Saturday, January 20, 2018

Sphenisciformes: the Penguins

Penguins are flightless, marine birds belonging to the order Sphenisciformes in the family Spheniscidae. They have existed since the beginning of the Cenozoic with roots possibly dating back to the late Cretaceous, and they appear to have always been restricted to the Southern Hemisphere; both modern penguins and their extinct relatives are distributed along the coasts of Antarctica, Australia, New Zealand, sub-Saharan Africa, and South America.
A pair of Emperor Penguins (Aptenodytes forsteri) tobogganing across
an Antarctic snowscape. Wiki.
The Paleogene fossil record of penguins has been intensively studied in the last few decades with numerous new taxa having been described during that time, with argueably the most interesting discoveries coming out of New Zealand. Although the penguin lineage is believed to have split away from other birds as early as the late Cretaceous (71mya), the earliest-known and most basal penguins date back to the early to middle Paleocene (62 to 58mya) and belong to the genus Waimanu from New Zealand. The South American genus Perudyptes and a slightly older, unnamed taxon, demonstrate that penguins had expanded their range to encompass the entire southern Pacific Ocean by the middle Eocene and were probably present throughout the Atlantic Ocean as well. There are four accepted penguin subfamilies (discussed below), only one of which is still alive today.

Map showing the collective distribution of modern penguins.
 Penguins throughout their evolutionary history appear to have
been confined to the Southern Hemisphere, so this distribution
map is also applicable to fossil penguins. Wiki.
The Palaeeudyptinae, or “giant penguins”, are the most well-studied of the extinct penguin subfamilies. The earliest known member, Crossvallia unienwillia, lived during the late Paleocene (59.2–56mya) in what is now Antarctica, and therefore would have coexisted with basal penguins like Waimanu. The group persisted throughout the Paleogene and until the late Oligocene, and potentially even lasting until the middle Miocene if the tentatively placed Anthropodyptes proves to be a true member. Palaeeudyptines differ from modern penguins in a number of aspects. For example, the forelimb had not yet developed into a rigid flipper and would have retained a degree of flexibility. The beak also differs from those of modern penguins in being relatively long and spear-like, similar to that of a heron.

By far the most frequent topic addressed when discussing members of Palaeeudyptinae, is body size. True to their colloquial name, these early Cenozoic penguins were quite large by modern standards; the 20 species of modern penguin range in size from the 40cm tall and 1kg Little Blue Penguin (Eudyptula minor), up to the 110cm tall and 35kg Emperor Penguin (Aptenodytes forsteri). The early palaeeudyptine Crossvallia unienwillia was only slightly larger than an Emperor Penguin with much larger penguins, with body lengths of over 130cm, occurring from the middle Paleocene to the late Oligocene. Due to the often fragmentary and incomplete nature of penguin fossils, physical aspects such as body size and mass must be calculated by measuring individual bones and scaling them against modern specimens. Through this method, lengths of 180cm and 160cm have been calculated for Anthropornis nordenskjoeldi and Palaeeudyptes klekowskii respectively, both being widely regarded as the largest penguins known to have ever lived (the latter even being given the common name of “Colossus Penguin”).

Giant penguins appear to have become extinct early in the Neogene. The reason for their disappearance is not currently understood, although a likely explanation may be a combination of climatic changes and feeding competition from pinnipeds (seals, sea lions, and walruses) which were undergoing an adaptive radiation at the time these giant penguins had declined.

Palaeospheniscinae & Paraptenodytinae
Smaller than the large-bodied palaeeudyptines with whom they coexisted for a time, the Palaeospheniscinae and Paraptenodytinae may have been the ecological analogues of the modern spheniscine penguins, though they were probably not directly ancestral to them. Both subfamilies are relatively poorly understood at the timing of this blog post. Palaeospheniscines, also known as “slender-footed penguins”, contain five species within the genus Palaeospheniscus. They were small to medium-sized penguins that ranged in body length from 50 to 75cm, and occurred from the early Miocene to early Pliocene of southern Africa and South America. Paraptenodytines, also known as the “stout-footed penguins”, contains four known species within the genera Arthrodytes and Paraptenodytes. These occurred from the late Eocene to the middle Miocene of South America

The group containing all modern penguins, all of which belong to the subfamily Spheniscinae, arose during the late Paleogene and recognizable members of today’s genera began to appear during the middle Miocene (14 to 13mya). The radiation of this group appears to correspond to two episodes of global climatic cooling.

Examples of each of the modern genera within the penguin subfamily Sheniscinae.
From left-to-right and top-to-bottom: King Penguin (Aptenodytes patagonicus),
Adelie Penguin (Pygoscelis adeliae), Megallanic Penguin (Spheniscus magellanicus),
Little Blue Penguin (Eudyptula minor), Macaroni Penguin (Eudyptes chrysolophus),
Yellow-eyed Penguin (Megadyptes antipodes).

Among modern penguins, the genus Aptenodytes (“great penguins”) are the most basal, with DNA evidence showing that these birds split from other spheniscines around 40mya. The earliest known fossil evidence for this genus comes from the early Pliocene of New Zealand and ascribed to the species A. ridgeni. The two modern members of the genus are characterized by yellow-orange neck, breast, and beak patches. Chicks are almost naked upon hatching and brooding adults incubate their eggs on their feet beneath a specialized fold of skin.

The genus Pygoscelis (“brush-tailed penguins”) are the next to diverge with DNA evidence showing that a split occurred at about 38mya and are said to most closely resemble the common ancestor of the Spheniscinae in physical form. Fossils from the genus date to the late Miocene of South America and New Zealand. There are three modern species which breed in Antarctica and southern South America.

The oldest fossils of the genus Spheniscus (“banded penguins”) date back to the middle Miocene. There are four modern species distributed through southern Africa and the southern and western coasts of South America up to the equatorial islands of Galapagos. Members of the genus are characterized by a single band of black that runs around their bodies bordering their black dorsal coloring, black beaks with a small vertical white band, distinct spots on their bellies, and a small patch of unfeathered or thinly feathered skin around their eyes that can be either white or pink. All members of this genus raise their young in nests situated in burrows or natural depressions in the earth. They are also renowned for their loud, braying vocalizations which earn them the nickname of the "jack-ass penguins".

The genus Eudyptula (“little penguins”) contains three modern species which are distributed through southern Australia and New Zealand. They are burrow-nesters distinguished by their small body size and iridescent dorsal plumage which gives them a bluish-black appearance.

Members of the genus Eudyptes (“crested penguins”) are characterized by their hair-like yellow ornamental head feathers and their reddish-colored beaks. This is the most specious of the modern penguin genera, containing eight species. These form a clade with the New Zealand endemic genus Megadyptes, which has a single living species and one recently extinct after first contact with the Maori. The two genera apparently diverged from each other during the middle Miocene (15 or 14mya) and the modern species of Eudyptes all radiated between the late Miocene and late Pliocene (about 8 to 3mya).

Skeleton & Movement
Penguins are superbly adapted for an aquatic environment and are remarkably swift and agile when traveling through the water (video). Unlike volant (flying) birds which have lightweight, hollowed bones to reduce body weight, penguin bones are solid and heavy which decreases buoyancy, making diving easier. Unlike other flightless birds, the penguin sternum is keeled to support well-developed pectoral musculature. The flight stroke of the wings has been modified into a swimming stroke, enabling these birds to “fly” through the water. The wings of their ancestors have been modified into rigid flippers, which has been achieved by the broadening, flattening, and densification of the bones of the forelimb. While swimming, the feet and tail trail backwards and function as steering aids or rudders. The body itself is fusiform in shape for streamlining.
Articulated skeleton of the Magellanic Penguin (Sheniscus magellanicus)
shown in swimming posture. Wiki.

In contrast to their efficient aquatic locomotion, terrestrial locomotion can be rather slow and clumsy (video). Penguins primarily adopt an upright waddling gait during terrestrial locomotion during which they use their flippers and tails to maintain balance and an upright stance. A hopping motion is utilized for travel over uneven or rocky terrain. The penguin tarsometatarsus (the fusion of three metatarsals and some of the tarsal bones in birds) is very distinctive in that it is extremely short, broad, and flat. possibly to strengthen the foot. This unique shape was present in penguins by the middle Paleocene, as demonstrated by Anthropornis. This shape may correlate functionally with the aforementioned upright posture and gait adopted by modern penguins. The one known subversion of this morphology is the very basal genus Waimanu, which was contemporaneous with Anthropornis but had a tarsometatarsus that was morphologically more comparable to that of a cormorant. This suggests that these archaic penguins had a somewhat different style of terrestrial locomotion and that they probably relied more on their feet for propulsion.

Foot bones of three penguin genera, two extinct and one modern. Compare
and contrast the morphology of the tarsometatarsi between
Anthropornis (a-c), Waimanu (f-g), and Aptendytes (j-k).
Figure 1 from Mayr et al. (2017).

Skin & Feathers
Modern penguins possess a layer of fat, or blubber, several centimeters thick for insulation in cold waters. Given the largely tropical to subtropical conditions of the Paleogene, it is hard to say whether early penguins such as the giant palaeeupytines would have needed this extra layer of insulation. If not, these penguins would have appeared relatively slender compared to modern penguins despite many of them being considerably larger. What is almost certain is that, like their modern relatives, fossil penguins had a dense covering of short, rigid feathers. This plumage insulates penguins against cold air and water by trapping air near the skin. Waterproofing is achieved through the application of a special oil during grooming. When hot, penguins ruffle their feathers to allow cool air to reach their skin, thus lowering the body temperature.

Preserved skin and feathers are known for the late Eocene species Inkayacu paracasensis confirms that the penguin feather morphology was in development very early in the group’s evolutionary history. The feathers of the flipper and body, which were densely packed together, had large shafts that made them very rigid. The feathers were also short compared to those of modern penguins, with a maximum length of 3cm. This, again, may reflect the warm Paleogene environment in which these early penguins lived and the need for insulation; I. paracasensis was a larger animal which lived in tropical waters, while modern penguins are smaller and, with the exception of the Galapagos Penguin (Spheniscus mendiculus), generally inhabit colder Antarctic and Subantarctic waters.

Photograph showing the wing feathering of the late Eocene Inkayacu paracasensis
(scale = 1 cm). Figure 2 from Clarke et al. (2010).

All modern penguins have a countershaded color scheme as adults, with black or dark brown dorsal plumage and white ventral plumage. This coloration provides camouflage when viewed from the top and bottom; a predator looking up from below has difficulty distinguishing between a white penguin underside and the reflective water at the surface, while the dark plumage on the back blends the darker water seen at deeper depths. The known feathers for I. paracasensis are different in that they appear to have been gray or reddish-brown in color as determined by the shape and structure of the melanosomes. It is unclear how these colors were distributed; they may have covered the whole body or perhaps this penguin was similar to the modern Aptenodytes in having special identifying markings while the rest of the body was the more conventional black-and-white pattern. Nonetheless, this fossil eludes to the potential variability that could have existed in the plumage of fossil penguins.

Penguin eyes are specialized for underwater vision and the sense of sight is their primary means of hunting. The sense of hearing is said to be average by the standards of other birds, but nonetheless appears to be sensitive enough for individuals to identify and locate their mates and offspring within huge, densely populated nesting colonies. The first ever study published study to determine the importance of scent for individual recognition in birds demonstrated that Humbolt Penguins (Speniscus humbolti) are able to discern familial and non-familial odors, a finding which holds many implications for penguin social behavior. Interestingly, endocasts of the braincases of multiple genera demonstrate that fossil penguins such as Paraptenodytes had larger olfactory lobes than modern penguins, suggesting that the sense of smell was even more important to them behaviorally.

Endocasts of the brain (blue) and semicircular canals (pink) in several
extinct and modern penguins:
(A) an unnamed fossil from Atarctica
(B) Paraptenodytes antarcticus
(C) Emperor Penguin (Aptenodytes forsteri)
(D) African Penguin (Spheniscus demersus)
(E) Magellanic Penguin (Spheniscus magellanicus)
(F) Little Blue Penguin (Eudyptula minor)
(G) Chinstrap Penguin (Pygoscelis antarctica)
(H) Adélie Penguin (Pygoscelis adeliae)
Figure 9 from Tambussi et al. (2015)

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References & Further Reading
Mayr G, De Pietri VL, Scofield PR (2017). "A new fossil from the mid-Paleocene of New Zealand reveals an unexpected diversity of world’s oldest penguins". The Science of Nature 104(9): DOI 10.1007/s00114-017-1441-0 <Abstract>

Mayr G, Scofield PR, De Pietri VL, Tennyson AJD (2017). "A Paleocene penguin from New Zealand substantiates multiple origins of gigantism in fossil Sphenisciformes". Nature Communications 9(1927): DOI: 10.1038/s41467-017-01959-6 <Full Article>

Gavryushkina A, Heath TA, Ksepka DT, Stadler T, Welch D, Drummond AJ (2017). "Bayesian total evidence dating reveals the recent crown radiation of penguins". Systematic Biology 66(1): 57-73 <Full Article>

Tambussi CP, Degrange FJ, Ksepka DT (2015). "Endocranial anatomy of Antarctic Eocene stem penguins: implications for sensory system evolution in Sphenisciformes (Aves)". Journal of Vertebrate Paleontology: e981635 <Abstract>

Coffin HR, Watters JV, Mateo JM (2011). "Odor-based recognition of familiar and related conspecifics: a first test conducted on captive Humbolt penguins (Spheniscus humbolti)". PLoS ONE 6(9): e25003 <Full Article>

Clarke JA, Ksepka DT, Salas-Gismondi R, Altamirano AJ, Shawkey MD, D'Alba L, Vinther J, DeVries TJ, Baby P (2010). "Fossil evidence for evolution of the shape and color of penguin feathers". Science 330(6006): 954-957 <Full Article>

Baker AJ, Pereira SL, Haddrath OP, Edge KA (2006). "Multiple gene evidence for expansion of extant penguins out of Antarctica due to global cooling". Proceedings of the Royal Society B 273: 11-17 <Full Article>

Slack KE, Jones CM, Ando T, Harrison Gl, Fordyce RE, Arnason U, Penny D (2006). "Early penguin fossils, plus mitochondrial genomes, calibrate avian evolution". Molecular Biology and Evolution 23(6): 1144-1155 <Full Article>

Ksepka DT, Bertelli S, Giannini NP (2006). "The phylogeny of the living and fossil Sphenisciformes (penguins)". Cladistics 22(5): 412-441 <Abstract>

Jouventin P, Aubin T, Lengangne T (1999). "Finding a parent in a king penguin colony: the acoustic system of individual recognition". Animal Behavior 57(6): 1175-1183 <Abstract>

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