predecessors, might have been like. At some point, primates or their ancestors moved into the trees and adapted to an arboreal environment. Two theories regarding the evolution of some primate characteristics, such as grasping or prehensile hands, forward-oriented eyes, and depth perception, are the Arboreal and Visual Predation Theories. The Arboreal Theory posits that primate characteristics, such as grasping hands and feet and the presence of nails instead of claws, are the result of moving into and adapting to an arboreal environment. (Imagine the casualties!) The Visual Predation Theory asserts that characteristics that were well-suited to scurrying around in trees and visual features in particular, such as convergent orbits, are adaptations to insect predation. Short of a butterfly net, grasping hands, visual acuity, and depth perception are essential for catching insects, but I guess they would be kind of handy for using a butterfly net as well!

Figure 3.4 Tree shrew. “Tupaia cf javanica 050917 manc” by W. Djatmiko is licensed under CC BY-SA 3.0.

Figure 3.5 Hands and feet of apes and monkeys. “Hands and Feet of Apes and Monkeys” by Richard Lydekker is in the public domain.

While primates are thought to have evolved in Asia, the majority of the early fossil material is found in North America and Europe, dating to the Eocene Epoch (~56–34 mya). The map in Figure 3.6 indicates both living and fossil strepsirrhine sites. They are divided into two superfamilies, Adapoidea and Omomyoidea. In general, the adapoids were diurnal, lemur-like animals that are thought to be the ancestors of the strepsirrhine primates, i.e. the lemurs of Madagascar and the lorisids of Africa and Southeast Asia (i.e. bushbabies and pottos of Africa and lorises of Southeast Asia) (see Figure 3.7). The smaller, nocturnal omomyoids are good candidates for the ancestors of modern-day tarsiers. However, due to the early dates for ancestral tarsiers, it is possible that the omomyoids and tarsiers were sister lineages.

Figure 3.6 Range of living strepsirrhine primates (green) and Eocene-Miocene fossil sites (red). “Extant strepsirrhine range with fossil sites,” a derivative work by Maky, is in the public domain.

During the Eocene Epoch, the early strepsirrhine-like primates experienced an adaptive radiation and expanded into numerous niches over a broad geographic area. The northern expansion of early primates into Europe and North America was possible because Eurasia and North America were joined as the large landmass known as Laurasia and, as mentioned, it was warm enough for tropical animals to move into northern latitudes. Due to subsequent global cooling, the early primates in North America and Europe eventually went extinct. Strepsirrhine primates spread into Africa after it docked with Laurasia. They are also hypothesized to have “rafted” on floating mats of vegetation to Madagascar, where they evolved into the great diversity of extinct and extant lemur species.

Figure 3.7 Strepsirrhini. Notes: Top left: ring-tailed lemurs (Madagascar); top right: diademed sifaka (Madagascar); top middle left: aye-aye (Madagascar); top middle right: ruffed lemur (Madagascar); bottom middle left: mouse lemur (Madagascar); bottom middle right: slow loris (Asia); bottom left: slender loris (Asia); bottom right: greater bushbaby (Africa).

By at least the late Eocene, the first anthropoid primates had evolved. There is debate over the origin of the anthropoids, i.e. the ancestor of the monkeys and apes. There are four different theories of our ancestry, each with its share of supporters: (1) adapoid, (2) omomyoid, (3) tarsier, or (4) independent origin as yet undiscovered. Remains of early anthropoids dating to the late Eocene are found in Africa and Asia. A possible stem or basal anthropoid, meaning the original ancestor of all monkeys and apes, comes from the Shanghuang deposits of China. Termed genus: Eosimias (see Figure 3.8), it was as small as the smallest living anthropoid, the pygmy marmoset monkey of South America. While ring-tailed lemurs have striped tails, I do not know of any other striped primates so am not sure why the artist gave them stripes … but it sure is an intriguing little creature! Other late Eocene fossils have been discovered in Myanmar (genus: Pondaungia), Thailand (genus: Siamopithecus), Libya (genus: Biretia), Algeria, and the Fayum Beds of Egypt.

Figure 3.8 Eosimias sinesis. Illustration by Keenan Taylor.

During the Oligocene Epoch (~34–23 mya), the anthropoid primates underwent a great adaptive radiation. The richest location for Oligocene anthropoid fossils is the Fayum Beds of Egypt. Oligocene anthropoids are divided into three families: Parapithecidae, Oligopithecidae, and Propliopithecidae, from most primitive to most derived over time. The New World monkeys are thought to have branched off from the parapithecids, with which they share some characteristics. Genus: Apidium is a prime contender for a possible ancestor. Once again, a rafting hypothesis is proposed for the migration of that ancestor from Africa to South America.

The ancestors of the Old World monkeys and apes diverged from the family: Propliopithecidae. The propliopithecid, Aegyptopithecus zeuxis (also known as Propliopithecus zeuxis) is thought to be a common ancestor of the ape and Old World monkey lineages (see Figure 3.9). While the earliest anthropoids were more monkey- than ape-like, the apes (or hominoids) were the first to successfully adapt to changing environmental conditions in Africa.

Figure 3.9 Aegyptopithecus or Propliopithecus zeuxis. “Aegyptopithecus NT” by Nobu Tamura is licensed under CC BY-SA 3.0.

For years, people have asked me, “Barbara, you don’t really believe that we came from monkeys, do you?” and I always answered, “No, we came from apes!” However, our common anthropoid ancestor was more monkey- than ape-like…. So, “YEAH, I suppose I do!”

During the Miocene Epoch (~23–5.3 mya), the adaptive radiation of the apes or hominoids can be observed in the fossil record. The earliest fossils are from Kenya and Uganda. There were 20 or more genera of apes during the Miocene and they exhibited a wide range of body sizes and adaptive strategies. Proconsul is a possible stem ape, dating to ~18 mya (see Figure 3.10 and 3.11). The ancestry of the lesser apes is unclear but they are thought to have branched off 18–16 mya. The great apes diversified and spread from Africa to Asia and Europe. The ancestors of the orangutans, the sivapithecines, moved into western and subsequently eastern Asia. Remains in Turkey have been dated to 14 mya. The largest primate that ever lived, i.e. the now extinct genus: Gigantopithecus (known only from isolated dental and mandibular fragments), also had a sivapithecine ancestry. Dryopithecine apes moved into Europe during the late Miocene. Generally referred to as “dental apes,” due to the scanty remains of jaws and teeth, that evolutionary side branch eventually went extinct due to global cooling, as with the earlier strepsirrhines in the northern latitudes.

Figure 3.10 “Proconsul NT” by Nobu Tamura is licensed under CC BY-SA 3.0.

While there were Old World monkeys in the Miocene Epoch, such as genus: Victoriapithecus from Kenya, the adaptive radiation of the Old World monkeys lagged behind the hominoids. However, the same environmental conditions that drove most ape genera to extinction in Africa led to an explosion of monkey species. Monkeys could more quickly adapt due to their shorter life stages and greater number of offspring. A baboon can give birth every two years versus four or five years for gorillas and chimps, respectively. While the leaf-eating ancestor of the colobines stayed in the trees, the ancestor of the cercopithecine or cheek pouch monkeys, such as macaques and baboons, adapted to traveling on the ground as well as in the trees. The ability to exploit both arboreal and terrestrial resources expanded their niche and they survived and thrived in Africa and Asia. With only two extant genera, the African colobines did not diversify to the same extent, having been confined to forests. However, the Asian colobines did not experience the same forest loss as their African cousins did and are thus much more diverse. When African forests later expanded, the ancestors of some cercopithecine species, such as the colorful arboreal guenons, went back to the trees.

It has been difficult to trace the origin of the human/chimp/gorilla lineage during the mid-Miocene due to a paucity of fossils from that time and many conflicting viewpoints. Some of the contenders for the stem African great ape are Nakalipithecus (10 mya) and Samburupithecus (9.5 mya) from Kenya. Other possible ancestors or related species are Afropithecus (18–16 mya) and Nacholapithecus (15 mya) from Kenya and Otavipithecus (13 mya) from Namibia.

The chimp and human lineages are thought to have diverged by the late Miocene. Global cooling in the latter part of the Miocene led to the extinction of all ape genera in northern latitudes. Forest cover in Africa was vastly reduced over time due to climatic fluctuations and while most apes went extinct, the newly emerged hominins thrived. Hominins experienced an adaptive radiation during the Pliocene Epoch (~5.3–2.6 mya), and late in the Pleistocene Epoch (~2.6 mya–11.7 kya) our own species, Homo sapiens, evolved (≤200 kya).

Figure 3.11 Proconsul africanus by Keenan Taylor.

 

Published on July 31st, 2020 | by David Marshall

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Episode 114: Horseshoe Crabs

The horseshoe crabs (Xiphosura) are a group of large aquatic arthropods known from the East coast of the USA, and the Southern and Eastern coasts of Asia. Despite their name, they are not actually crabs at all, but are chelicerates (the group containing spiders and scorpions). As a group, the horseshoe crabs possess an extremely long fossil record, reaching as far back as the Ordovician Period, some 480 million years ago. Since that time, they would appear to have undergone very little change, leading the horseshoe crabs to become the archetypal ‘living fossils’.

Joining us for this two-part episode is Dr Russell Bicknell, University of New England, Australia. We discuss what makes a horseshoe crab, before taking questions from our listeners as to all aspects of horseshoe crab ecology and what we can infer from them about other extinct arthropods.

Paper: Bicknell RDC and Pates S (2020) Pictorial Atlas of Fossil and Extant Horseshoe Crabs, With Focus on Xiphosurida. Front. Earth Sci. 8:98.

Horseshoe crabs are named after the shape of their head (prosoma/cephalothorax). Their bodies are roughly divided into three functional units: the head, abdomen (opisthosoma/thoracetron) and the tail spine (telson).

Only four species of horseshoe crab exist today. Pictured are the two species of Tachypleus: T. tridentatus, the Chinese/Japanese horseshoe crab and T. gigas, the Indo-Pacific horseshoe crab.

Image: (A,B) Male T. tridentatus. (A) Dorsal view. (B) Ventral view. (C,D) Male T. gigas. (C) Dorsal view. (D) Ventral view. (E,F) Female T. tridentatus. (E) Dorsal view. (F) Ventral view. (G,H) Female T. gigas. (G) Dorsal view. (H) Ventral view.

That’s it! Those are all the horseshoe crabs, male and female, that you can see alive today.
Pictured are Limulus polyphemus, the American horseshoe crab and Carcinoscorpius rotundicauda, the mangrove horseshoe crab.

Image: (A,B) Male C. rotundicauda. (A) Dorsal view. (B) Ventral view. (C,D) Male L. polyphemus. (C) Ventral view. (D) Dorsal view. (E,F) Female C. rotundicauda. (E) Dorsal view. (F)Ventral view. (G,H) Female L. polyphemus. (G) Ventral view. (H) Dorsal view.

Whilst just four species are alive today, the evolutionary history of the horseshoe crabs reveals several periods, notably the Triassic and Carboniferous when the number of different species, their diversity, was much higher.

Some of the earliest horseshoe crabs, such as Belinurus carterae, are instantly recognisable as horseshoe crabs. This is because there isn’t a huge amount of morphological disparity (differences in shape) between the species, however important differences still exist. Here, it is easy to spot that the abdomen is composed of individual segments. In later species, these would have fused together, functionally forming one big segment we call the thoracetron today.

During the Carboniferous period, a relatively large number of horseshoe crab species are known. The reason for this might be due to horseshoe crabs experimenting with more freshwater environments and living in places where they might stand a better chance of being preserved. It may be due to the fact more Carboniferous rocks were excavated in the search of coal, or it might simply be an actual increase in the number of species alive at that time.

Image: Examples of the Carboniferous horseshoe crab Euproops danae.

Another period during which there was incrased diversity and disparity was the Triassic. Here wonderful forms such as the pick axe-shaped Austrolimulus fletcheri existed.

Image: Austrolimulids from Australia. (A) Austrolimulus fletcheri from the Triassic-aged Beacon Hill Shale, NSW, Australia. (B) Tasmaniolimulus patersoni from the Permian-aged Jackey Shale, Tasmania, Australia. (C) Dubbolimulus peetae from the Triassic-aged Ballimore Formation, NSW, Australia.

Reconstruction of Tasmaniolimulus patersoni.

The Triassic is also host to the famous Vaderlimulus tricki whose pointed head resembles the helmet of Darth Vader. Image: Joschua Knüppe.

Even by the Triassic, species belong to the modern genera Tachypleus (A) and Limulus (E) were known. That is a duration of over 200 million years for these genera!

In the Jurassic, fantastic specimens can be found in the Solnhofen Limestone. These fossils are so well-preserved that they are often shown side by side with modern horseshoe crabs to illustrate how little they have evolved over time, or even to try and disprove evolution.
Examples of the iconic Jurassic-aged Mesolimulus walchi from Germany

It’s true that horseshoe crabs have changed very little in superficial shape over 480 million years and they can still be found on beaches, probably living their lives not too dissimilar to how they have been doing for almost half a billion years!

Throughout their evolutionary history they have survived nearly every mass extinction event, however humans pose perhaps their greatest threat. Horseshoe crabs gathering on the beaches represent a fantastic source of free fertiliser.

Unfortunately, the blood of the horseshoe crab is a vital component of the pharmaceutical industry and live individuals are captured and their blood harvested. This is obviously a traumatic procedure and is having a negative effect on horseshoe crab populations.

Dr Russell Bicknell with a horseshoe crab.

  

Published on September 16th, 2015 | by Liz Martin-Silverstone

História de crescimento inicial de primatas

A evolução dos primatas é algo muito debatido e não muito bem compreendido na paleontologia, mas ainda é muito estudado. Em 2009, um incrível fóssil de primata foi encontrado em Messel, datando de aproximadamente 47 milhões de anos atrás, e foi denominado Darwinius masillae. Apenas um único fóssil de Darwinius é conhecido, e é pequeno, completo e muito bem preservado. A descrição inicial anunciou-o como uma espécie de "elo perdido" na evolução dos primatas, uma forma de transição no ramo para primatas antropoides, incluindo humanos. No entanto, estudos desde então discordaram da classificação original, colocando-a em outros ramos da árvore dos primatas.

Um novo estudo que analisa a história de crescimento de Darwinius revelou novos detalhes sobre o padrão de erupção dos dentes e a posição do fóssil nos primatas. O autor principal do estudo, Sergi López-Torres, é um aluno de doutorado da Universidade de Toronto e nos deu uma breve descrição do estudo:

“Os adapoides eram primatas arbóreos de médio porte, espalhados por toda a Europa, Ásia, África e América do Norte, variando no tempo entre 55,8 e 9 milhões de anos atrás. Pertencem a uma radiação de primatas bastante bem-sucedida, formando 6 famílias e mais de 100 espécies. As relações evolutivas dos adapoides com grupos modernos de primatas têm recebido considerável atenção na literatura científica há mais de um século. Jacob Wortman sugeriu pela primeira vez em 1904 que os adapoides estavam intimamente relacionados aos antropoides (o grupo que inclui macacos, macacos e humanos), agora conhecido como a hipótese antropóide-adapóide. Em 1920, William Gregory propôs ao contrário que os primatas adapóides eram mais intimamente relacionados aos estrepsirrinos (o grupo que inclui lêmures e lóris), conhecido como a hipótese de estrepsirrina adapóide.

Embora a hipótese do Adapoid-Strepsirrhine tenha sido mais amplamente aceita nas últimas duas décadas, a descrição do adapóide juvenil Darwinius masillae (apelidado de “Ida”) em 2009 reacendeu a controvérsia. Seus descritores sugeriram uma relação mais próxima com os haplorrinos (o grupo que inclui társios e antropoides) e, mais tarde, especificamente com os antropóides.

Um tópico menos amplamente discutido é o modelo de crescimento usado para prever a idade de Ida ao morrer e sua massa corporal adulta final. Até agora, o único modelo proposto de crescimento e desenvolvimento para Darwinius era baseado em um primata antropoide vivo, o macaco-esquilo (Saimiri sciureus), um modelo que está de acordo com a hipótese antropoide-adapoide. No entanto, as descobertas recentes sugerem que a sequência de erupção dentária (ou seja, a ordem em que os dentes aparecem) de Darwinius compartilha semelhanças com três principais ancestrais primatas (o ancestral estrepsirrino, o ancestral haplorrino e o ancestral de todos os primatas), mas mostra uma grande diferença do ancestral antropoide. Os antropóides irrompem o terceiro molar muito tarde na sequência, e o Darwinius não, uma característica observada em lêmures. Isso levou à proposta de um novo modelo alternativo baseado em lêmures (Eulemur e Varecia). O novo modelo sugere uma idade maior no momento da morte para “Ida” (1,05-1,14 anos) e um peso adulto reconstruído menor (622-642g).

Embora os dados da sequência de erupção não possam refutar a hipótese do adapoide-antropóide, eles são menos consistentes com essa ideia do que com a hipótese do adapoide-estrepsirrina. Com relação a futuras descobertas, este novo modelo fornece uma abordagem alternativa para estimar os parâmetros da história de vida mais em conformidade com a visão consensual das relações adapóides. ”

O artigo é de acesso gratuito, publicado na Royal Society Open Science, então dê uma olhada!

Legenda da imagem: Fóssil de Darwinius masillae (imagem de Franzen et al. 2009).

Radiograph of the right side of skull of Darwinius masillae showing the deciduous (indicated with a ‘d’) and permanent teeth. Image from López-Torres et al. 2015

 

Tags: Eocene, Evolution, mammals, News, primate

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Primate Origins and the Plesiadapiforms

By: Mary T. Silcox Ph. D (Associate Professor, University of Toronto Scarborough) © 2014 Nature Education 

Citation: Slicox, M. T. (2014) Primate Origins and the Plesiadapiforms. Nature Education Knowledge 5(3):1

Sixty-five million years ago, as the last of the non-avian dinosaurs were going extinct, our earliest recognized ancestors appeared in the fossil record. When, where, and why did they evolve?

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What is a primate?

The first problem that one inevitably faces in thinking about primate origins is where to draw the line between primates and other types of mammals. If one considers only living primates (i.e., monkeys, apes, lemurs, lorises, and humans), then the Order Primates is not that difficult to define. Living primates are unusual in relying very heavily on vision — this is reflected in the skull by orbits that are close together (convergent) and bounded by a postorbital bar (Figure 1a). Primates also typically have hands and feet that are well designed for grasping (with long fingers, opposable thumbs and big toes, and nails rather than claws on most digits), and other skeletal traits that are beneficial for moving around in the trees. Finally, living primates eat a lot of fruit, and members of the order generally have features of the teeth that reflect this, including fat, round (bunodont) cusps and broad talonid basins, for squishing fruit in a mortar-and-pestle type arrangement.

Figure 1: Crania of a euprimate (a,b Lemur catta, FMNH 23977) and a plesiadapiform (c,d Ignacius graybullianus, USNM 421608) in rostral (a,c) and dorsal (b,d) views.

Note the presence of the postorbital bar (red asterisk) and convergent orbits in the euprimate, missing in the plesiadapiform. Image of Ignacius graybullianus is based on a 3D reconstruction using high resolution X-ray computed tomography data (see Silcox, 2003). Scales = 1cm.

© 2013 Nature Education (a,b) Photos courtesy of Zachary Randall. All rights reserved.

Species in the fossil record that exhibit all these features will be easy to recognize as primates. However, at some point primates must have evolved from ancestors that lacked these characteristics. And unless all of these features evolved in an evolutionary instant (which seems unlikely), there must have been some species more closely related to living primates than to any other mammalian group that nonetheless lacked some of these traits. Recognizing such forms as being related to primates is a much more difficult problem, but is critical to understanding where, when, and why primates branched off from the rest of Mammalia.

The group of fossils that is the focus of debates over primate origins is called the plesiadapiforms. This is an incredibly diverse group including more than 140 named species arranged into 11 different families. The first record of plesiadapiforms appears just as the non-avian dinosaurs were going extinct about 65 million years ago, near the beginning of the Paleocene. Some plesiadapiforms persist well into the Eocene, with the last species going extinct around 37 million years ago (Silcox & Gunnell, 2008). Over the course of its more than 25-million-year history, the group underwent an impressive adaptive radiation, producing forms with very distinctive dental adaptations including weird, multi-cusped upper incisors, and a diversity of very specialized lower premolars (Figure 2).

Figure 2: Scanning electron micrographs of plesiadapiform P4s and M1s in occlusal view: a) Purgatorius janisae (UCMP 107406, image reversed); b) Elphidotarsius florencae (USNM 9411); c) Tinimomys graybulliensis (YPM 33895); d) Picrodus silberlingi (AMNH 35456).

Note the wide range of different P₄ morphologies, reflecting the adaptive diversity of plesiadapiforms. The white asterisk indicates the broad talonid basin in Purgatorius. Scales = 1 mm.

© 2013 Nature Education All rights reserved.

Plesiadapiforms share some traits with living primates, including long fingers well designed for grasping, and other features of the skeleton that are related to arboreality (Bloch & Boyer, 2002). One species, Carpolestes simpsoni, even had a divergent big toe with a nail (Bloch & Boyer, 2002). Dentally, plesiadapiforms look quite similar to definitive primates, with broad talonid basins and a similar pattern of cusps and crests. However, no known plesiadapiform exhibits the features of living primates associated with specialized vision, such as the postorbital bar or convergent orbits. Even though they exhibit some features for primate-like grasping, known plesiadapiforms still retain claws on most of their digits. These contrasts have led some workers (e.g., Martin, 1968; Cartmill, 1974) to suggest that plesiadapiforms do not belong in the Order Primates. Others (e.g., Kay et al., 1990; Beard, 1990) have further argued that plesiadapiforms are not most closely related to primates, but instead are closer kin to another order, Dermoptera.

The most comprehensive analysis to date of the relationships among plesiadapiforms, primates, and closely related mammals is by Bloch and colleagues (2007; Figure 3). The results of this analysis support the idea that plesiadapiforms are more closely related to primates than to any other group. These authors argued that plesiadapiforms should therefore be considered stem primates; they adopted the name Euprimates (Hofstetter, 1977) for living primates and any fossil forms that exhibit all of the features of modern primates listed above. It is worth noting, however, that not all researchers are convinced by Bloch et al.'s (2007) results. These workers generally equate the Order Primates with Euprimates, excluding plesiadapiforms from the order. This leads to a very different conception of what constitutes primate origins than discussed here (e.g., Soligo & Martin, 2007), with the order not appearing until nearly 10 million years later, when the first forms that share traits such as the postorbital bar and nails on most digits (e.g., adapoids and omomyoids) are first found in the fossil record (e.g., Rose et al., 2012).

The broader relationships of Primates in Mammalia

Although there are continuing disagreements about where to draw the primate/non-primate line in the fossil record, more consensus exists about the identity of the closest living relatives of primates. Molecular analyses of mammalian relationships have fairly consistently placed primates in a group called Euarchonta with two other living orders: Scandentia and Dermoptera (Springer et al., 2004). Scandentians are small, quadrupedal animals that live in Southeast Asia. They are more commonly referred to as treeshrews, although not all of them live in the trees, and taxonomically they are not shrews. From 1922 (Carlsson, 1922) until 1980, treeshrews were often classified as primates. This idea was effectively refuted in a landmark edited volume (Luckett, 1980) that assessed the primate hypothesis using multiple lines of evidence. Even though trees shrews are no longer considered to be primates, familiarity with them is very important for researchers interested in primate origins because they provide the best living models for the earliest members of the Order. Indeed, the most primitive plesiadapiform known from a reasonably complete skeleton (Dryomomys szalayi) closely resembles the pen-tailed treeshrew, Ptilocercus lowii (Figure 4), below the head (Bloch et al., 2007). These similarities suggest that the first primate probably looked quite a bit like Ptilocercus, and supports the idea (Szalay & Drawhorn, 1980) that primates evolved from an already arboreal ancestor.

Figure 4: Ptilocercus lowii (the pen-tailed treeshrew).

Ptilocercus is the most primitive living treeshrew (Sargis, 2004) and represents the best extant model for the common ancestor of all Primates (including plesiadapiforms).

© 2013 Public Domain Zoological Society of London, 1848 Some rights reserved.

Part of what makes treeshrews so important to modeling early primates is that the other living group generally agreed to be closely related to primates, Dermoptera, is so profoundly weird. Dermopterans are gliding mammals also known from Southeast Asia. They are often referred to as flying lemurs (though they do not fly and taxonomically are not lemurs), but a better name for them is mitten gliders, because dermopterans have a very extensive gliding membrane (patagium) that extends between their fingers, creating the appearance of wearing mittens. In addition to numerous postcranial specializations for gliding, dermopterans also exhibit peculiar features of the teeth, including incisors that literally have tines like a comb (Figure 5). Unfortunately, neither Scandentia nor Dermoptera is known from very complete fossil material (see summary in Silcox et al., 2005), although there is one extinct family from the Paleocene and Eocene of North America (Plagiomenidae) which may be related to dermopterans (Bloch et al., 2007; but see MacPhee et al., 1989).

Figure 5: Labial view of the mandible of a dermopteran, Galeopterus variegatus (AMNH/Mammalogy: 106628).

Note the distinctive, comb-like morphology of the first two lower incisors. Scale = 2 mm.

© 2013 Nature Education Photo courtesy of Zachary Randall. All rights reserved.

Although most researchers agree that primates are closely related to dermopterans and scandentians, there are several other issues of debate with respect to the broader relationships of primates in Mammalia. Studies disagree on which of the two orders is primates' sister taxon. Three possibilities have been supported in different analyses: Dermoptera (e.g., Janečka et al., 2007), Scandentia (e.g., Novacek, 1992), or Sundatheria (which is Dermoptera + Scandentia; e.g., Bloch et al., 2007). Also in debate is the relationship of bats to primates—morphological studies generally position them in a group with euarchontans (Archonta; e.g., Novacek, 1992) while molecular results have consistently found them to be much more distantly related (Springer et al., 2004). Finally, molecular results generally place Euarchonta in a group with rodents and rabbits (Euarchontoglires; Springer et al., 2004) whereas this result is not usually found in morphological studies (but see Silcox et al., 2010).

Where, when, and why did primates evolve?

The earliest known plesiadapiforms are placed in the genus Purgatorius. Although there is a fragment of a tooth for this genus that has been described as coming from the same deposit as a Triceratops skeleton (Van Valen & Sloan, 1965), this deposit includes a mixture of material of different ages (Clemens, 2004). More firmly dated specimens are known from the earliest part of the Paleocene, however, so it is clear that even if Purgatorius did not overlap in time with the non-avian dinosaurs, it was one of the first groups to exploit the new opportunities created by their extinction (Johnston & Fox, 1984; Clemens, 2004).

Although the age and primitive morphology of Purgatorius would suggest an origin of primates around 65 million years ago, molecular dates for the origin of the order are generally at least 10 million years earlier, well before the Cretaceous-Tertiary boundary (Springer et al., 2003). If this is the case, we may be missing or not recognizing a significant part of the early history of the order. Alternatively, this discrepancy could reflect problems with the assumptions underlying molecular clock models, such as constancy of evolutionary rate (Ho, 2008), and the difficulty of calibrating the clock (e.g., see discussion in Stauffer et al., 2001).

While the Southeast Asian location of the closest living relatives of primates might suggest an Asian origin for the order (Beard, 2004), the North American location of most primitive plesiadapiforms supports a North American origin instead (Bloch et al., 2007). This may be a product, however, of much greater sampling of the fossil record in North America. Indeed, there is now a relatively primitive plesiadapiform known from Asia (Asioplesiadapis youngi Fu et al., 2002; see discussion in Silcox, 2008), which suggests that further discoveries on that continent may make it seem a more plausible place of origin for Primates.

In terms of why primates evolved, most of the scenarios for primate origins that discuss plesiadapiforms (rather than those restricted to Euprimates) suggest a relationship to diet. Szalay, in his landmark 1968 paper, "The beginnings of Primates," wrote: "It is only an increasing occupation of feeding on fruits, leaves, and other herbaceous matter that explains the first radiation of primates" (p. 32), and Sussman and Raven (1978) included plesiadapiforms in their proposed co-evolutionary scenario between primates and flowering plants. Eriksson et al. (2000) document an increase in the volume of fruit and proportion of animal-dispersed flowering plants through the late Cretaceous into the Paleocene; early primates may have been one of the forces driving this change. These suggestions may seem surprising in light of the small, rather pointy teeth of Purgatorius (Figure 2a). Certainly in comparisons with modern primates this morphology would suggest a diet richer in insects than in plant materials (Kay & Cartmill, 1977). However, it is important to note that the features we use to differentiate Purgatorius and the other early plesiadapiforms from contemporaneous small mammals (e.g., broad talonid basins) are precisely those which can be related to a more diverse diet. So while we still have much to learn about the ecological profile of the earliest primates, a shift to a more omnivorous diet may have been one part of their success. A diffuse co-evolutionary relationship with flowering plants may have subsequently played a critical role in shaping the diversification of plesiadapiforms throughout the Paleocene (Sussman, 1991; Bloch et al., 2007).

References and Recommended Reading

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