Friday, April 12, 2019

Avian Pachyostotic Bones; seeking a clear definition.

The gap between amateur and professional ornithologist is often painfully apparent to me, especially when wording is concerned.

This was the situation the other evening while I was reading Gerald Mayr’s 2017 book, Avian Evolution; The Fossil Record of Birds and its Paleo-biological Significance. In it, I came upon a peculiar word that I never saw, or heard of before.

Mayr starts by writing that the foot-propelled hesperornithiforms were highly specialized diving birds and mainly occurred in the northern latitudes of the Northern Hemisphere, most of which were flightless and lived in marine environments. 

Hesperornis regalis
Photo Credit: Paul Cianfaglione
Within hesperornithiforms a large size was gained several times independently, and in particular some species, like the Late Cretaceous Hesperornis were very large, reaching a length of 1.5 meters.

As an adaptation for their diving habits, the limb bones of hesperornithiforms are pachyostotic; that is, they have very thick bone walls. Pachyostotic bones also occur in other flightless diving birds, such as penguins, and reduce buoyancy by increasing the weight of the bird (source: Mayr, G. 2017 Avian Evolution). 

Pachyostotic, or Pachyostosis, is further described as a non-pathological condition in vertebrate animals in which the bones experience a thickening, generally caused by extra layers of lamellar bone. It often occurs together with bone densification (osteosclerosis), reducing inner cavities. This joint occurrence is called pachyosteosclerosis. However, especially in the older literature, "pachyostosis" is often used loosely, referring to all osseous specializations characterized by an increase in bone compactness and/or volume (source: Wikipedia).

Removed from the definition of Pachyostosis are two related words, including osteosclerosis and pachyosteosclerosis.

Frustrated, I decided to look deeper into other researcher’s scientific work, and see how they used and defined these terms regarding high bone compactness.

One paper I found titled; Osteosclerosis in the Extinct Cayaoa bruneti (Aves, Anseriformes): Insights on Behavior and Flightlessness (De Mendoza and Tambussi 2015), provided a slightly different description of bone histology in extant and extinct birds.    

De Mendoza and Tambussi write;

“There is a strong association between bone micro-anatomy and lifestyle. In general terms, divers have skeletons with greater bone density due to a thickening of the cortex by higher deposition of periosteal bone which is known as pachyostosis. There may be also a reduction in the medullary resorption, a process called osteosclerosis. Both of these processes may occur in combination.”

“Among wing-propelled divers, several studies showed that penguins have highly osteosclerotic limbs (Ksepka, 2007; Meister, 1962; Cerda et al., 2014). Such high osteosclerosis levels were also verified in foot-propelled divers such as the Cretaceous flightless bird Hesperornis (Marsh, 1872) and the Antarctic Cretaceous loon Polarornis gregorii (Chatterjee, 2002).” 

Even more uncertain was information provided in the 2015 paper, Bone Histology in Extant and Fossil Penguins (Aves: Sphenisciformes), by Daniel T Ksepka. He writes; High bone density in penguins results from compaction of the internal cortical tissues, and thus penguin bones are best considered osteosclerotic rather than pachyostotic.

The author also mentions that penguin limb bones differ from those of nearly all other birds in their greater bone density, near complete lack of pneumatization, unusually thick periosteum and greatly reduced medullary cavities. 

Bone Regions
Image Property:
Thick periosteum and greatly reduced medullary cavities? To the average reader like myself, the last sentence sounds a bit ambivalent, but Ksepka goes on to elaborate by stating;

“The bone structure of penguins has been variably referred to as pachyostotic (Meister, 1962; Mayr, 2005; Ksepka et al. 2006; Livezey & Zusi, 2006), osteosclerotic (de Ricqlès & de Buffrénil, 2001; Ksepka et al. 2008; Cerda et al. 2015) or pachyosteoscleortic (Houssaye, 2009). The term pachyostotic does not apply to penguins, because they do not exhibit periosteal hyperplasy: the external dimensions of the hind limb bones of penguins remain unexpanded, with hind limb bone cross-sectional area remaining within the range observed in similarly sized volant birds (DTK personal observation). Comparisons of the external dimensions of wing bones between penguins and volant birds are more difficult due to the highly flattened and shortened morphology of the penguin flipper. However, Meister (1962) demonstrated that the extremely high bone density of the penguin humerus is achieved solely through compaction of the internal tissues, without expansion of the external circumferential layer. This finding was further supported by observations from an ontogenetic series of Aptenodytes patagonicus specimens by Canoville (2010). Thus, the term ‘osteosclerotic’ is more accurately applied to penguin limb bones (de Ricqlès & de Buffrénil, 2001).”

It is also important to point out why this condition is important to certain bird species. Ksepka continues;

“Dense bone commonly has been interpreted as a means of reducing buoyancy and thus conserving energy during diving (e.g. Nopcsa, 1923; Wall, 1983; Lovvorn et al. 1999). Additionally, increased thickness of the limb bone cortices can enhance resistance to bending or torsional loads, which are substantially higher in wing-propelled diving relative to most conditions encountered during aerial flight (Habib & Ruff, 2008). Because extant penguin wing bones exhibit cortical thickness far above that required for bending resistance alone, it is plausible that this increase in thickness serves both to increase ballast and to increase resistance (Habib, 2010).”

Below is a comparison in weight of two similar sized bones; the humerus of a Miocene Banded Penguin (Spheniscus urbinai), the other of a Pleistocene Wild Turkey (Meleagris gallopavo). Despite the penguin bones smaller dimensions, its weight is nearly double that of the turkeys. 

Banded Penguin and Wild Turkey Humerus
Photo Credit: Paul Cianfaglione
Banded Penguin Humerus Weight in Grams
Photo Credit: Paul Cianfaglione
Wild Turkey Humerus Weight in Grams
Photo Credit: Paul Cianfaglione

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