泰坦鸟属

泰坦鸟（Titanis，命名自古希腊神话中的神明泰坦）又称泰坦巨鸟、泰坦恐鹤、泰坦窃鹤，是一种生存于上新世至早更新世北美的大型不飞鸟，属于叫鹤目下的恐鹤科。
泰坦鸟的化石最早由业余考古学家本杰明·沃勒（Benjamin Waller）和罗伯特·艾伦（Robert Allen）在佛罗里达州圣菲河发现，1963年，鸟类学家皮尔斯·布罗德科布（Pierce Brodkorb）将其命名为沃氏泰坦鸟（Titanis walleri），以表彰其发现者沃勒。泰坦鸟的正模标本包含一段不完整的跗跖骨和一节趾骨，该个体是恐鹤科下最大的个体之一。在正模标本描述后的数年内，佛罗里达的其他地区、德克萨斯和加利福尼亚也陆续发现了其他零碎的泰坦鸟标本。泰坦鸟传统上被归入恐鹤亚科，该亚科包括一些最晚、最大的恐鹤，例如卡林肯恐鹤和德文森恐鹤。
作为恐鹤科的一员，泰坦鸟有著较长的后肢、较窄的骨盆、较小的翅膀和巨大的、生有钩状喙的头骨。泰坦鸟是最大的恐鹤之一，体型可能与曲带恐鹤相近，一项2005年的研究估算泰坦鸟身高可达2-2.5米、体重可达200公斤，而近期的研究则认为其身高为1.4-2米，体重为150公斤。虽然泰坦鸟的骨骼标本较为残破，研究人员对其解剖学细节知之甚少，但仍能观察到其跗跖存在显著特征。根据已有标本，泰坦鸟的头骨长度可能在32至54厘米之间。
恐鹤常被视为地栖捕食者或食腐动物，在有胎盘类较少的南美洲担任顶级捕食者。与其他恐鹤不同的是，泰坦鸟是一种生活在北美的顶级捕食者，与有胎盘类共享着栖息地。泰坦鸟像近亲卡林肯恐鹤一样有着较长的跗跖，使其可以快速奔跑；根据对安达尔加拉恐鹤的研究，大型恐鹤有着坚硬且抗压的头骨，使其可以整个吞下较小的猎物，也能通过反复凿击杀死体型较大的猎物。泰坦鸟生活在上新世佛罗里达州、加利福尼亚州南部和德克萨斯州东南的开阔稀树草原上，以捕食哺乳动物为生。泰坦鸟的潜在猎物包括雕齿兽类、马类、貘类和水豚。泰坦鸟是当前已知的唯一分布于北美的恐鹤，它们或许在中美洲陆桥完全形成前就已经抵达了北美洲。
发现.
泰坦鸟最早发现于1961/1962年的冬天，当时业余考古学家本杰明·沃勒和罗伯特·艾伦正在美国佛罗里达州吉尔克里斯特县和哥伦比亚县交界处的圣菲河中使用潜水装备寻找文物与化石。两位收藏家随后将他们的发现捐赠给了佛罗里达自然历史博物馆（UF），这些发现包括马类、象类以及很多其他生活在晚上新世和晚更新世佛罗里达的物种化石。沃勒与艾伦发现的鸟类化石仅包括一块跗跖骨的远端和一块趾骨，这两块标本的编号分别为UF 4108和UF 4109。1962年前，这两块标本一直没有经过分析，直到这一年古生物学家克莱顿·雷（Clayton Ray）分析了这组标本，他意识到这两块鸟骨相当巨大，或许应当属于某种恐鹤。雷还指出了它们所属的年代：这两块标本发现自含有矮三趾马和恐犬的沉积层，表明它们应当生存于晚布兰卡期（220万年–180万年前）。
雷将圣菲河化石赠予博物馆的鸟类学家皮尔斯·布罗德科布（Pierce Brodkorb）研究，他错误地认为这些化石来自Rancholabrean时期，并认为这只鸟是生活在南美洲的美洲鸵的近亲。尽管雷试图说服布罗德科布这只鸟属于一种恐鹤，但布罗德科布还是在其手稿中将其归类为了美洲鸵的近亲。布罗德科布于1963年发表了对该标本的描述，他基于该标本命名了新的属种"Titanis walleri"（译为：沃氏泰坦鸟）。属名"Titanis"指的是希腊的泰坦巨人，意指这种鸟巨大的体型； 种加词"walleri"则是为了表彰该模式标本的发现者沃勒。如雷所建议的一样，布罗德科布将泰坦鸟属作为曲带恐鹤属和德文森恐鹤属的近亲放在了恐鹤亚科之下。这是南美洲之外有关恐鹤类鸟类的首次描述。
佛罗里达的发现.
佛罗里达发现有五处泰坦鸟化石点：锡特勒斯县圣菲河的1a点、1b点和英格利斯的1b的，夏洛特县的夏洛特港以及萨拉索塔县的一个贝壳坑。佛罗里达州的40个泰坦鸟标本中，有27个发掘自圣菲河，其中有许多标本是在20世纪60年代和70年代根据布罗德科布的描述收集的。圣菲河的标本产自河流内的两个地点，1a和1b。前者产出的骨骼更加丰富，包括泰坦鸟的脊椎骨、肢骨甚至部分头骨。化石点英格利斯1b最初是上新世时期的一处地陷，20世纪60年代，联邦政府修建佛罗里达驳船运河（Cross Florida Barge Canal）时，这一黏土沉积层得到了挖掘。佛罗里达大学的一对研究生于1967年首次发现了粘土中的化石，这激起了由佛罗里达自然历史博物馆馆长大卫·韦伯（David Webb）发动的大规模挖掘的热潮。发掘工作从1967年持续到了1973年，在此期间研究人员收集了超过一万八千件化石标本。尽管化石数量众多，只有12件标本属于泰坦鸟，包括包括颈椎、一件腕掌骨和一件跖骨。夏洛特港仅发现了一枚化石，即第四脚趾的部分趾骨，该化石于1990年捐赠给了佛罗里达大学。据报道，萨拉索塔县的一个贝壳坑中发现了一块泰坦鸟的跗跖骨残片，这是该物种已发现的唯一一块非正模跗跖骨标本。
德克萨斯和加利福尼亚的发现.
泰坦鸟化石的更晚的发现来自1995年，研究人员描述了一块发现自德克萨斯州圣帕特里西奥县努埃塞斯河附近的奥德姆的沙砾坑中的单一脚趾骨，这是首次在佛罗里达之外的地区发现泰坦鸟的化石。坑洞中的化石杂乱无章，早上新世和晚更新世的化石混合在一起，描述文献沿用了布罗德科布错误的年代评估，认为该泰坦鸟化石产自晚更新世。对化石中稀土元素的进一步分析表明，德克萨斯州的泰坦鸟化石产自约500万年前的上新世赫姆菲尔期（Hemphillian stage），此时巴拿马地峡尚未形成，远远早于220-180万年前的佛罗里达州泰坦鸟化石。
1961年，G·戴维森·伍德沃德（G. Davidson Woodward）在加利福尼亚州安沙波利哥沙漠州立公园上新世（距今370万年）的奥拉组（Olla Formation）地层中收集化石时，发现了几块鸟类化石，其中有一块来自一只大鸟的与前颌骨相连的翼骨化石。这块翼骨最初被认为属于一种名为风神鹫（学名："Aiolornis"）的异鹫，这一评估于1972年得到了鸟类学家希尔德加德·霍华德（Hildegarde Howard）的支持，随后的研究也支持了这一观点。但在2013年，古生物学家罗伯特·钱德勒（Robert Chandler）及其同事凭借其包含的恐鹤特征和化石年代，将该标本归入了泰坦鸟。此块前颌骨标本可能来自370万年前，这是除德克萨斯标本外的泰坦鸟化石中最古老的一块。
分类.
新生代早期，哺乳动物得益于非鸟恐龙的灭绝开始了辐射演化，作为恐龙仅存的后裔，鸟类也开始了辐射演化，包括加斯东鸟类、驰鸟类、古颚类和恐鹤类在内的多种鸟类开始了大型化的进程。恐鹤类是叫鹤目下的已灭绝类群，这一类群仅有两种叫鹤延续到了今天。恐鹤是叫鹤目下物种多样性最高的化石类群，但是由于大部分恐鹤的化石完整性欠佳，研究人员很难明晰不同恐鹤之间的亲缘关系。除恐鹤之外，叫鹤目的掠食者谱系还包括高鸟类，这是一类生活在始新世至中新世北美洲的地栖鸟类，占据着类似叫鹤的生态位。
最古老的恐鹤化石来自古新世的南美，这一类群一直延续到了更新世，并最终通过泰坦鸟这一物种扩散到了北美洲，除此之外在欧洲和非洲也有发现可能的恐鹤科鸟类。 叫鹤目和恐鹤的起源仍然是个迷，它们可能起源于南美洲，也可能是通过陆桥或跳岛从其他地方迁徙而来。 恐鹤科鸟类一直存活到了更新世，因此它们似乎比在上新世灭绝的南美掠食性有袋类袋剑虎更成功，恐鹤有可能在更新世时与从北美迁入的真兽类掠食者在生态上进行了竞争。 泰坦鸟与许多掠食性哺乳动物共存，包括刃齿虎、北美猎豹、恐狼和短面熊。 所有这些物种，包括最后的恐鹤在内全部灭绝于第四纪灭绝事件中。
几十年来，恐鹤科的系统发育关系一直难以确定，研究人员命名了大量分类单元来归纳不同的恐鹤，进入21世纪，恐鹤的系统关系得到了更多的分析。泰坦鸟曾被广泛归入“恐鹤亚科”（Phorusrhacinae），该亚科除泰坦鸟之外还包括曲带恐鹤、卡林肯恐鹤和德文森恐鹤。 巴西古生物学家赫尔库拉诺·阿尔瓦伦加（Herculano Alvarenga）及其同事在2011年发布的有关恐鹤科的系统发育分析中没有将恐鹤亚科、雷鸣鸟亚科和巴塔哥恐鹤亚科区分开来，使泰坦鸟成为了多分支之一（见支序树一）。 2015年，在壮丽鸟的描述论文中，阿根廷古生物学家费德里科·J·德格兰格（Federico J. Degrange）及其同事对恐鹤科进行了系统分析，他们发现“恐鹤亚科”并不是自然分类群，而是个多系群（见支序树二）
支序树一： Alvarenga "et al." (2011) 
蓝色示“巴塔哥恐鹤亚科”、红色示“恐鹤亚科”、黄色示“雷鸣鸟亚科”或“鸣恐鹤亚科”
支序树二： Degrange "et al." (2015) 
蓝色示“巴塔哥恐鹤亚科”、红色示“恐鹤亚科”、黄色示“雷鸣鸟亚科”或“鸣恐鹤亚科”
描述.
恐鹤是一类不会飞的大型鸟类，它们后肢长，骨盆窄，翅膀小得不成比例。它们有拉长的头骨，头骨的末端是细长的钩状喙。总的来说，考虑到亲缘关系，泰坦鸟可能与南美洲的曲带恐鹤和德文森恐鹤非常相似。人们对泰坦鸟的身体结构知之甚少，但通过分析已知的标本，我们可以得知泰坦鸟的脚似乎没有德文森恐鹤那么宽，且其中间的脚趾按比例来说要粗壮得多。 泰坦鸟的体型数据经历了多次计算。早期的复原认为泰坦鸟身高，但基于新材料的新复原则认为泰坦鸟只有高，重。泰坦鸟得以成为人类已知的最大的恐鹤之一，只有其近亲德文森恐鹤、卡林肯恐鹤以及部分古颚类和加斯东鸟类可能比泰坦鸟还巨大。
Postcranial skeleton.
As for the postcranial anatomy, "Titanis" and other phorusrhacines were heavily built. They all preserve an elongated, thin tarsometatarsus that was at least 60% the length of the tibiotarsus. "Titanis" is distinguished from other phorusrhacines by the anatomy of its tarsometatarsus; the distal end of the mid-trochlea is spread out onto its sides and its slenderness compared to related genera of the same size. The pes is large and had three digits, the third of which had an enlarged ungual akin to that of dromaeosaurid dinosaurs. The spinal column is poorly known from "Titanis", though several vertebrae have been collected. The cervical vertebrae are elongated anteroposteriorly and somewhat flexible, whereas the dorsal, sacral, and caudal vertebrae were more boxy and rigid. The dorsal vertebrae have tall neural spines atop the centra. The dorsal ribs connected to the sacral ribs, creating a basketed underbelly.
The wings are small and could not have been used for flight, but were much more strongly built than those of living ratites. It also had a relatively rigid wrist, which would not have allowed the hand to fold back against the arm to the same degree as other birds. This led R. M. Chandler to suggest in a 1994 paper that the wings may have supported some type of clawed, mobile hand similar to the hands of non-avian theropod dinosaurs, such as the dromaeosaurs. It was later pointed out by Gould and Quitmyer in a 2005 study that demonstrated that this wing joint is not unique and is present in seriemas, which do not have specialized grasping hands. The wing bones articulated in an unusual joint-like structure, suggesting the digits could flex to some degree. Evidence of elongated quill-feathers are known from "Patagornis" and "Llallawavis", with large tubercles called quill knobs present on their ulnae. These quill knobs would have supported long flight feathers.
古生物学.
Little is known about the paleobiology of "Titanis" due to a scarcity of fossil remains. Many of its habits are inferred based on related taxa like "Kelenken" and "Andalgalornis." Features such as the pointed premaxillary beak tip and recurved pedal unguals are direct evidence of its carnivorous lifestyle.
食性.
Phorusrhacids are thought to have been terrestrial predators or scavengers, and have often been considered apex predators that dominated Cenozoic South America in the absence of placental mammalian predators. They co-existed with some large, carnivorous borhyaenid mammals for much of their existence. Earlier hypotheses of phorusrhacid feeding ecology were mainly inferred from their large skulls with hooked beaks rather than through detailed hypotheses and biomechanical studies. Detailed analyses of their running and predatory adaptations were only conducted from the beginning of the 21st century through the use of computer technology.
Alvarenga and Elizabeth Höfling made some general remarks about phorusrhacid habits in a 2003 article. They were flightless, as evidenced by the proportional size of their wings and body mass, and the wing-size was more reduced in larger members of the group. These researchers pointed out that the narrowing of the pelvis, upper maxilla, and thorax could have been adaptations to enable the birds to search for and take smaller animals in tall plant growth or broken terrain. The large expansions above the eyes formed by the lacrimal bones (similar to what is seen in modern hawks) would have protected the eyes against the sun, and enabled keen eyesight, which indicates they hunted by sight in open, sunlit areas, and not shaded forests.
腿部功能.
In 2005, Rudemar Ernesto Blanco and Washington W. Jones examined the strength of the tibiotarsus (shin bone) of phorusrhacids to determine their speed, but conceded that such estimates can be unreliable even for extant animals. The tibiotarsal strength of "Patagornis" and an indeterminate large phorusrhacine suggested a speed of , and that of "Mesembriornis" suggested ; the latter is greater than that of a modern ostrich, approaching that of a cheetah, . They found these estimates unlikely due to the large body size of these birds, and instead suggested the strength could have been used to break the long-bones of medium-sized mammals, the size for example of a saiga or Thomson's gazelle. This strength could be used for accessing the marrow inside the bones, or by using the legs as kicking weapons (like some modern ground birds do), consistent with the large, curved, and sideways compressed claws known in some phorusrhacids. They also suggested future studies could examine whether they could have used their beaks and claws against well-armored mammals such as armadillos and glyptodonts. In a 2006 news article, Luis Chiappe, an Argentine paleontologist, stated that "Kelenken", a similar genus to "Titanis", would have been as quick as a greyhound, and that while there were other large predators in South America at the time, they were limited in numbers and not as fast and agile as the phorusrhacids, and the many grazing mammals would have provided ample prey. Chiappe remarked that phorusrhacids crudely resembled earlier predatory dinosaurs like "Tyrannosaurus", in having gigantic heads, very small forelimbs, and very long legs, and thereby similar carnivore adaptations.
头颈部功能.
A 2010 study by Degrange and colleagues of the medium-sized phorusrhacid "Andalgalornis", based on Finite Element Analysis using CT scans, estimated its bite force and stress distribution in its skull. They found its bite force to be 133 Newtons at the bill tip, and showed it had lost a large degree of intracranial immobility (mobility of skull bones in relation to each other), as was also the case for other large phorusrhacids such as "Titanis". These researchers interpreted this loss as an adaptation for enhanced rigidity of the skull; compared to the modern red-legged seriema and white-tailed eagle, the skull of the phorusrhacid showed relatively high stress under sideways loadings, but low stress where force was applied up and down, and in simulations of "pullback" where the head returned to its normal position. Due to the relative weakness of the skull at the sides and midline, these researchers considered it unlikely that "Andalgalornis" engaged in potentially risky behavior that involved using its beak to subdue large, struggling prey. Instead, they suggested that it either fed on smaller prey that could be killed and consumed more safely by swallowing it whole. Degrange "et al." also postulated that it used a series of well-targeted repetitive strikes with the beak in an "attack-and-retreat" strategy. Struggling prey could also have been restrained with the feet, despite the lack of sharp talons.
A 2012 follow-up study by Claudia Tambussi and colleagues analyzed the flexibility of the neck of "Andalgalornis" based on the morphology of its neck vertebrae, finding the neck to be divided into three sections. By manually manipulating the vertebrae, they concluded that the neck musculature and skeleton of "Andalgalornis" were adapted to carrying a large head and for raising the head after the neck had been fully extended. The researchers assumed same would be true for other large, big-headed phorusrhacids. A 2020 study of phorusrhacid skull morphology by Degrange found that there were two main morphotypes within the group, derived from a seriema-like ancestor. These were the "Psilopterine Skull Type", which was plesiomorphic (more similar to the ancestral type), and the "Terror Bird Skull Type", which included "Titanis" and other large members, that was more specialized, with more rigid skulls. Despite the differences, studies have shown the two types handled prey similarly; the more rigid skulls and resulting larger bite force of the "Terror Bird" type would have been an adaptation to handling larger prey.
古环境学.
During the Blancan stage, "Titanis" lived alongside both endemic mammals as well as new immigrants from Asia and South America. Because of this, the fauna of the Blancan starkly contrasted with the fauna of the Pleistocene and Holocene. Localities in which "Titanis" is known are all tropical or subtropical in climate, with dense forests and a variety of flora. In Inglis 1a specifically, longleaf pine flatwoods and pine-oak scrub are known to have occupied the area, similar to the modern flora. During the Pliocene-Miocene climatic transition, the climate was cooler but temperatures did not reach those of the Pleistocene, creating a warm period. Sea levels were higher, but this was reversed at the end of the Pliocene during the beginning of large glaciations that fostered the Pleistocene's "Ice Age".
The Blancan age strata of Florida from "Titanis" sites preserve over a hundred species and many different mammals. This includes extinct proboscideans and perissodactyls represented by grazing equids and browsing tapirs"." A wide array of artiodactyls existed, including peccaries, camelids, pronghorns, and the extant white-tailed deer"." Armadillos and their relatives are also known such as a pampathere, a glyptodont, and dasypodids"." One of the largest groups known from the Blancan of Florida is the ground sloths represented by three families"." The carnivorans include borophagins, hyaenids, and "saber-toothed" cats. Large rodents are represented by capybaras and porcupines. Many fossils of smaller mammals like shrews, rabbits, and muskrats have been found associated with "Titanis." Along with mammals, a menagerie of reptiles including lizards, turtles, and snakes is known from fossils. There are abundant remains of avifauna, with thousands of known fossils, including birds of prey, the teratorn "Teratornis", one of the largest flight-capable birds known, and turkeys.
南北美洲生物大迁徙.
South America, the continent where phorusrhacids originated, was isolated after the breakup of the landmass Gondwana at the end of the Mesozoic era. This period of separation from the rest of the Earth's continents led to an age of unique mammalian and avian evolution, with the dominance of phorusrhacids and sparassodonts as predators in contrast to the North American placental carnivores. The fauna of North America was composed of living groups like canids, felids, ursids, tapirids, antilocaprids, and equids populating the region alongside now extinct families like the gomphotheres, amphicyonids, and mammutids. Phorusrhacids evolved in South America to fill gaps in niches otherwise filled by placentals in other continents, such as that of apex predator. Flight-capable birds could more easily migrate between continents, creating a more homogenous avian fauna.
The Great American Interchange took place between the Paleogene and Pliocene, though most species crossed at around 2.7 million years ago. The momentous final stage witnessed the movement of glyptodonts, capybaras, pampatheres, and marsupials to North America via the Isthmus of Panama, which connected South America to the rest of the Americas, and a reverse migration of ungulates, proboscideans, felids, canids, and many other mammal groups to South America. The oldest fossil of "Titanis" is estimated to be 5 million years old, at least half a million years older than the earliest date for the Isthmus' formation about 4.5–3.5 million years ago. How "Titanis" was able to traverse the gap to North America is unknown. A hypothesis made by a 2006 article stipulated that it could have island-hopped through Central America and the Caribbean islands. "Titanis" is possibly not the only large animal to have done this; two genera of large ground sloth and a procyonid made it to North America millions of years before the volcanic formation of Panama. The period following the Isthmus' foundation saw the extinction of many groups, including the South American phorusrhacids, the last phorusrhacids went extinct in the Pleistocene. Human settlement in the Americas, climate change, and other factors likely led to the extinction of most of the remaining native South American mammal families.
灭绝.
The extinction of "Titanis" and other phorusrhacids throughout the Americas was originally theorized to have been due to competition with large placental (canid, felid, and possibly ursid) carnivores that occupied the same ancient terrestrial ecosystems during the Great American Interchange. However, this has been contested as "Titanis" had competed successfully against both groups for several million years upon entering North America. Brodkorb's description of "Titanis" as being from the latest Pleistocene, an error followed by later studies, postulated that it went extinct as recent as 15,000 BP (before present). The rare earth element analysis of "Titanis" fossils by MacFadden and colleagues in 2007 dispelled this, demonstrating that the genus lived during the Pliocene and earliest Pleistocene. Notably, some phorusrhacid material from South America dates to the Late Pleistocene, younger than "Titanis", close to the time of human arrival.-->