Phorusrhacos (meaning "bearer of scars" or "bearer of wrinkles") is an extinct genus of phorusrhacid, a group of predominantly large, predatory flightless birds commonly known as terror birds. It lived during the Early to Middle Miocene, approximately 18–13 million years ago, in what is now southern Argentina. The first fossil attributed to the genus, an isolated fragment of the mandible (lower jaw), was discovered in 1887 by Argentine paleontologist Carlos Ameghino in sediments of the Santa Cruz Formation in Santa Cruz Province, Argentina. Later that year, his brother, Florentino Ameghino, formally described the specimen as the new genus and species Phorusrhacos longissimus. Florentino initially interpreted the fossil as belonging to a giant, toothless sloth and assigned the genus to its own family, Phorusrhacidae. In 1891, however, Argentine naturalist Francisco Moreno recognized that the specimen was avian rather than mammalian. Since its initial description, Phorusrhacos has become one of the best-known phorusrhacid genera, with fossils documented from numerous localities in southern Argentina.
Phorusrhacos is estimated to have stood tall and weighed around , placing it among the largest known phorusrhacids. Despite its size, it was more lithe and cursorial (built for running) than the similarly sized Kelenken and Devincenzia. Like other phorusrhacids, it had a tall, elongated skull with a sharply hooked beak. The skull was laterally compressed, particularly when viewed from the front. Its cervical vertebrae were short, thick, and broad, features that may have strengthened the neck for delivering powerful strikes. The hindlimbs were long and slender, with the tarsometatarsus measuring approximately 70% the length of the tibiotarsus and possessing a relative slender shaft.
During the Miocene, the region of present-day Santa Cruz Province was characterized by a mosaic of savanna-like grasslands, shrublands, and woodlands. At the time, South America remained geographically isolated from other continents, supporting a distinctive assemblage of mammalian, avian, and reptilian fauna before the Great American Interchange. Phorusrhacos occupied an apex predator role in these Santacrucian ecosystems, likely preying on a variety of native mammals, such as ground sloths, notoungulates, and litopterns.
Discovery and naming
Initial description
The taxonomic history of Phorusrhacos is muddled due to the fragmentary nature of fossils, few of which overlap, and little consensus between scientists. In the 19th century, interest in the rich "Tertiary" (now Neogene) fossil sites of Argentina boomed. This included exploration by two teams of Argentine paleontologists: Florentino and Carlos Ameghino, who operated independently, and Francisco Pascasio Moreno and Alcides Mercerat of the Museo de La Plata (MLP). Their discoveries comprised thousands of fossils from Cenozoic fossil deposits throughout Patagonia and central Argentina. Initially, Moreno hired Florentino and Carlos Ameghino as staff at the MLP in 1887; Florentino operated as a researcher and Deputy Director, while his brother conducted expeditions and assisted with preparation. However, the relationship between Moreno and Florentino Ameghino deteriorated, culminating in the Ameghinos' resignation from the MLP in 1888. Operating independently of any institutions until 1902, the Ameghinos competed with Moreno and the Museo de La Plata to discover and describe as many new fossils as they could. This led to the description of many genera and species based on fragmentary, undiagnostic, or poorly described material.
The first known discovery of Phorusrhacos was made in 1887 by Carlos Ameghino during an expedition to an outcrop of the Miocene-aged Santa Cruz Formation along the Santa Cruz River in Santa Cruz Province, Argentine Patagonia. This formation dates to the Santacrucian South American Land Mammal Age (a series of mammalian fauna-based geologic ages), which lasted from 22 to 16 million years ago, and preserves a diverse array of endemic birds, mammals, reptiles, and more. This trip alone produced hundreds of fossils, which were described as over 120 novel species of fossil mammals in 1887. The descriptions produced by Florentino were short, vague, and sought to establish priority for new taxa before his competitor, Moreno, could. Along the Santa Cruz River, Carlos unearthed an isolated, incomplete edentulous (toothless) mandible (lower jaw), which was later deposited at the MLP. Florentino believed this fossil belonged to a new genus and species of edentate mammal, around the size of Mylodon, which he named Phorusrhacos longissimus. He chose the mandible (specimen number MLP-118) as the holotype (name-bearing) specimen of P. longissimus. The etymology of Phorusrhacos is convoluted. Ameghino did not explain the etymology of the generic name Phorusrhacos or the specific name longissimus. The former is derived from the Greek -φόρος, (-phoros), meaning "bearer" in word combinations, and ῥάκος, (rhakos), "rag" or "wrinkle", probably in reference to the wrinkled jaw surface of the holotype. This creates the literal meaning "wrinkle bearer (jaw)" for Phorusrhacos. When the original derivation was no longer understood, other translations were proposed, such as the literal translation "rag-thief" and "branch-holder" based on mistaken assumption that the name was intended to derive from the Greek rhakis, "branch". The specific name means "very long" in Latin, again in reference to the holotype.
Competition
In 1889, Ameghino published a detailed monograph on Argentine fossil mammals, including Phorusrhacos. Here, he went into greater detail on its anatomy, classification, and attempted to amend the generic name to Phororhracos. However, the original name Phorusrhacos has priority. Despite this, Phororhacos continued to be used for decades until 1967, when American paleontologist Pierce Brodkorb corrected its use. Ameghino (1891) also erected the family Phororhacosidae (now Phorusrhacidae) and classified it in Edentata. Not knowing it was avian, Ameghino theorized that Phorusrhacos had a bird or turtle-like keratin sheath on its jaws.
At the same time as Ameghino's discoveries, Moreno and the MLP sent out its own teams to explore the Santacrucian deposits of Patagonia. In 1888, Moreno briefly mentioned that "large birds" had been found in Monte Hermoso by these expeditions. The next year, he classified them as belonging to two new genera of lamellirostral web-footed birds: Mesembriornis and Palaeociconia. Though these fossils do not belong to Phorusrhacos. This led Carlos Ameghino to reconsider the classification of Phorusrhacos; in an 1890 letter to his brother, he was the first to propose that Phorusrhacos is a bird, based on Moreno's discoveries. However, Florentino reaffirmed that it was mammalian in his response.
In 1891, Moreno and Alcides Mercerat published a monograph describing many avian fossils from the Tertiary of Argentina. They named several genera and species, such as Owenornis and Stereornis, based on fragmentary material. Furthermore, they placed these taxa in the now-defunct order Stereornithes. Unlike Ameghino's work, this monograph features figures and illustrations, giving a better impression of the anatomy of "stereornithine" birds. This featured a figure of the P. longissimus holotype, which they misinterpreted as being a (front jaw bone), and assigned to a bird rather than an edentate mammal. Later studies have considered Stereornithes a synonym of Phorusrhacidae. Many of the genera and species described in the paper are synonyms of P. longissimus.In 1891, Ameghino named the genus and species Tolmodus inflatus, which he believed was also a ground sloth. That same year, Carlos unearthed a poorly preserved skull and mandible (NHMUK-A529) of Phorusrhacos from a locality in Santa Cruz. This skull was noted to measure in length in the field, and it soon fragmented, leaving only the mandible, beak tip, and a fragment from the of the (a ridge forming the top part of the orbit) to be recovered. Following its discovery, Ameghino gave a comprehensive description of Phorusrhacos and corrected his previous error by classifying it as a bird. In this paper, Ameghino figured and described the holotype of P. longissimus in detail, assigned Tolmodus to Phorusrhacos, and named additional Phorusrhacos species. In 1895, Florentino described the mandible in detail and noted the discovery of the fragmented skull, including an illustration. Additionally, the paper had illustrations and detailed analysis of many P. longissimus fossils for the first time, including vertebrae and limb bones.
In response to Moreno and Mercerat (1891), Ameghino stated that many of the species they had named were synonyms of Phorusrhacos and were inadequately described. In the same paper, Ameghino classified Phorusrhacidae within Ratites, a group of giant birds like ostriches (Struthio), Aepyornis and Dinornis. Ameghino went on to claim that Carlos was the first to recognize the existence of giant birds in the Tertiary of South America in 1891, when in actuality Moreno had in 1889. In 1895 and 1896, Ameghino and Moreno sold much of their respective fossil collections to the Natural History Museum of London. By 1911, Ameghino had died, and Moreno had retired, leaving behind a legacy of convoluted phorusrhacid taxonomy. Many European naturalists did not study Ameghino's or Moreno's taxa, especially their phorusrhacids, on account of their confusing taxonomy.
Later study American expeditions
At the close of the 19th century, American universities and institutions began taking an interest in the Neogene fauna of Patagonia. Inspired by Ameghino's papers and descriptions, these universities and institutions sought to establish their own collections of Santacrucian mammals and birds, including those of Phorusrhacos. Between 1896 and 1899, Princeton University students, under the leadership of American paleontologist John Bell Hatcher, led a series of expeditions to the same outcrops in Santa Cruz Province. Princeton was joined by the American Museum of Natural History on the last of these expeditions, altogether collecting a trove of new fossil bird and mammal material. Furthermore, in 1902, American professor William B. Scott visited the MLP and Ameghino's collections to photograph and review the taxa named by Ameghino, Moreno, and Mercerat. His studies, along with those of Hatcher, American paleontologist William John Sinclair, and others, culminated in a series of monographs published between 1903 and 1912 by Princeton University. Although they reviewed Ameghino's mammal taxa in detail, their description of some of the phorusrhacid remains was brief. Meanwhile, in 1926, American paleontologist Marcus Farr traveled to the Natural History Museum of London in order to study the Ameghino collection, with the assistance of Dr. F. A. Bather, the museum's curator.