翼竜の長い首、支えたのは「自転車スポーク」様の骨

【ワシントンＡＦＰ＝時事】恐竜が生息していた時代に空を舞っていたアズダルコ科に属する巨大な翼竜。興味深い特徴の一つは、キリンをもしのぐその極めて長い首だ。最長３メートルにおよぶと推定され、どのようにして首を折ることなく頭の重さを支えていたのか、という疑問が上がっていた。（写真は自転車のスポークのような構造をした翼竜の脊椎。英ポーツマス大学提供）
　学術誌「アイサイエンス」に掲載された論文によると、英ポーツマス大学の研究チームは、モロッコで発掘された保存状態の良い脊椎（背骨）の標本を分析。極めて軽量だが負荷に耐え得るスポーク状の骨が複雑に配置された、脊椎内側の構造がその答えだと考えた。
　現生種、絶滅種にかかわらず、動物界でこれに相当する構造は他に知られていない。
　脊椎の中央には、神経が通る神経管がある。神経管と脊椎の外壁をつなぐのは、自転車のスポークのように放射状に重なり合った複数の細い骨だ。
　このスポーク状の細い骨は脊椎の縦方向にもらせん状に走っており、これが骨の強度をさらに増している。
　研究チームが生体力学技術者と協力して実施した計算では、わずか５０本ほどのスポーク状の骨が、翼竜の首の耐荷重を９０％増やしたとの結果が示唆された。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2021/04/16-12:33）

Azhdarchid pterosaurs were massive flying reptiles that soared across the skies in the age of the dinosaurs, using their long bills to pick out their prey of fish and other river animals.
One of the most intriguing things about them has been the extreme length of their necks -- estimated at up to three meters (10 feet), which is longer than a giraffe's and raised questions about how the animals could support its weight without snapping.
By studying well-preserved vertebrae specimens excavated from Morocco, a team of scientists think they have the answer: a complex assemblage of extremely light yet weight-bearing spokes inside the bones.
Cariad Williams, first author of a new paper that appeared in iScience, told AFP that the team had an idea that the inside of the vertebral column housed a sophisticated internal structure.
They sent the specimens off for CT scan, and we just couldn't believe what we'd found -- it is one of the most unique structures that we've ever seen, said the Phd student at the University of Illinois at Urbana-Champaign.
It has no known equivalent in the animal kingdom, either modern or extinct, and I'm just surprised nobody found it sooner, added Williams.
The neural tube, which carries nerves through the back bone, is in the center.
It connects to the outer walls of the vertebrae via fine bones called trabeculae, that are radially arranged and cross over each other, just like the spokes of a bicycle.
These fine bones also run up and down the length of the vertebrae in a helix-like arrangement, adding further strength.
The team then collaborated with biomechanical engineers whose calculations suggested that as few as 50 of the spoke-like bones increased the amount of weight the animals could carry by 90 percent.
Co-author David Martill from the University of Portsmouth said in a statement that the discovery resolved many concerns about the biomechanics of how these creatures were able to support massive heads -- longer than 1.5 meters -- on necks longer than the modern-day giraffe, all whilst retaining the ability of powered flight.
Relatively little is known about pterosaurs, and they have previously been dismissed as evolutionary dead ends, instead of being a research priority.
Yet the new findings show them to be fantastically complex and sophisticated, and worthy of much deeper study, added Martill and the team.